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In der vorliegenden Arbeit werden verschiedene Spektrometer für 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. Für 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 über einen Wellenlängenbereich von 1500 nm – 1700 nm aufgenommen. Der qualitative Vergleich mit zu erwartenden Spektren, welche auf der HITRAN-Datenbank basieren, zeigte eine gute Übereinstimmung. Die quantitative Interpretierbarkeit der Daten war jedoch stark eingeschränkt aufgrund des hohen zufälligen und systematischen Fehlers der Messungen. Als Konsequenz aus der als nicht zufriedenstellend bewerteten quantitativen Interpretierbarkeit der Daten wurde eine alternative Messmethode gesucht, welche eine höhere Sensitivität und Genauigkeit ermöglicht. Die Wahl fiel auf die Cavity-Ring-Down-Spektroskopie, eine resonatorgestützte Variante der Absorptionsspektroskopie. Wesentliche Vorteile dieser Technik sind a) die Unabhängigkeit von Leistungsschwankungen der Strahlquelle, b) ein effektiver Absorptionsweg von bis zu mehreren Kilometern, welcher sich unmittelbar auf die Sensitivität der Messungen auswirkt, c) die Ermittlung absoluter Absorberkonzentrationen, ohne die Notwendigkeit einer Kalibrierung oder den Vergleich mit einer Referenzzelle und d) die Vernachlässigbarkeit von Absorptionen außerhalb des Resonators. Als notwendiger Zwischenschritt auf dem Weg zu einem breitbandigen CRD-Spektrometer wurde zunächst ein monochromatisches CRD-Spektrometer designt, aufgebaut und charakterisiert. Für 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 gestört wurde. Daraufhin wurden systematisch mögliche Ursachen dieser Störung untersucht und das Spektrometer optimiert. Mit diesem optimierten Spektrometer wurden Spektren gemessen, welche sowohl qualitativ als auch quantitativ gut mit den zu erwartenden Spektren übereinstimmen. Als Nachweisgrenze dieses Spektrometers wurde ein Wert für den Absorptionskoeffizienten alpha von 10^-8 cm-1 bestimmt. Mit dem monochromatischen CRD-Spektrometer war es zudem möglich, isotopenspezifische Messungen durchzuführen. Für das breitbandige Spektrometer wurde als Strahlquelle eine ASE-Diode (amplified spontaneous emission) verwendet. Dabei handelt es sich um eine inkohärente Strahlquelle. Mittels Messungen nach dem Prinzip der Cavity-Enhanced-Absorptionsspektroskopie wurde die generelle Funktionalität des resonatorgestützten Spektrometers überprüft. Anschließend wurden die wellenlängenabhängigen Abklingsignale des leeren und des mit einem CO2-Luft-Gemisch gefüllten Resonators gemessen und ebenfalls mit den zu erwartenden Spektren verglichen. Qualitativ stimmen die experimentellen Spektren gut mit den zu erwartenden Spektren überein. Für die quantitative Interpretation der Daten wurde ein spezieller Algorithmus entwickelt, der die spektrale Auflösung des Systems berücksichtigt. Mit dem vorgestellten Spektrometer ist so die qualitative und quantitative Interpretation der Spektren möglich. Die Nachweisgrenze des breitbandigen Cavity-Ring-Down-Spektrometers wurde zu einem Wert von alpha = 8x10^-7 cm-1 bestimmt. Der systematischen und der zufällige 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.
Im Rahmen dieser Arbeit wird anhand von neuartigen Materialien das Potential der Europium-Lumineszenz für 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 dafür hergestellten Reagenzien erfolgt und hat zu sehr kleinen, amorphen Nanopartikeln geführt. Durch eine nachfolgende Temperaturbehandlung hat sich die Kristallinität erhöht. Damit verbunden haben sich auch die Kristallstruktur sowie die Position des Dotanden Europium verändert.
