@phdthesis{Schifferle2024,
  author    = {Schifferle, Lukas},
  title     = {Optical properties of (Mg,Fe)O at high pressure},
  doi       = {10.25932/publishup-62216},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-622166},
  school      = {Universit{\"a}t Potsdam},
  pages     = {XIV, 90},
  year      = {2024},
  abstract  = {Large parts of the Earth's interior are inaccessible to direct observation, yet global geodynamic processes are governed by the physical material properties under extreme pressure and temperature conditions. It is therefore essential to investigate the deep Earth's physical properties through in-situ laboratory experiments. With this goal in mind, the optical properties of mantle minerals at high pressure offer a unique way to determine a variety of physical properties, in a straight-forward, reproducible, and time-effective manner, thus providing valuable insights into the physical processes of the deep Earth. This thesis focusses on the system Mg-Fe-O, specifically on the optical properties of periclase (MgO) and its iron-bearing variant ferropericlase ((Mg,Fe)O), forming a major planetary building block. The primary objective is to establish links between physical material properties and optical properties. In particular the spin transition in ferropericlase, the second-most abundant phase of the lower mantle, is known to change the physical material properties. Although the spin transition region likely extends down to the core-mantle boundary, the ef-fects of the mixed-spin state, where both high- and low-spin state are present, remains poorly constrained. In the studies presented herein, we show how optical properties are linked to physical properties such as electrical conductivity, radiative thermal conductivity and viscosity. We also show how the optical properties reveal changes in the chemical bonding. Furthermore, we unveil how the chemical bonding, the optical and other physical properties are affected by the iron spin transition. We find opposing trends in the pres-sure dependence of the refractive index of MgO and (Mg,Fe)O. From 1 atm to ~140 GPa, the refractive index of MgO decreases by ~2.4\% from 1.737 to 1.696 (±0.017). In contrast, the refractive index of (Mg0.87Fe0.13)O (Fp13) and (Mg0.76Fe0.24)O (Fp24) ferropericlase increases with pressure, likely because Fe Fe interactions between adjacent iron sites hinder a strong decrease of polarizability, as it is observed with increasing density in the case of pure MgO. An analysis of the index dispersion in MgO (decreasing by ~23\% from 1 atm to ~103 GPa) reflects a widening of the band gap from ~7.4 eV at 1 atm to ~8.5 (±0.6) eV at ~103 GPa. The index dispersion (between 550 and 870 nm) of Fp13 reveals a decrease by a factor of ~3 over the spin transition range (~44-100 GPa). We show that the electrical band gap of ferropericlase significantly widens up to ~4.7 eV in the mixed spin region, equivalent to an increase by a factor of ~1.7. We propose that this is due to a lower electron mobility between adjacent Fe2+ sites of opposite spin, explaining the previously observed low electrical conductivity in the mixed spin region. From the study of absorbance spectra in Fp13, we show an increasing covalency of the Fe-O bond with pressure for high-spin ferropericlase, whereas in the low-spin state a trend to a more ionic nature of the Fe-O bond is observed, indicating a bond weakening effect of the spin transition. We found that the spin transition is ultimately caused by both an increase of the ligand field-splitting energy and a decreasing spin-pairing energy of high-spin Fe2+.},
  language  = {en}
}
@phdthesis{Haubitz2021,
  author    = {Haubitz, Toni},
  title     = {Transient absorption spectroscopy},
  doi       = {10.25932/publishup-53509},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-535092},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xiii, 176},
  year      = {2021},
  abstract  = {The optical properties of chromophores, especially organic dyes and optically active inorganic molecules, are determined by their chemical structures, surrounding media, and excited state behaviors. The classical optical go-to techniques for spectroscopic investigations are absorption and luminescence spectroscopy. While both techniques are powerful and easy to apply spectroscopic methods, the limited time resolution of luminescence spectroscopy and its reliance on luminescent properties can make its application, in certain cases, complex, or even impossible. This can be the case when the investigated molecules do not luminesce anymore due to quenching effects, or when they were never luminescent in the first place. In those cases, transient absorption spectroscopy is an excellent and much more sophisticated technique to investigate such systems. This pump-probe laser-spectroscopic method is excellent for mechanistic investigations of luminescence quenching phenomena and photoreactions. This is due to its extremely high time resolution in the femto- and picosecond ranges, where many intermediate or transient species of a reaction can be identified and their kinetic evolution can be observed. Furthermore, it does not rely on the samples being luminescent, due to the active sample probing after excitation. In this work it is shown, that with transient absorption spectroscopy it was possible to identify the luminescence quenching mechanisms and thus luminescence quantum yield losses of the organic dye classes O4-DBD, S4-DBD, and pyridylanthracenes. Hence, the population of their triplet states could be identified as the competitive mechanism to their luminescence. While the good luminophores O4-DBD showed minor losses, the S4-DBD dye luminescence was almost entirely quenched by this process. However, for pyridylanthracenes, this phenomenon is present in both the protonated and unprotonated forms and moderately effects the luminescence quantum yield. Also, the majority of the quenching losses in the protonated forms are caused by additional non-radiative processes introduced by the protonation of the pyridyl rings. Furthermore, transient absorption spectroscopy can be applied to investigate the quenching mechanisms of uranyl(VI) luminescence by chloride and bromide. The reduction of the halides by excited uranyl(VI) leads to the formation of dihalide radicals X^(·-2). This excited state redox process is thus identified as the quenching mechanism for both halides, and this process, being diffusion-limited, can be suppressed by cryogenically freezing the samples or by observing these interactions in media with a lower dielectric constant, such as ACN and acetone.},
  language  = {en}
}
@phdthesis{Fritzewski2021,
  author    = {Fritzewski, Dario Jasper},
  title     = {From fast to slow rotation in the open clusters NGC 2516 and NGC 3532},
  doi       = {10.25932/publishup-53135},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-531356},
  school      = {Universit{\"a}t Potsdam},
  pages     = {viii, 137},
  year      = {2021},
  abstract  = {Angular momentum is a particularly sensitive probe into stellar evolution because it changes significantly over the main sequence life of a star. In this thesis, I focus on young main sequence stars of which some feature a rapid evolution in their rotation rates. This transition from fast to slow rotation is inadequately explored observationally and this work aims to provide insights into the properties and time scales but also investigates stellar rotation in young open clusters in general. I focus on the two open clusters NGC 2516 and NGC 3532 which are ~150 Myr (zero-age main sequence age) and ~300 Myr old, respectively. From 42 d-long time series photometry obtained at the Cerro Tololo Inter-American Observatory, I determine stellar rotation periods in both clusters. With accompanying low resolution spectroscopy, I measure radial velocities and chromospheric emission for NGC 3532, the former to establish a clean membership and the latter to probe the rotation-activity connection. The rotation period distribution derived for NGC 2516 is identical to that of four other coeval open clusters, including the Pleiades, which shows the universality of stellar rotation at the zero-age main sequence. Among the similarities (with the Pleiades) the "extended slow rotator sequence" is a new, universal, yet sparse, feature in the colour-period diagrams of open clusters. From a membership study, I find NGC 3532 to be one of the richest nearby open clusters with 660 confirmed radial velocity members and to be slightly sub-solar in metallicity. The stellar rotation periods for NGC 3532 are the first published for a 300 Myr-old open cluster, a key age to understand the transition from fast to slow rotation. The fast rotators at this age have significantly evolved beyond what is observed in NGC 2516 which allows to estimate the spin-down timescale and to explore the issues that angular momentum models have in describing this transition. The transitional sequence is also clearly identified in a colour-activity diagram of stars in NGC 3532. The synergies of the chromospheric activity and the rotation periods allow to understand the colour-activity-rotation connection for NGC 3532 in unprecedented detail and to estimate additional rotation periods for members of NGC 3532, including stars on the "extended slow rotator sequence". In conclusion, this thesis probes the transition from fast to slow rotation but has also more general implications for the angular momentum evolution of young open clusters.},
