TY - JOUR A1 - Oberg, H. A1 - Gladh, Jörgen A1 - Anniyev, Toyli A1 - Beye, Martin A1 - Coffee, Ryan A1 - Föhlisch, Alexander A1 - Katayama, T. A1 - Kaya, Sarp A1 - LaRue, Jerry A1 - Mogelhoj, Andreas A1 - Nordlund, Dennis A1 - Ogasawara, Hirohito A1 - Schlotter, William F. A1 - Sellberg, Jonas A. A1 - Sorgenfrei, Florian A1 - Turner, Joshua J. A1 - Wolf, Martin A1 - Wurth, W. A1 - Ostrom, Henrik A1 - Nilsson, Anders A1 - Norskov, Jens K. A1 - Pettersson, Lars G. M. T1 - Optical laser-induced CO desorption from Ru(0001) monitored with a free-electron X-ray laser: DFT prediction and X-ray confirmation of a precursor state JF - Surface science N2 - We present density functional theory modeling of time-resolved optical pump/X-ray spectroscopic probe data of CO desorption from Ru(0001). The BEEF van der Waals functional predicts a weakly bound state as a precursor to desorption. The optical pump leads to a near-instantaneous (<100 fs) increase of the electronic temperature to nearly 7000 K. The temperature evolution and energy transfer between electrons, substrate phonons and adsorbate is described by the two-temperature model and found to equilibrate on a timescale of a few picoseconds to an elevated local temperature of similar to 2000K. Estimating the free energy based on the computed potential of mean force along the desorption path, we find an entropic barrier to desorption (and by time-reversal also to adsorption). This entropic barrier separates the chemisorbed and precursor states, and becomes significant at the elevated temperature of the experiment (similar to 1.4 eV at 2000 K). Experimental pump-probe X-ray absorption/X-ray emission spectroscopy indicates population of a precursor state to desorption upon laser-excitation of the system (Dell'Angela et al., 2013). Computing spectra along the desorption path confirms the picture of a weakly bound transient state arising from ultrafast heating of the metal substrate. (C) 2015 Elsevier B.V. All rights reserved. KW - CO desorption KW - Potential of mean force KW - Two-temperature model KW - Pump-probe KW - X-ray spectroscopy KW - Density functional theory Y1 - 2015 U6 - https://doi.org/10.1016/j.susc.2015.03.011 SN - 0039-6028 SN - 1879-2758 VL - 640 SP - 80 EP - 88 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Kröner, Dominik A1 - Götze, Jan Philipp T1 - Modeling of a violaxanthin-chlorophyll b chromophore pair in its LHCII environment using CAM-B3LYP JF - Journal of photochemistry and photobiology : B, Biology N2 - Collecting energy for photosystem II is facilitated by several pigments, xanthophylls and chlorophylls, embedded in the light harvesting complex II (LHCII). One xanthophyll, violaxanthin (Vio), is loosely bound at a site close to a chlorophyll b (Chl). No final answer has yet been found for the role of this specific xanthophyll. We study the electronic structure of Vio in the presence of Chl and under the influence of the LHCII environment, represented by a point charge field (PCF). We compare the capability of the long range corrected density functional theory (DFT) functional CAM-B3LYP to B3LYP for the modeling of the UV/vis spectrum of the Vio + Chl pair. CAM-B3LYP was reported to allow for a very realistic reproduction of bond length alternation of linear polyenes, which has considerable impact on the carotenoid structure and spectrum. To account for the influence of the LHCII environment, the chromophore geometries are optimized using an ONIOM(DFT/6-31G(d):PM6) scheme. Our calculations show that the energies of the locally excited states are almost unaffected by the presence of the partner chromophore or the PCF. There are, however, indications for excitonic coupling of the Chl Soret band and Vio. We propose that Vio may accept energy from blue-light excited Chl. KW - CAM-B3LYP KW - Density functional theory KW - Light-harvesting complex II KW - Photosynthesis KW - Violaxanthin KW - Chlorophyll b Y1 - 2012 U6 - https://doi.org/10.1016/j.jphotobiol.2011.12.007 SN - 1011-1344 VL - 109 IS - 2 SP - 12 EP - 19 PB - Elsevier CY - Lausanne ER -