Während die etablierte Methode der Röntgendiffraktometrie einen Blick auf das Kristallgitter als Gesamtes ermöglicht, so trifft die Lumineszenz des Europiums durch die Sichtbarkeit einzelner Stark-Aufspaltungen Aussagen über dessen lokale Symmetrien. Die Symmetrie wird durch Sauerstofffehlstellen verändert, welche die Sauerstoffleitfähigkeit der Nanopartikel beeinflussen. Diese ist für 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 mikroporösen Struktur für 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 mikroporösen Strukturen der Metallorganischen Netzwerke die Einlagerung von Lanthanoid-Ionen, so können diese bei der entsprechenden Kombination als Weißlicht-Emittierer fungieren. Dabei ist neben den Verhältnissen 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 Umgebungssensitivität der Europium-Lumineszenz ausgenutzt. Die auf diese Weise festgestellte Formiat-Bildung hängt von zahlreichen Parametern ab.
Insgesamt stellt sich die im Rahmen dieser Arbeit verwendete Methodik des Einsatzes von Europium als strukturelle Sonde in höchstem Maße vielseitig dar und zeigt seine größte Stärke in der Kombination mit weiteren Methoden der Strukturanalytik. Die auf diese Weise genauestens charakterisierten neuartigen Materialien können nun gezielt und anwendungsfokussiert weiterentwickelt werden.
We describe how inversion symmetry separation of electronic state manifolds in resonant inelastic soft X-ray scattering (RIXS) can be applied to probe excited-state dynamics with compelling selectivity. In a case study of Fe L-3-edge RIXS in the ferricyanide complex Fe(CN)(6)(3-), we demonstrate with multi-configurational restricted active space spectrum simulations how the information content of RIXS spectral fingerprints can be used to unambiguously separate species of different electronic configurations, spin multiplicities, and structures, with possible involvement in the decay dynamics of photo-excited ligand-to-metal charge-transfer. Specifically, we propose that this could be applied to confirm or reject the presence of a hitherto elusive transient Quartet species. Thus, RIXS offers a particular possibility to settle a recent controversy regarding the decay pathway, and we expect the technique to be similarly applicable in other model systems of photo-induced dynamics.
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.
Biological carbon dioxide (CO2) reduction is an important step by which organisms form valuable energy-richer molecules required for further metabolic processes. The Mo-dependent formate dehydrogenase (FDH) from Rhodobacter capsulatus catalyzes reversible formate oxidation to CO2 at a bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor. To elucidate potential substrate binding sites relevant for the mechanism, we studied herein the interaction with the inhibitory molecules azide and cyanate, which are isoelectronic to CO2 and charged as formate. We employed infrared (IR) spectroscopy in combination with density functional theory (DFT) and inhibition kinetics. One distinct inhibitory molecule was found to bind to either a non-competitive or a competitive binding site in the secondary coordination sphere of the active site. Site-directed mutagenesis of key amino acid residues in the vicinity of the bis-MGD cofactor revealed changes in both non-competitive and competitive binding, whereby the inhibitor is in case of the latter interaction presumably bound between the cofactor and the adjacent Arg587.
Silicate melts are major components of the Earth’s interior and as such they make an essential contribution in igneous processes, in the dynamics of the solid Earth and the chemical development of the entire Earth. Macroscopic physical and chemical properties such as density, compressibility, viscosity, degree of polymerization etc. are determined by the atomic structure of the melt. Depending on the pressure, but also on the temperature and the chemical composition, silicate melts show different structural properties. These properties are best described by the local coordination environment, i.e. symmetry and number of neighbors (coordination number) of an atom, as well as the distance between the central atom and its neighbors (inter-atomic distance). With increasing pressure and temperature, i.e. with increasing depth in the Earth, the density of the melt increases, which can lead to changes in coordination number and distances. If the coordination number remains the same, the distance usually decreases. If the coordination number increases, the distance can increase. These general trends can, however, vary greatly, which can be attributed in particular to the chemical composition.
Due to the fact that natural melts of the deep earth are not accessible to direct investigations, in order to understand their properties under the relevant conditions, extensive experimental and theoretical investigations have been carried out so far. This has often been studied using the example of amorphous samples of the end-members SiO2 and GeO2 , with the latter serving as a structural and chemical analog model to SiO2. Commonly, the experiments were carried out at high pressure and at room temperature. Natural melts are chemically much more complex than the simple end-member SiO2 and GeO2, so that observations made on them may lead to incorrect compression models. Furthermore, the investigations on glasses at room temperature can show potentially strong deviations from the properties of melts under natural thermodynamic conditions.