  language  = {en}
}
@phdthesis{Bouma2021,
  author    = {Bouma, Sietske},
  title     = {The circum- and intergalactic medium and its connection to the large scale structure in the nearby universe},
  doi       = {10.25932/publishup-52085},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-520852},
  school      = {Universit{\"a}t Potsdam},
  pages     = {iii, 102},
  year      = {2021},
  abstract  = {The majority of baryons in the Universe is believed to reside in the intergalactic medium (IGM). This makes the IGM an important component in understanding cosmological structure formation. It is expected to trace the same dark matter distribution as galaxies, forming structures like filaments and clusters. However, whereas galaxies can be observed to be arranged along these large-scale structures, the spatial distribution of the diffuse IGM is not as easily unveiled. Absorption line studies of quasar (QSO) spectra can help with mapping the IGM, as well as the boundary layer between IGM and galaxies: the circumgalactic medium (CGM). By studying gas in the Local Group, as well as in the IGM, this study aims to get a better understanding of how the gas is linked to the large-scale structure of the local Universe and the galaxies residing in that structure. Chapter 1 gives an introduction to the CGM and IGM, while the methods used in this study are explained in Chapter 2. Chapter 3 starts on a relatively small cosmological scale, namely that of our Local Group, which includes i.a. the Milky Way (MW) and the M31. Within the CGM of the MW, there exist denser clouds, some of which are infalling while others are moving away from the Galactic disc. To study these clouds, 29 QSO spectra obtained with the Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope (HST) were analysed. Abundances of Si II, Si III, Si IV, C II, and C IV were measured for 69 HVCs belonging to two samples: one in the direction of the LG's barycentre and the other in the anti-barycentre direction. Their velocities range from -100 ≥ vLSR ≥ -400 km/s for the barycentre sample and between +100 ≤ vLSR ≤ +300 km/s for the anti-barycentre sample. By using Cloudy models, these data could then be used to derive gas volume densities for the HVCs. Because of the relationship between density and pressure of the ambient medium, which is in turn determined by the Galactic radiation field, the distances of the HVCs could be estimated. From this, a subsample of absorbers located in the direction of M31 was found to exist outside of the MW's virial radius, their low densities (log nH ≤ -3.54) making it likely for them to be part of the gas in between the MW and M31. No such low-density absorbers were found in the anti-barycentre sample. Our results thus hint at gas following the dark matter potential, which would be deeper between the MW and M31 as they are by far the most massive members of the LG. From this bridge of gas in the LG, this study zooms out to the large-scale structure of the local Universe (z ~ 0) in Chapter 4. Galaxy data from the V8k catalogue and QSO spectra from COS were used to study the relation between the galaxies tracing large-scale filaments and the gas existing outside of those galaxies. This study used the filaments defined in Courtois et al. (2013). A total of 587 Lyman α (Lyα) absorbers were found in the 302 QSO spectra in the velocity range 1070 - 6700 km/s. After selecting sightlines passing through or close to these filaments, model spectra were made for 91 sightlines and 215 (227) Lyα absorbers (components) were measured in this sample. The velocity gradient along each filament was calculated and 74 absorbers were found within 1000 km/s of the nearest filament segment. In order to find whether the absorbers are more tied to galaxies or to the large-scale structure, equivalent widths of the Lyα absorbers were plotted against both galaxy and filament impact parameters. While stronger absorbers do tend to be closer to either galaxies or filaments, there is a large scatter in this relation. Despite this large scatter, this study found that the absorbers do not follow a random distribution either. They cluster less strongly around filaments than galaxies, but stronger than random distributions, as confirmed by a Kolmogorov-Smirnov test. Furthermore, the column density distribution function found in this study has a slope of -β = 1.63±0.12 for the total sample and -β =1.47±0.24 for the absorbers within 1000 km/s of a filament. The shallower slope for the latter subsample could indicate an excess of denser absorbers within the filament, but they are consistent within errors. These values are in agreement with values found in e.g. Lehner et al. (2007); Danforth et al. (2016). The picture that emerges from this study regarding the relation between the IGM and the large-scale structure in the local Universe fits with what is found in other studies: while at least part of the gas traces the same filamentary structure as galaxies, the relation is complex. This study has shown that by taking a large sample of sightlines and comparing the data gathered from those with galaxy data, it is possible to study the gaseous large-scale structure. This approach can be used in the future together with simulations to get a better understanding of structure formation and evolution in the Universe.},
  language  = {en}
}
@phdthesis{Krstulovic2021,
  author    = {Krstulovic, Marija},
  title     = {Local structure of network formers and network modifiers in silicate melts at high pressure and temperature conditions},
  doi       = {10.25932/publishup-51641},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-516415},
  school      = {Universit{\"a}t Potsdam},
  pages     = {137},
  year      = {2021},
  abstract  = {Silikatische Schmelzen sind wichtiger Bestandteil des Erdinneren und als solche leisten sie in magmatischen Prozessen einen wesentlichen Beitrag in der Dynamik der festen Erde und der chemischen Entwicklung des gesamten Erdk{\"o}pers. Makroskopische physikalische und chemische Eigenschaften wie Dichte, Kompressibilit{\"a}t, Viskosit{\"a}t, Polymerisationsgrad etc. sind durch die atomare Struktur der Schmelzen bestimmt. In Abh{\"a}ngigkeit vom Druck, aber auch von der Temperatur und der chemischen Zusammensetzung zeigen silikatische Schmelzen unterschiedliche strukturelle Eigenschaften. Diese Eigenschaften sind am besten durch die lokale Koordinationsumgebung, d.h. Symmetrie und Anzahl der Nachbarn (Koordinationszahl) eines Atoms, sowie dem Abstand zwischen Zentralatom und Nachbarn (atomarer Abstand) beschrieben. Mit steigendem Druck und Temperatur, das heißt mit der zunehmenden Tiefe in der Erde, nimmt die Dichte der Schmelzen zu, welches zur Ver{\"a}nderung von Koordinationszahl und Abst{\"a}nden f{\"u}hren kann. Bei gleichbleibender Koordinationszahl nimmt der Abstand in der Regel zu. Kommt es zu Erh{\"o}hung der Koordinationszahl kann der Abstand zunehmen. Diese allgemeinen Trends k{\"o}nnen allerdings stark variieren, welches insbesondere auf die chemische Zusammensetzung zur{\"u}ckzuf{\"u}hren ist. Dadurch, dass nat{\"u}rliche Schmelzen der tiefen Erde f{\"u}r direkte Untersuchungen nicht zug{\"a}nglich sind, um ihre Eigenschaften unter den relevanten Bedingungen zu verstehen, wurden umfangreiche experimentelle und theoretische Untersuchungen bisher durchgef{\"u}hrt. Dies wurde h{\"a}ufig am Beispiel von amorphen Proben der Endglieder SiO2, und GeO2 studiert, wobei letzteres als strukturelles und chemisches Analogmodell zu SiO2 dient. Meistens wurden die Experimente bei hohem Druck und bei Raumtemperatur durchgef{\"u}hrt. Nat{\"u}rliche Schmelzen sind chemisch deutlich komplexer als die einfachen Endglieder SiO2 und GeO2, so dass die Beobachtungen an diesen m{\"o}glicherweise zu falschen Verdichtungsmodellen f{\"u}hren k{\"o}nnen. Weiterhin k{\"o}nnen die Untersuchungen an Gl{\"a}sern bei Raumtemperatur potentiell starke Abweichungen zu Eigenschaften von Schmelzen bei nat{\"u}rlichen thermodynamischen Bedingungen aufweisen. Das Ziel dieser Dissertation war es zu erl{\"a}utern, welchen Einfluss die Zusammensetzung und die Temperatur auf die strukturelle Eigenschaften der Schmelzen unter hohen Dr{\"u}cken haben. Um das zu verstehen, haben wir komplexe alumino-germanatische und alumino-silikatische Gl{\"a}ser studiert. Genauer gesagt, wir haben synthetische Gl{\"a}ser studiert, die eine Zusammensetzung wie das Mineral Albit und wie eine Mischung von Albit-Diopsid im eutektischen Punkt haben. Das Albitglas {\"a}hnelt strukturell einer vereinfachten granitischen Schmelze, w{\"a}hrend das Albit-Diopsid-Glas eine vereinfachte basaltische Schmelze simuliert. Um die lokale Koordinationsumgebung der Elemente zu studieren, haben wir die R{\"o}ntgenabsorptionsspektroskopie in Kombination mit einer Diamantstempelzelle benutzt. Dadurch, dass die Diamanten eine hohe Absorption f{\"u}r R{\"o}ntgenstrahlung mit Energien unterhalb von 10 keV aufweisen, ist die unmittelbare Untersuchung der geologisch sehr relevanten Elemente wie Si, Al, Ca, Mg etc. mit dieser Spektroskopie in Kombination mit einer Diamantstempelzelle nicht m{\"o}glich. Deswegen wurden die Gl{\"a}ser mit Ge und Sr dotiert. Diese Elemente dienen teilweise oder vollst{\"a}ndig als Ersatzelemente f{\"u}r wichtige Hauptelemente. In diesem Sinne, dient Ge als Ersatzelement f{\"u}r Si und andere Netzwerkbildner, w{\"a}hrend Sr Netzwerkwandler wie Z.B. Ca, Na, Mg etc., sowie andere Kationen mit großem Ionenradius ersetzt. Im ersten Schritt haben wir die Ge K-Kante im Ge-Albit-Glass, NaAlGe3O8, bei Raumtemperatur bis 131 GPa untersucht. Dieses Glas hat eine h{\"o}here chemische Komplexit{\"a}t als SiO2 und GeO2, aber es ist immer noch vollst{\"a}ndig polymerisiert. Die Unterschiede im Verdichtungsmechanismus zwischen diesem Glas und den einfachen Oxiden k{\"o}nnen so eindeutig auf h{\"o}here chemische Komplexit{\"a}t zur{\"u}ckgef{\"u}hrt werden. Die partiell mit Ge und Sr dotierten Albit und Albit-Diopsid-Zusammensetzungen wurden bei Raumtemperatur f{\"u}r Ge bis 164 GPa und f{\"u}r Sr bis 42 GPa untersucht. W{\"a}hrend das Albitglass wie NaAlGe3O8 nominelll vollst{\"a}ndig polymerisiert ist, ist das Albit-Diopsid Glas teilweise depolymerisiert. Die Ergebnisse zeigen, dass in allen drei Gl{\"a}sern strukturelle An̈derungen in den ersten 25 bis maximal 30 GPa stattfinden, wobei beide Ge und Sr die maximale Koordinationszahl 6 bzw. ∼9 erreichen. Bei h{\"o}heren Dr{\"u}cken findet in den Gl{\"a}sern nur eine isostrukturelle Schrumpfung der Koordinationspolyeder statt. Der wichtigste Befund der Hochdruckstudien an den alumino-silikatischen und alumino-germanatischen Gl{\"a}sern ist, dass in diesen komplexen Gl{\"a}sern die Polyeder eine viel h{\"o}here Kompressibilit{\"a}t aufweisen als bei den Endgliedern zu beobachten. Das zeigt sich insbesondere durch die starke Verk{\"u}rzung der Ge-O Abst{\"a}nde in dem amorphen NaAlGe3O8 und Albit-Diopsid-Glas bei Dr{\"u}cken {\"u}ber 30 GPa. Zus{\"a}tzlich zu den Effekten der Zusammensetzung auf den Verdichtungsprozess, haben wir den Einfluss der Temperatur auf die strukturelle {\"A}nderungen untersucht. Dazu haben wir das Albit-Diopsid-Glas untersucht, da es den Schmelzen im unteren Mantel chemisch am {\"a}hnlichsten ist. Wir haben die Ge K-Kante der Probe mit einer resistiv-geheizten und einer Laser-geheizter Diamantstempelzelle untersucht, f{\"u}r einen Druckbereich bis zu 48 GPa, sowie einen Temperaturbereich bis 5000 K. Hohe Temperaturen, bei denen die Probe fl{\"u}ssig ist und die f{\"u}r den Erdmantel relevant sind, haben einen bedeutenden Einfluss auf die strukturelle Transformation. Diese wird um ca. 30\% zu deutlich niedrigeren Dr{\"u}cken verschoben, im Vergleich zu den Gl{\"a}sern bei Raumtemperatur und unterhalb von 1000 K. Die Ergebnisse dieser Dissertation stellen einen wichtigen Beitrag fur das Verst{\"a}ndnis der Eigenschaften von Schmelzen unter Bedingungen des unteren Mantels dar. Im Kontext der Diskussion {\"u}ber die Existenz und den Ursprung von silikatischen Schmelzen mit ultrahoher Dichte, welche an der Grenze zwischen Mantel und Erdkern aufgrund seismologischer Daten vermutet werden, zeigen diese Untersuchugen, dass die im Vergleich zur Umgebung h{\"o}here Dichte nicht durch strukturelle Besonderheiten, sondern durch eine besondere chemische Zusammensetzung erkl{\"a}rt werden m{\"u}ssen. Außerdem legen die Ergebnisse nahe, dass f{\"u}r Schmelzen im unteren Erdmantel nur sehr geringe L{\"o}slichkeiten von Edelgasen zu erwarten sind, so dass die strukturellen Eigenschaften deutlich den Gesamthaushalt und Transport der Edelgase im Erdmantel beeinflussen.},
  language  = {en}
}
@phdthesis{Ramachandran2019,
  author    = {Ramachandran, Varsha},
  title     = {Massive star evolution, star formation, and feedback at low metallicity},
  doi       = {10.25932/publishup-43245},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-432455},
  school      = {Universit{\"a}t Potsdam},
  pages     = {291},
  year      = {2019},
  abstract  = {The goal of this thesis is to broaden the empirical basis for a better, comprehensive understanding of massive star evolution, star formation and feedback at low metallicity. Low metallicity massive stars are a key to understand the early universe. Quantitative information on metal-poor massive stars was sparse before. The quantitative spectroscopic studies of massive star populations associated with large-scale ISM structures were not performed at low metallicity before, but are important to investigate star-formation histories and feedback in detail. Much of this work relies on spectroscopic observations with VLT-FLAMES of ~500 OB stars in the Magellanic Clouds. When available, the optical spectroscopy was complemented by UV spectra from the HST, IUE, and FUSE archives. The two representative young stellar populations that have been studied are associated with the superbubble N 206 in the Large Magellanic Cloud (LMC) and with the supergiant shell SMC-SGS 1 in the Wing of the Small Magellanic Cloud (SMC), respectively. We performed spectroscopic analyses of the massive stars using the nonLTE Potsdam Wolf-Rayet (PoWR) model atmosphere code. We estimated the stellar, wind, and feedback parameters of the individual massive stars and established their statistical distributions. The mass-loss rates of N206 OB stars are consistent with theoretical expectations for LMC metallicity. The most massive and youngest stars show nitrogen enrichment at their surface and are found to be slower rotators than the rest of the sample. The N 206 complex has undergone star formation episodes since more than 30 Myr, with a current star formation rate higher than average in the LMC. The spatial age distribution of stars across the complex possibly indicates triggered star formation due to the expansion of the superbubble. Three very massive, young Of stars in the region dominate the ionizing and mechanical feedback among hundreds of other OB stars in the sample. The current stellar wind feedback rate from the two WR stars in the complex is comparable to that released by the whole OB sample. We see only a minor fraction of this stellar wind feedback converted into X-ray emission. In this LMC complex, stellar winds and supernovae equally contribute to the total energy feedback, which eventually powered the central superbubble. However, the total energy input accumulated over the time scale of the superbubble significantly exceeds the observed energy content of the complex. The lack of energy along with the morphology of the complex suggests a leakage of hot gas from the superbubble. With a detailed spectroscopic study of massive stars in SMC-SGS 1, we provide the stellar and wind parameters of a large sample of OB stars at low metallicity, including those in the lower mass-range. The stellar rotation velocities show a broad, tentatively bimodal distribution, with Be stars being among the fastest. A few very luminous O stars are found close to the main sequence, while all other, slightly evolved stars obey a strict luminosity limit. Considering additional massive stars in evolved stages, with published parameters and located all over the SMC, essentially confirms this picture. The comparison with single-star evolutionary tracks suggests a dichotomy in the fate of massive stars in the SMC. Only stars with an initial mass below 30 solar masses seem to evolve from the main sequence to the cool side of the HRD to become a red supergiant and to explode as type II-P supernova. In contrast, more massive stars appear to stay always hot and might evolve quasi chemically homogeneously, finally collapsing to relatively massive black holes. However, we find no indication that chemical mixing is correlated with rapid rotation. We measured the key parameters of stellar feedback and established the links between the rates of star formation and supernovae. Our study demonstrates that in metal-poor environments stellar feedback is dominated by core-collapse supernovae in combination with winds and ionizing radiation supplied by a few of the most massive stars. We found indications of the stochastic mode of star formation, where the resulting stellar population is fully capable of producing large-scale structures such as the supergiant shell SMC-SGS 1 in the Wing. The low level of feedback in metal-poor stellar populations allows star formation episodes to persist over long timescales. Our study showcases the importance of quantitative spectroscopy of massive stars with adequate stellar-atmosphere models in order to understand star-formation, evolution, and feedback. The stellar population analyses in the LMC and SMC make us understand that massive stars and their impact can be very different depending on their environment. Obviously, due to their different metallicity, the massive stars in the LMC and the SMC follow different evolutionary paths. Their winds differ significantly, and the key feedback agents are different. As a consequence, the star formation can proceed in different modes.},