The aim of this thesis was to explain the influence of the composition and the temperature on the structural properties of the melts at high pressures. To understand this, we studied complex alumino-germanate and alumino-silicate glasses. More precisely, we studied synthetic glasses that have a composition like the mineral albite and like a mixture of albite-diopside at the eutectic point. The albite glass is structurally similar to a simplified granitic melt, while the albite-diopside glass simulates a simplified basaltic melt. To study the local coordination environment of the elements, we used X-ray absorption spectroscopy in combination with a diamond anvil cell. Because the diamonds have a high absorbance for X-rays with energies below 10 keV, the direct investigation of the geologically relevant elements such as Si, Al, Ca, Mg etc. with this spectroscopic probe technique in combination with a diamond anvil cell is not possible. Therefore the glasses were doped with Ge and Sr. These elements serve partially or fully as substitutes for important major elements. In this sense, Ge serves as an a substitute for Si and other network formers, while Sr replaces network modifiers such as Ca, Na, Mg etc.,
as well as other cations with a large ionic radius.
In the first step we studied the Ge K-edge in Ge-Albit-glass, NaAlGe3O8, at room temperature up to 131 GPa. This glass has a higher chemical complexity than SiO2 and GeO2, but it is still fully polymerized. The differences in the compression mechanism between this glass and the simple oxides can clearly be attributed to higher chemical complexity. The albite and albite-diopside compositions partially doped with Ge and Sr were probed at room temperature for Ge up to 164 GPa and for Sr up to 42 GPa. While the albite glass is nominally fully polymerized like NaAlGe3O8, the albite-diopside glass is partially depolymerized. The results show that structural changes take place in all three glasses in the first 25 to a maximum of 30 GPa, with both Ge and Sr reaching the maximum coordination number 6 and ∼9, respectively. At higher pressures, only isostructural shrinkage of the coordination polyhedra takes place in the glasses. The most important finding of the high pressure studies on the alumino-silicate and alumino-germanate glasses is that in these complex glasses the polyhedra show a much higher compressibility than what can be observed in the end-members. This is shown in particular by the strong shortening of the Ge-O distances in the amorphous NaAlGe3O8 and albite-diopside glass at pressures above 30 GPa.
In addition to the effects of the composition on the compaction process, we investigated the influence of temperature on the structural changes. To do this, we probed the albite-diopside glass, as it is chemically most similar to the melts in the lower mantle. We studied the Ge K edge of the sample with a resistively heated and a laser-heated diamond anvil cell, for a pressure range of up to 48 GPa and a temperature range of up to 5000 K. High temperatures at which the sample is liquid and that are relevant for the Earth mantle, have a significant impact on the structural transformation, with a shift of approx. 30% to significantly lower pressures, compared to the glasses at room temperature and below 1000 K.
The results of this thesis represent an important contribution to the understanding of the properties of melts at conditions of the lower mantle. In the context of the discussion about the existence and origin of ultra-dense silicate melts at the core-mantle boundary, these investigations show that the higher density compared to the surrounding material cannot be explained by only structural features, but by a distinct chemical composition. The results also suggest that only very low solubilities of noble gases are to be expected for melts in the lower mantle, so that the structural properties clearly influence the overall budget and transport of noble gases in the Earth’s mantle.
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.
This dissertation focuses on the understanding of the optical manipulation of microgels dispersed in aqueous solution of azobenzene containing surfactant. The work consists of three parts where each part is a systematic investigation of the (1) photo-isomerization kinetics of the surfactant in complex with the microgel polymer matrix, (2) light driven diffusiosmosis (LDDO) in microgels and (3) photo-responsivity of microgel on complexation with spiropyran.
The first part comprises three publications where the first one [P1] investigates the photo-isomerization kinetics and corresponding isomer composition at a photo-stationary state of the photo-sensitive surfactant conjugated with charged polymers or micro sized polymer networks to understand the structural response of such photo-sensitive complexes. We report that the photo-isomerization of the azobenzene-containing cationic surfactant is slower in a polymer complex compared to being purely dissolved in an aqueous solution. The surfactant aggregates near the polyelectrolyte chains at concentrations much lower than the bulk critical micelle concentration. This, along with the inhibition of the photo-isomerization kinetics due to steric hindrance within the densely packed aggregates, pushes the isomer-ratio to a higher trans-isomer concentration for all irradiation wavelengths.