  language  = {en}
}
@phdthesis{Sablowski2019,
  author    = {Sablowski, Daniel},
  title     = {Spectroscopic analysis of the benchmark system Alpha Aurigae},
  doi       = {10.25932/publishup-43239},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-432396},
  school      = {Universit{\"a}t Potsdam},
  pages     = {169},
  year      = {2019},
  abstract  = {Binaries play an important role in observational and theoretical astrophysics. Since the mass and the chemical composition are key ingredients for stellar evolution, high-resolution spectroscopy is an important and necessary tool to derive those parameters to high confidence in binaries. This involves carefully measured orbital motion by the determination of radial velocity (RV) shifts and sophisticated techniques to derive the abundances of elements within the stellar atmosphere. A technique superior to conventional cross-correlation methods to determine RV shifts in known as spectral disentangling. Hence, a major task of this thesis was the design of a sophisticated software package for this approach. In order to investigate secondary effects, such as flux and line-profile variations, imprinting changes on the spectrum the behavior of spectral disentangling on such variability is a key to understand the derived values, to improve them, and to get information about the variability itself. Therefore, the spectral disentangling code presented in this thesis and available to the community combines multiple advantages: separation of the spectra for detailed chemical analysis, derivation of orbital elements, derivation of individual RVs in order to investigate distorted systems (either by third body interaction or relativistic effects), the suppression of telluric contaminations, the derivation of variability, and the possibility to apply the technique to eclipsing binaries (important for orbital inclination) or in general to systems that undergo flux-variations. This code in combination with the spectral synthesis codes MOOG and SME was used in order to derive the carbon 12C/13C isotope ratio (CIR) of the benchmark binary Capella. The observational result will be set into context with theoretical evolution by the use of MESA models and resolves the discrepancy of theory and observations existing since the first measurement of Capella's CIR in 1976. The spectral disentangling code has been made available to the community and its applicability to completely different behaving systems, Wolf-Rayet stars, have also been investigated and resulted in a published article. Additionally, since this technique relies strongly on data quality, continues development of scientific instruments to achieve best observational data is of great importance in observational astrophysics. That is the reason why there has also been effort in astronomical instrumentation during the work on this thesis.},
  language  = {en}
}
@phdthesis{Menski2019,
  author    = {Menski, Antonia Isabell},
  title     = {Europium als strukturelle Sonde zur Analyse neuartiger Materialien},
  doi       = {10.25932/publishup-42714},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-427141},
  school      = {Universit{\"a}t Potsdam},
  pages     = {181},
  year      = {2019},
  abstract  = {Im Rahmen dieser Arbeit wird anhand von neuartigen Materialien das Potential der Europium-Lumineszenz f{\"u}r die strukturelle Analyse dargestellt. Bei diesen Materialien handelt es sich zum einen um Nanopartikel mit Matrizes aus mehreren Metall-Mischoxiden und Dotierungen durch die Sonde Europium und zum anderen um Metallorganische Netzwerke (MOFs), die mit Neodym , Samarium- und Europium-Ionen beladen sind. Die Synthese der aus der Kombination von Metalloxiden enthaltenen Nanopartikel ist unter milden Bedingungen mithilfe von speziell daf{\"u}r hergestellten Reagenzien erfolgt und hat zu sehr kleinen, amorphen Nanopartikeln gef{\"u}hrt. Durch eine nachfolgende Temperaturbehandlung hat sich die Kristallinit{\"a}t erh{\"o}ht. Damit verbunden haben sich auch die Kristallstruktur sowie die Position des Dotanden Europium ver{\"a}ndert. W{\"a}hrend die etablierte Methode der R{\"o}ntgendiffraktometrie einen Blick auf das Kristallgitter als Gesamtes erm{\"o}glicht, so trifft die Lumineszenz des Europiums durch die Sichtbarkeit einzelner Stark-Aufspaltungen Aussagen {\"u}ber dessen lokale Symmetrien. Die Symmetrie wird durch Sauerstofffehlstellen ver{\"a}ndert, welche die Sauerstoffleitf{\"a}higkeit der Nanopartikel beeinflussen. Diese ist f{\"u}r die Anwendung als Katalysatoren in industriellen Prozessen und ebenso als Sensoren und Therapeutika in biologischen Systemen von Bedeutung. Zur ersten katalytischen Charakterisierung werden die Proben mittels Temperatur-programmierter Reduktion untersucht. Des Weiteren werden die Mischoxid-Nanopartikel auch hinsichtlich ihrer Verwendbarkeit als Matrix in Aufkonversionsprozessen untersucht. Die Metallorganischen Netzwerke eignen sich aufgrund ihrer mikropor{\"o}sen Struktur f{\"u}r Anwendungen in der Speicherung gleichermaßen von Nutzgasen wie auch von Schadstoffen. Ebenfalls ist eine biologische Anwendung denkbar, die insbesondere den Bereich der drug delivery-Reagenzien betrifft. Erfolgt in die mikropor{\"o}sen Strukturen der Metallorganischen Netzwerke die Einlagerung von Lanthanoid-Ionen, so k{\"o}nnen diese bei der entsprechenden Kombination als Weißlicht-Emittierer fungieren. Dabei ist neben den Verh{\"a}ltnissen zwischen den Lanthanoid-Ionen auch die genaue Position innerhalb des Netzwerks sowie die Distanz zu anderen Ionen von Interesse. Zur Untersuchung dieser Fragestellungen wird die Umgebungssensitivit{\"a}t der Europium-Lumineszenz ausgenutzt. Die auf diese Weise festgestellte Formiat-Bildung h{\"a}ngt von zahlreichen Parametern ab. Insgesamt stellt sich die im Rahmen dieser Arbeit verwendete Methodik des Einsatzes von Europium als strukturelle Sonde in h{\"o}chstem Maße vielseitig dar und zeigt seine gr{\"o}ßte St{\"a}rke in der Kombination mit weiteren Methoden der Strukturanalytik. Die auf diese Weise genauestens charakterisierten neuartigen Materialien k{\"o}nnen nun gezielt und anwendungsfokussiert weiterentwickelt werden.},
  language  = {de}
}
@phdthesis{Beamish2019,
  author    = {Beamish, Alison Leslie},
  title     = {Hyperspectral remote sensing of the spatial and temporal heterogeneity of low Arctic vegetation},
  doi       = {10.25932/publishup-42592},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-425922},
  school      = {Universit{\"a}t Potsdam},
  pages     = {v, 102},
  year      = {2019},
  abstract  = {Arctic tundra ecosystems are experiencing warming twice the global average and Arctic vegetation is responding in complex and heterogeneous ways. Shifting productivity, growth, species composition, and phenology at local and regional scales have implications for ecosystem functioning as well as the global carbon and energy balance. Optical remote sensing is an effective tool for monitoring ecosystem functioning in this remote biome. However, limited field-based spectral characterization of the spatial and temporal heterogeneity limits the accuracy of quantitative optical remote sensing at landscape scales. To address this research gap and support current and future satellite missions, three central research questions were posed: • Does canopy-level spectral variability differ between dominant low Arctic vegetation communities and does this variability change between major phenological phases? • How does canopy-level vegetation colour images recorded with high and low spectral resolution devices relate to phenological changes in leaf-level photosynthetic pigment concentrations? • How does spatial aggregation of high spectral resolution data from the ground to satellite scale influence low Arctic tundra vegetation signatures and thereby what is the potential of upcoming hyperspectral spaceborne systems for low Arctic vegetation characterization? To answer these questions a unique and detailed database was assembled. Field-based canopy-level spectral reflectance measurements, nadir digital photographs, and photosynthetic pigment concentrations of dominant low Arctic vegetation communities were acquired at three major phenological phases representing early, peak and late season. Data were collected in 2015 and 2016 in the Toolik Lake Research Natural Area located in north central Alaska on the North Slope of the Brooks Range. In addition to field data an aerial AISA hyperspectral image was acquired in the late season of 2016. Simulations of broadband Sentinel-2 and hyperspectral Environmental and Mapping Analysis Program (EnMAP) satellite reflectance spectra from ground-based reflectance spectra as well as simulations of EnMAP imagery from aerial hyperspectral imagery were also obtained. Results showed that canopy-level spectral variability within and between vegetation communities differed by phenological phase. The late season was identified as the most discriminative for identifying many dominant vegetation communities using both ground-based and simulated hyperspectral reflectance spectra. This was due to an overall reduction in spectral variability and comparable or greater differences in spectral reflectance between vegetation communities in the visible near infrared spectrum. Red, green, and blue (RGB) indices extracted from nadir digital photographs and pigment-driven vegetation indices extracted from ground-based spectral measurements showed strong significant relationships. RGB indices also showed moderate relationships with chlorophyll and carotenoid pigment concentrations. The observed relationships with the broadband RGB channels of the digital camera indicate that vegetation colour strongly influences the response of pigment-driven spectral indices and digital cameras can track the seasonal development and degradation of photosynthetic pigments. Spatial aggregation of hyperspectral data from the ground to airborne, to simulated satel-lite scale was influenced by non-photosynthetic components as demonstrated by the distinct shift of the red edge to shorter wavelengths. Correspondence between spectral reflectance at the three scales was highest in the red spectrum and lowest in the near infra-red. By artificially mixing litter spectra at different proportions to ground-based spectra, correspondence with aerial and satellite spectra increased. Greater proportions of litter were required to achieve correspondence at the satellite scale. Overall this thesis found that integrating multiple temporal, spectral, and spatial data is necessary to monitor the complexity and heterogeneity of Arctic tundra ecosystems. The identification of spectrally similar vegetation communities can be optimized using non-peak season hyperspectral data leading to more detailed identification of vegetation communities. The results also highlight the power of vegetation colour to link ground-based and satellite data. Finally, a detailed characterization non-photosynthetic ecosystem components is crucial for accurate interpretation of vegetation signals at landscape scales.},
  language  = {en}
}
@phdthesis{Anders2017,
  author    = {Anders, Friedrich},
  title     = {Disentangling the chemodynamical history of the Milky Way disc with asteroseismology and spectroscopy},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-396681},
  school      = {Universit{\"a}t Potsdam},
  pages     = {121},
  year      = {2017},
  abstract  = {Galaxies are among the most complex systems that can currently be modelled with a computer. A realistic simulation must take into account cosmology and gravitation as well as effects of plasma, nuclear, and particle physics that occur on very different time, length, and energy scales. The Milky Way is the ideal test bench for such simulations, because we can observe millions of its individual stars whose kinematics and chemical composition are records of the evolution of our Galaxy. Thanks to the advent of multi-object spectroscopic surveys, we can systematically study stellar populations in a much larger volume of the Milky Way. While the wealth of new data will certainly revolutionise our picture of the formation and evolution of our Galaxy and galaxies in general, the big-data era of Galactic astronomy also confronts us with new observational, theoretical, and computational challenges. This thesis aims at finding new observational constraints to test Milky-Way models, primarily based on infra-red spectroscopy from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and asteroseismic data from the CoRoT mission. We compare our findings with chemical-evolution models and more sophisticated chemodynamical simulations. In particular we use the new powerful technique of combining asteroseismic and spectroscopic observations that allows us to test the time dimension of such models for the first time. With CoRoT and APOGEE (CoRoGEE) we can infer much more precise ages for distant field red-giant stars, opening up a new window for Galactic archaeology. Another important aspect of this work is the forward-simulation approach that we pursued when interpreting these complex datasets and comparing them to chemodynamical models. The first part of the thesis contains the first chemodynamical study conducted with the APOGEE survey. Our sample comprises more than 20,000 red-giant stars located within 6 kpc from the Sun, and thus greatly enlarges the Galactic volume covered with high-resolution spectroscopic observations. Because APOGEE is much less affected by interstellar dust extinction, the sample covers the disc regions very close to the Galactic plane that are typically avoided by optical surveys. This allows us to investigate the chemo-kinematic properties of the Milky Way's thin disc outside the solar vicinity. We measure, for the first time with high-resolution data, the radial metallicity gradient of the disc as a function of distance from the Galactic plane, demonstrating that the gradient flattens and even changes its sign for mid-plane distances greater than 1 kpc. Furthermore, we detect a gap between the high- and low-[\$\alpha\$/Fe] sequences in the chemical-abundance diagram (associated with the thin and thick disc) that unlike in previous surveys can hardly be explained by selection effects. Using 6D kinematic information, we also present chemical-abundance diagrams cleaned from stars on kinematically hot orbits. The data allow us to confirm without doubt that the scale length of the (chemically-defined) thick disc is significantly shorter than that of the thin disc. In the second part, we present our results of the first combination of asteroseismic and spectroscopic data in the context of Galactic Archaeology. We analyse APOGEE follow-up observations of 606 solar-like oscillating red giants in two CoRoT fields close to the Galactic plane. These stars cover a large radial range of the Galactic disc (4.5 kpc \$\lesssim R_{\rm Gal}\lesssim15\$ kpc) and a large age baseline (0.5 Gyr \$\lesssim \tau\lesssim\$ 13 Gyr), allowing us to study the age- and radius-dependence of the [\$\alpha\$/Fe] vs. [Fe/H] distributions. We find that the age distribution of the high-[\$\alpha\$/Fe] sequence appears to be broader than expected from a monolithically-formed old thick disc that stopped to form stars 10 Gyr ago. In particular, we discover a significant population of apparently young, [\$\alpha\$/Fe]-rich stars in the CoRoGEE data whose existence cannot be explained by standard chemical-evolution models. These peculiar stars are much more abundant in the inner CoRoT field LRc01 than in the outer-disc field LRc01, suggesting that at least part of this population has a chemical-evolution rather than a stellar-evolution origin, possibly due to a peculiar chemical-enrichment history of the inner disc. We also find that strong radial migration is needed to explain the abundance of super-metal-rich stars in the outer disc. Finally, we use the CoRoGEE sample to study the time evolution of the radial metallicity gradient in the thin disc, an observable that has been the subject of observational and theoretical debate for more than 20 years. By dividing the CoRoGEE dataset into six age bins, performing a careful statistical analysis of the radial [Fe/H], [O/H], and [Mg/Fe] distributions, and accounting for the biases introduced by the observation strategy, we obtain reliable gradient measurements. The slope of the radial [Fe/H] gradient of the young red-giant population (\$-0.058\pm0.008\$ [stat.] \$\pm0.003\$ [syst.] dex/kpc) is consistent with recent Cepheid data. For the age range of \$1-4\$ Gyr, the gradient steepens slightly (\$-0.066\pm0.007\pm0.002\$ dex/kpc), before flattening again to reach a value of \$\sim-0.03\$ dex/kpc for stars with ages between 6 and 10 Gyr. This age dependence of the [Fe/H] gradient can be explained by a nearly constant negative [Fe/H] gradient of \$\sim-0.07\$ dex/kpc in the interstellar medium over the past 10 Gyr, together with stellar heating and migration. Radial migration also offers a new explanation for the puzzling observation that intermediate-age open clusters in the solar vicinity (unlike field stars) tend to have higher metallicities than their younger counterparts. We suggest that non-migrating clusters are more likely to be kinematically disrupted, which creates a bias towards high-metallicity migrators from the inner disc and may even steepen the intermediate-age cluster abundance gradient.},