The second publication [P2] combines experimental results and non-adiabatic dynamic simulations for the same surfactant molecules embedded in the micelles with absorption spectroscopy measurements of micellar solutions to uncover the reasons responsible for the slowdown in photo induced trans → cis azobenzene isomerization at concentrations higher than the critical micelle concentration (CMC). The simulations reveal a decrease of isomerization quantum yields for molecules inside the micelles and observes a reduction of extinction coefficients upon micellization. These findings explain the deceleration of the trans → cis switching in micelles of the azobenzene-containing surfactants.
Finally, the third publication [P3] focusses on the kinetics of adsorption and desorption of the same surfactant within anionic microgels in the dark and under continuous irradiation. Experimental data demonstrate, that microgels can serve as a selective absorber of the trans isomers. The interaction of the isomers with the gel matrix induces a remotely controllable collapse or swelling on appropriate irradiation wavelengths. Measuring the kinetics of the microgel size response and knowing the exact isomer composition under light exposure, we calculate the adsorption rate of the trans-isomers.
The second part comprises two publications. The first publication [P4] reports on the phenomenon of light-driven diffusioosmotic (DO) long-range attractive and repulsive interactions between micro-sized objects, whose range extends several times the size of microparticles and can be adjusted to point towards or away from the particle by varying irradiation parameters such as intensity or wavelength of light. The phenomenon is fueled by the aforementioned photosensitive surfactant. The complex interaction of dynamic exchange of isomers and photo-isomerization rate yields to relative concentrations gradients of the isomers in the vicinity of micro-sized object inducing a local diffusioosmotic (DO) flow thereby making a surface act as a micropump.
The second publication [P5] exclusively aims the visualization and investigation of the DO flows generated from microgels by using small tracer particles. Similar to micro sized objects, the flow is able to push adjacent tracers over distances several times larger than microgel size. Here we report that the direction and the strength of the l-LDDO depends on the intensity, irradiation wavelength and the amount of surfactant adsorbed by the microgel. For example, the flow pattern around a microgel is directed radially outward and can be maintained quasi-indefinitely under exposure at 455 nm when the trans:cis ratio is 2:1, whereas irradiation at 365 nm, generates a radially transient flow pattern, which inverts at lower intensities.
Lastly, the third part consists of one publication [P6] which, unlike the previous works, reports on the study of the kinetics of photo- and thermo-switching of a new surfactant namely, spiropyran, upon exposure with light of different wavelengths and its interaction with p(NIPAM-AA) microgels. The surfactant being an amphiphile, switches between its ring closed spiropyran (SP) form and ring open merocyanine (MC) form which results in a change in the hydrophilic–hydrophobic balance of the surfactant as MC being a zwitterionic form along with the charged head group, generates three charges on the molecule. Therefore, the MC form of the surfactant is more hydrophilic than in the case of the neutral SP state. Here, we investigate the initial shrinkage of the gel particles via charge compensation on first exposure to SP molecules which results from the complex formation of the molecules with the gel matrix, triggering them to become photo responsive. The size and VPTT of the microgels during irradiation is shown to be a combination of heating up of the solution during light absorption by the surfactant (more pronounced in the case of UV irradiation) and the change in the hydrophobicity of the surfactant.
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+.
With the present theoretical study of the photochemical switching of E-methylfurylfulgide we contribute an important step towards the understanding of the photochemical processes in furylfulgide-related molecules. We have carried out large-scale, full-dimensional direct semiempirical configuration-interaction surface-hopping dynamics of the photoinduced ring-closure reaction. Simulated static and dynamical UV/Vis-spectra show good agreement with experimental data of the same molecule. By a careful investigation of our dynamical data, we were able to identify marked differences to the dynamics of the previously studied E-isopropylfurylfulgide. With our simulations we can not only reproduce the experimentally observed quantum yield differences qualitatively but we can also pinpoint two reasons for them: kinematics and pre-orientation. With our analysis, we thus offer straightforward molecular explanations for the high sensitivity of the photodynamics towards seemingly minor changes in molecular constitution. Beyond the realm of furylfulgides, these insights provide additional guidance to the rational design of photochemically switchable molecules.