  language  = {en}
}
@phdthesis{Schroeder2016,
  author    = {Schr{\"o}der, Henning},
  title     = {Ultrafast electron dynamics in Fe(CO)5 and Cr(CO)6},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-94589},
  school      = {Universit{\"a}t Potsdam},
  pages     = {v, 87},
  year      = {2016},
  abstract  = {In this thesis, the two prototype catalysts Fe(CO)₅ and Cr(CO)₆ are investigated with time-resolved photoelectron spectroscopy at a high harmonic setup. In both of these metal carbonyls, a UV photon can induce the dissociation of one or more ligands of the complex. The mechanism of the dissociation has been debated over the last decades. The electronic dynamics of the first dissociation occur on the femtosecond timescale. For the experiment, an existing high harmonic setup was moved to a new location, was extended, and characterized. The modified setup can induce dynamics in gas phase samples with photon energies of 1.55eV, 3.10eV, and 4.65eV. The valence electronic structure of the samples can be probed with photon energies between 20eV and 40eV. The temporal resolution is 111fs to 262fs, depending on the combination of the two photon energies. The electronically excited intermediates of the two complexes, as well as of the reaction product Fe(CO)₄, could be observed with photoelectron spectroscopy in the gas phase for the first time. However, photoelectron spectroscopy gives access only to the final ionic states. Corresponding calculations to simulate these spectra are still in development. The peak energies and their evolution in time with respect to the initiation pump pulse have been determined, these peaks have been assigned based on literature data. The spectra of the two complexes show clear differences. The dynamics have been interpreted with the assumption that the motion of peaks in the spectra relates to the movement of the wave packet in the multidimensional energy landscape. The results largely confirm existing models for the reaction pathways. In both metal carbonyls, this pathway involves a direct excitation of the wave packet to a metal-to-ligand charge transfer state and the subsequent crossing to a dissociative ligand field state. The coupling of the electronic dynamics to the nuclear dynamics could explain the slower dissociation in Fe(CO)₅ as compared to Cr(CO)₆.},
  language  = {en}
}
@phdthesis{Brosinsky2015,
  author    = {Brosinsky, Arlena},
  title     = {Spectral fingerprinting},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-83369},
  school      = {Universit{\"a}t Potsdam},
  pages     = {VI, 117},
  year      = {2015},
  abstract  = {Current research on runoff and erosion processes, as well as an increasing demand for sustainable watershed management emphasize the need for an improved understanding of sediment dynamics. This involves the accurate assessment of erosion rates and sediment transfer, yield and origin. A variety of methods exist to capture these processes at the catchment scale. Among these, sediment fingerprinting, a technique to trace back the origin of sediment, has attracted increasing attention by the scientific community in recent years. It is a two-step procedure, based on the fundamental assumptions that potential sources of sediment can be reliably discriminated based on a set of characteristic 'fingerprint' properties, and that a comparison of source and sediment fingerprints allows to quantify the relative contribution of each source. This thesis aims at further assessing the potential of spectroscopy to assist and improve the sediment fingerprinting technique. Specifically, this work focuses on (1) whether potential sediment sources can be reliably identified based on spectral features ('fingerprints'), whether (2) these spectral fingerprints permit the quantification of relative source contribution, and whether (3) in situ derived source information is sufficient for this purpose. Furthermore, sediment fingerprinting using spectral information is applied in a study catchment to (4) identify major sources and observe how relative source contributions change between and within individual flood events. And finally, (5) spectral fingerprinting results are compared and combined with simultaneous sediment flux measurements to study sediment origin, transport and storage behaviour. For the sediment fingerprinting approach, soil samples were collected from potential sediment sources within the Is{\´a}bena catchment, a meso-scale basin in the central Spanish Pyrenees. Undisturbed samples of the upper soil layer were measured in situ using an ASD spectroradiometer and subsequently sampled for measurements in the laboratory. Suspended sediment was sampled automatically by means of ISCO samplers at the catchment as well as at the five major subcatchment outlets during flood events, and stored fine sediment from the channel bed was collected from 14 cross-sections along the main river. Artificial mixtures of known contributions were produced from source soil samples. Then, all source, sediment and mixture samples were dried and spectrally measured in the laboratory. Subsequently, colour coefficients and physically based features with relation to organic carbon, iron oxide, clay content and carbonate, were calculated from all in situ and laboratory spectra. Spectral parameters passing a number of prerequisite tests were submitted to principal component analyses to study natural clustering of samples, discriminant function analyses to observe source differentiation accuracy, and a mixing model for source contribution assessment. In addition, annual as well as flood event based suspended sediment fluxes from the catchment and its subcatchments were calculated from rainfall, water discharge and suspended sediment concentration measurements using rating curves and Quantile Regression Forests. Results of sediment flux monitoring were interpreted individually with respect to storage behaviour, compared to fingerprinting source ascriptions and combined with fingerprinting to assess their joint explanatory potential. In response to the key questions of this work, (1) three source types (land use) and five spatial sources (subcatchments) could be reliably discriminated based on spectral fingerprints. The artificial mixture experiment revealed that while (2) laboratory parameters permitted source contribution assessment, (3) the use of in situ derived information was insufficient. Apparently, high discrimination accuracy does not necessarily imply good quantification results. When applied to suspended sediment samples of the catchment outlet, the spectral fingerprinting approach was able to (4) quantify the major sediment sources: badlands and the Villacarli subcatchment, respectively, were identified as main contributors, which is consistent with field observations and previous studies. Thereby, source contribution was found to vary both, within and between individual flood events. Also sediment flux was found to vary considerably, annually as well as seasonally and on flood event base. Storage was confirmed to play an important role in the sediment dynamics of the studied catchment, whereas floods with lower total sediment yield tend to deposit and floods with higher yield rather remove material from the channel bed. Finally, a comparison of flux measurements with fingerprinting results highlighted the fact that (5) immediate transport from sources to the catchment outlet cannot be assumed. A combination of the two methods revealed different aspects of sediment dynamics that none of the techniques could have uncovered individually. In summary, spectral properties provide a fast, non-destructive, and cost-efficient means to discriminate and quantify sediment sources, whereas, unfortunately, straight-forward in situ collected source information is insufficient for the approach. Mixture modelling using artificial mixtures permits valuable insights into the capabilities and limitations of the method and similar experiments are strongly recommended to be performed in the future. Furthermore, a combination of techniques such as e.g. (spectral) sediment fingerprinting and sediment flux monitoring can provide comprehensive understanding of sediment dynamics.},
  language  = {en}
}
@phdthesis{Herenz2014,
  author    = {Herenz, Peter},
  title     = {A study of the absorption characteristics of gaseous galaxy halos in the local Universe},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-70513},
  school      = {Universit{\"a}t Potsdam},
  year      = {2014},
  abstract  = {Today, it is well known that galaxies like the Milky Way consist not only of stars but also of gas and dust. The galactic halo, a sphere of gas that surrounds the stellar disk of a galaxy, is especially interesting. It provides a wealth of information about in and outflowing gaseous material towards and away from galaxies and their hierarchical evolution. For the Milky Way, the so-called high-velocity clouds (HVCs), fast moving neutral gas complexes in the halo that can be traced by absorption-line measurements, are believed to play a crucial role in the overall matter cycle in our Galaxy. Over the last decades, the properties of these halo structures and their connection to the local circumgalactic and intergalactic medium (CGM and IGM, respectively) have been investigated in great detail by many different groups. So far it remains unclear, however, to what extent the results of these studies can be transferred to other galaxies in the local Universe. In this thesis, we study the absorption properties of Galactic HVCs and compare the HVC absorption characteristics with those of intervening QSO absorption-line systems at low redshift. The goal of this project is to improve our understanding of the spatial extent and physical conditions of gaseous galaxy halos in the local Universe. In the first part of the thesis we use HST /STIS ultraviolet spectra of more than 40 extragalactic background sources to statistically analyze the absorption properties of the HVCs in the Galactic halo. We determine fundamental absorption line parameters including covering fractions of different weakly/intermediately/highly ionized metals with a particular focus on SiII and MgII. Due to the similarity in the ionization properties of SiII and MgII, we are able to estimate the contribution of HVC-like halo structures to the cross section of intervening strong MgII absorbers at z = 0. Our study implies that only the most massive HVCs would be regarded as strong MgII absorbers, if the Milky Way halo would be seen as a QSO absorption line system from an exterior vantage point. Combining the observed absorption-cross section of Galactic HVCs with the well-known number density of intervening strong MgII absorbers at z = 0, we conclude that the contribution of infalling gas clouds (i.e., HVC analogs) in the halos of Milky Way-type galaxies to the cross section of strong MgII absorbers is 34\%. This result indicates that only about one third of the strong MgII absorption can be associated with HVC analogs around other galaxies, while the majority of the strong MgII systems possibly is related to galaxy outflows and winds. The second part of this thesis focuses on the properties of intervening metal absorbers at low redshift. The analysis of the frequency and physical conditions of intervening metal systems in QSO spectra and their relation to nearby galaxies offers new insights into the typical conditions of gaseous galaxy halos. One major aspect in our study was to regard intervening metal systems as possible HVC analogs. We perform a detailed analysis of absorption line properties and line statistics for 57 metal absorbers along 78 QSO sightlines using newly-obtained ultraviolet spectra obtained with HST /COS. We find clear evidence for bimodal distribution in the HI column density in the absorbers, a trend that we interpret as sign for two different classes of absorption systems (with HVC analogs at the high-column density end). With the help of the strong transitions of SiII λ1260, SiIII λ1206, and CIII λ977 we have set up Cloudy photoionization models to estimate the local ionization conditions, gas densities, and metallicities. We find that the intervening absorption systems studied by us have, on average, similar physical conditions as Galactic HVC absorbers, providing evidence that many of them represent HVC analogs in the vicinity of other galaxies. We therefore determine typical halo sizes for SiII, SiIII, and CIII for L = 0.01L∗ and L = 0.05L∗ galaxies. Based on the covering fractions of the different ions in the Galactic halo, we find that, for example, the typical halo size for SiIII is ∼ 160 kpc for L = 0.05L∗ galaxies. We test the plausibility of this result by searching for known galaxies close to the QSO sightlines and at similar redshifts as the absorbers. We find that more than 34\% of the measured SiIII absorbers have galaxies associated with them, with the majority of the absorbers indeed being at impact parameters ρ ≤160 kpc.},
  language  = {en}
}
@phdthesis{Salffner2013,
  author    = {Salffner, Katharina},
  title     = {Entwicklung eines breitbandigen Cavity-Ring-Down-Spektrometers unter Verwendung nahinfraroter, inkoh{\"a}renter Strahlung},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-68952},
  school      = {Universit{\"a}t Potsdam},
  year      = {2013},
  abstract  = {In der vorliegenden Arbeit werden verschiedene Spektrometer f{\"u}r die Analyse von Gasen bzw. Gasgemischen vorgestellt und deren Design, Aufbau, Charakterisierung und Optimierung beschrieben. Das Resultat der Optimierung und Weiterentwicklungen ist ein spektral breitbandiges Cavity-Ring-Down-Spektrometer (CRD-Spektrometer). Ausgangspunkt der hier vorgestellten Arbeit ist ein Spektrometer auf Basis klassischer Absorptionsspektroskopie in einer Multireflexionszelle. F{\"u}r dieses Spektrometer wurde als Strahlquelle ein Superkontinuumlaser verwendet. Der Vorteil dieses Spektrometers liegt in seiner Kompaktheit. Mit diesem Spektrometer wurden Absorptionsspektren von mehreren Reingasen und einem Gasgemisch {\"u}ber einen Wellenl{\"a}ngenbereich von 1500 nm - 1700 nm aufgenommen. Der qualitative Vergleich mit zu erwartenden Spektren, welche auf der HITRAN-Datenbank basieren, zeigte eine gute {\"U}bereinstimmung. Die quantitative Interpretierbarkeit der Daten war jedoch stark eingeschr{\"a}nkt aufgrund des hohen zuf{\"a}lligen und systematischen Fehlers der Messungen. Als Konsequenz aus der als nicht zufriedenstellend bewerteten quantitativen Interpretierbarkeit der Daten wurde eine alternative Messmethode gesucht, welche eine h{\"o}here Sensitivit{\"a}t und Genauigkeit erm{\"o}glicht. Die Wahl fiel auf die Cavity-Ring-Down-Spektroskopie, eine resonatorgest{\"u}tzte Variante der Absorptionsspektroskopie. Wesentliche Vorteile dieser Technik sind a) die Unabh{\"a}ngigkeit von Leistungsschwankungen der Strahlquelle, b) ein effektiver Absorptionsweg von bis zu mehreren Kilometern, welcher sich unmittelbar auf die Sensitivit{\"a}t der Messungen auswirkt, c) die Ermittlung absoluter Absorberkonzentrationen, ohne die Notwendigkeit einer Kalibrierung oder den Vergleich mit einer Referenzzelle und d) die Vernachl{\"a}ssigbarkeit von Absorptionen außerhalb des Resonators. Als notwendiger Zwischenschritt auf dem Weg zu einem breitbandigen CRD-Spektrometer wurde zun{\"a}chst ein monochromatisches CRD-Spektrometer designt, aufgebaut und charakterisiert. F{\"u}r die effektive Einkopplung von Strahlungsenergie in einen Resonator ist die Anpassung der Strahlparameter an die Mode des Resonators notwendig. Voraussetzung dieser Anpassung ist die Kenntnis der Strahlparameter, welche experimentell ermittelt wurden. Im Laufe des Aufbaus des Spektrometers ergab sich, dass trotz der Modenanpassung die Einkopplung der Strahlungsenergie in den Resonator gest{\"o}rt wurde. Daraufhin wurden systematisch m{\"o}gliche Ursachen dieser St{\"o}rung untersucht und das Spektrometer optimiert. Mit diesem optimierten Spektrometer wurden Spektren gemessen, welche sowohl qualitativ als auch quantitativ gut mit den zu erwartenden Spektren {\"u}bereinstimmen. Als Nachweisgrenze dieses Spektrometers wurde ein Wert f{\"u}r den Absorptionskoeffizienten alpha von 10^-8 cm-1 bestimmt. Mit dem monochromatischen CRD-Spektrometer war es zudem m{\"o}glich, isotopenspezifische Messungen durchzuf{\"u}hren. F{\"u}r das breitbandige Spektrometer wurde als Strahlquelle eine ASE-Diode (amplified spontaneous emission) verwendet. Dabei handelt es sich um eine inkoh{\"a}rente Strahlquelle. Mittels Messungen nach dem Prinzip der Cavity-Enhanced-Absorptionsspektroskopie wurde die generelle Funktionalit{\"a}t des resonatorgest{\"u}tzten Spektrometers {\"u}berpr{\"u}ft. Anschließend wurden die wellenl{\"a}ngenabh{\"a}ngigen Abklingsignale des leeren und des mit einem CO2-Luft-Gemisch gef{\"u}llten Resonators gemessen und ebenfalls mit den zu erwartenden Spektren verglichen. Qualitativ stimmen die experimentellen Spektren gut mit den zu erwartenden Spektren {\"u}berein. F{\"u}r die quantitative Interpretation der Daten wurde ein spezieller Algorithmus entwickelt, der die spektrale Aufl{\"o}sung des Systems ber{\"u}cksichtigt. Mit dem vorgestellten Spektrometer ist so die qualitative und quantitative Interpretation der Spektren m{\"o}glich. Die Nachweisgrenze des breitbandigen Cavity-Ring-Down-Spektrometers wurde zu einem Wert von alpha = 8x10^-7 cm-1 bestimmt. Der systematischen und der zuf{\"a}llige Fehler der Messungen lagen bei Werten von ca. 1\%. Bei diesem Spektrometer handelt es sich um einen Prototyp. Mittels Optimierung des Systems lassen sich sowohl der Wert der Nachweisgrenze als auch die Fehler der Messungen verbessern.},
  language  = {de}
}
@phdthesis{Boeche2011,
  author    = {Boeche, Corrado},
  title     = {Chemical gradients in the Milky Way from unsupervised chemical abundances measurements of the RAVE spectroscopic data set},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-52478},
  school      = {Universit{\"a}t Potsdam},
  year      = {2011},
  abstract  = {The present thesis was born and evolved within the RAdial Velocity Experiment (RAVE) with the goal of measuring chemical abundances from the RAVE spectra and exploit them to investigate the chemical gradients along the plane of the Galaxy to provide constraints on possible Galactic formation scenarios. RAVE is a large spectroscopic survey which aims to observe spectroscopically ~10^6 stars by the end of 2012 and measures their radial velocities, atmospheric parameters and chemical abundances. The project makes use of the UK Schmidt telescope at Australian Astronomical Observatory (AAO) in Siding Spring, Australia, equipped with the multiobject spectrograph 6dF. To date, RAVE collected and measured more than 450,000 spectra. The precision of the chemical abundance estimations depends on the reliability of the atomic and atmosphere parameters adopted (in particular the oscillator strengths of the absorption lines and the effective temperature, gravity, and metallicity of the stars measured). Therefore we first identified 604 absorption lines in the RAVE wavelength range and refined their oscillator strengths with an inverse spectral analysis. Then, we improved the RAVE stellar parameters by modifying the RAVE pipeline and the spectral library the pipeline rely on. The modifications removed some systematic errors in stellar parameters discovered during this work. To obtain chemical abundances, we developed two different processing pipelines. Both of them perform chemical abundances measurements by assuming stellar atmospheres in Local Thermodynamic Equilibrium (LTE). The first one determines elements abundances from equivalent widths of absorption lines. Since this pipeline showed poor sensibility on abundances relative to iron, it has been superseded. The second one exploits the chi^2 minimization technique between observed and model spectra. Thanks to its precision, it has been adopted for the creation of the RAVE chemical catalogue. This pipeline provides abundances with uncertains of about ~0.2dex for spectra with signal-to-noise ratio S/N>40 and ~0.3dex for spectra with 20>S/N>40. For this work, the pipeline measured chemical abundances up to 7 elements for 217,358 RAVE stars. With these data we investigated the chemical gradients along the Galactic radius of the Milky Way. We found that stars with low vertical velocities |W| (which stay close to the Galactic plane) show an iron abundance gradient in agreement with previous works (~-0.07\$ dex kpc^-1) whereas stars with larger |W| which are able to reach larger heights above the Galactic plane, show progressively flatter gradients. The gradients of the other elements follow the same trend. This suggests that an efficient radial mixing acts in the Galaxy or that the thick disk formed from homogeneous interstellar matter. In particular, we found hundreds of stars which can be kinetically classified as thick disk stars exhibiting a chemical composition typical of the thin disk. A few stars of this kind have already been detected by other authors, and their origin is still not clear. One possibility is that they are thin disk stars kinematically heated, and then underwent an efficient radial mixing process which blurred (and so flattened) the gradient. Alternatively they may be a transition population" which represents an evolutionary bridge between thin and thick disk. Our analysis shows that the two explanations are not mutually exclusive. Future follow-up high resolution spectroscopic observations will clarify their role in the Galactic disk evolution.},
  language  = {en}
}
@phdthesis{Hildebrandt2006,
  author    = {Hildebrandt, Niko},
  title     = {Lanthanides and quantum dots : time-resolved laser spectroscopy of biochemical F{\"o}rster Resonance Energy Transfer (FRET) systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-12686},
  school      = {Universit{\"a}t Potsdam},
  year      = {2006},
  abstract  = {F{\"o}rster Resonance Energy Transfer (FRET) plays an important role for biochemical applications such as DNA sequencing, intracellular protein-protein interactions, molecular binding studies, in vitro diagnostics and many others. For qualitative and quantitative analysis, FRET systems are usually assembled through molecular recognition of biomolecules conjugated with donor and acceptor luminophores. Lanthanide (Ln) complexes, as well as semiconductor quantum dot nanocrystals (QD), possess unique photophysical properties that make them especially suitable for applied FRET. In this work the possibility of using QD as very efficient FRET acceptors in combination with Ln complexes as donors in biochemical systems is demonstrated. The necessary theoretical and practical background of FRET, Ln complexes, QD and the applied biochemical models is outlined. In addition, scientific as well as commercial applications are presented. FRET can be used to measure structural changes or dynamics at distances ranging from approximately 1 to 10 nm. The very strong and well characterized binding process between streptavidin (Strep) and biotin (Biot) is used as a biomolecular model system. A FRET system is established by Strep conjugation with the Ln complexes and QD biotinylation. Three Ln complexes (one with Tb3+ and two with Eu3+ as central ion) are used as FRET donors. Besides the QD two further acceptors, the luminescent crosslinked protein allophycocyanin (APC) and a commercial fluorescence dye (DY633), are investigated for direct comparison. FRET is demonstrated for all donor-acceptor pairs by acceptor emission sensitization and a more than 1000-fold increase of the luminescence decay time in the case of QD reaching the hundred microsecond regime. Detailed photophysical characterization of donors and acceptors permits analysis of the bioconjugates and calculation of the FRET parameters. Extremely large F{\"o}rster radii of more than 100 {\AA} are achieved for QD as acceptors, considerably larger than for APC and DY633 (ca. 80 and 60 {\AA}). Special attention is paid to interactions with different additives in aqueous solutions, namely borate buffer, bovine serum albumin (BSA), sodium azide and potassium fluoride (KF). A more than 10-fold limit of detection (LOD) decrease compared to the extensively characterized and frequently used donor-acceptor pair of Europium tris(bipyridine) (Eu-TBP) and APC is demonstrated for the FRET system, consisting of the Tb complex and QD. A sub-picomolar LOD for QD is achieved with this system in azide free borate buffer (pH 8.3) containing 2 \% BSA and 0.5 M KF. In order to transfer the Strep-Biot model system to a real-life in vitro diagnostic application, two kinds of imunnoassays are investigated using human chorionic gonadotropin (HCG) as analyte. HCG itself, as well as two monoclonal anti-HCG mouse-IgG (immunoglobulin G) antibodies are labeled with the Tb complex and QD, respectively. Although no sufficient evidence for FRET can be found for a sandwich assay, FRET becomes obvious in a direct HCG-IgG assay showing the feasibility of using the Ln-QD donor-acceptor pair as highly sensitive analytical tool for in vitro diagnostics.},
  language  = {en}
}
@phdthesis{Henkel2004,
  author    = {Henkel, Carsten},
  title     = {Coherence theory of atomic de Broglie waves and electromagnetic near fields},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001272},
  school      = {Universit{\"a}t Potsdam},
  year      = {2004},
  abstract  = {Die Arbeit untersucht theoretisch die Wechselwirkung neutraler Teilchen (Atome, Molek{\"u}le) mit Oberfl{\"a}chen, soweit sie durch das elektromagnetische Feld vermittelt wird. Spektrale Energiedichten und Koh{\"a}renzfunktionen werden hergeleitet und liefern eine umfassende Charakterisierung des Felds auf der sub-Wellenl{\"a}ngen-Skala. Die Ergebnisse finden auf zwei Teilgebieten Anwendung: in der integrierten Atomoptik, wo ultrakalte Atome an thermische Oberfl{\"a}chen koppeln, und in der Nahfeldoptik, wo eine Aufl{\"o}sung unterhalb der Beugungsbegrenzung mit einzelnen Molek{\"u}len als Sonden und Detektoren erzielt werden kann.},
  language  = {en}
}