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<i>In situ </i>soft x-ray absorption spectroscopic study of polycrystalline Fe/MgO interfaces
摘要： In situ soft x-ray absorption spectroscopic study of polycrystalline Fe/MgO interfaces. It is found that at the interface of iron and MgO film, about two monolayers of Fe3O4 is formed. Fe3O4 being the oxide of iron with the highest heat of formation, the reaction appears to be controlled thermodynamically. On the other hand, on the interface with MgO (001) surface, FeO is formed, suggesting that the reaction is limited by the availability of oxygen atoms. Magnetic behavior of the FeO layer gets modified significantly due to proximity effect of the bulk ferromagnetic iron layer.
关键词： FeO,soft x-ray absorption spectroscopy,Fe/MgO interfaces,Fe3O4,magnetic behavior
更新于2025-09-04 15:30:14
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Modeling core-level excitations with variationally optimized reduced-density matrices and the extended random phase approximation
摘要： The information contained within ground-state one- and two-electron reduced-density matrices (RDMs) can be used to compute wave functions and energies for electronically excited states through the extended random phase approximation (ERPA). The ERPA is an appealing framework for describing excitations out of states obtained via the variational optimization of the two-electron RDM (2-RDM), as the variational 2-RDM (v2RDM) approach itself can only be used to describe the lowest-energy state of a given spin symmetry. The utility of the ERPA for predicting near-edge features relevant to x-ray absorption spectroscopy is assessed for the case that the 2-RDM is obtained from a ground-state v2RDM-driven complete active space self-consistent field (CASSCF) computation. A class of killer conditions for the CASSCF-speci?c ERPA excitation operator is derived, and it is demonstrated that a reliable description of core-level excitations requires an excitation operator that ful?lls these conditions; the core-valence separation (CVS) scheme yields such an operator. Absolute excitation energies evaluated within the CASSCF/CVS-ERPA framework are slightly more accurate than those obtained from the usual random phase approximation (RPA), but the CVS-ERPA is not more accurate than RPA for predicting the relative positions of near-edge features. Nonetheless, CVS-ERPA is established as a reasonable starting point for the treatment of core-level excitations using variationally optimized 2-RDMs.
关键词： reduced-density matrices,extended random phase approximation,v2RDM,CASSCF,core-level excitations,x-ray absorption spectroscopy
更新于2025-09-04 15:30:14
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Fluorescence enhancement of a microbial rhodopsin via electronic reprogramming
摘要： The engineering of microbial rhodopsins with enhanced fluorescence is of great importance in the expanding field of optogenetics. Here we report the discovery of two mutants (W76S/Y179F and L83Q) of a sensory rhodopsin from the cyanobacterium Anabaena PCC7120 with opposite fluorescence behavior. In fact, while W76S/Y179F displays, with respect to the wild-type protein, a nearly ten-fold increase in red-light emission, the second is not emissive. Thus, the W76S/Y179F, L83Q pair offers an unprecedented opportunity for the investigation of fluorescence enhancement in microbial rhodopsins, which is pursued by combining transient absorption spectroscopy and multi-configurational quantum chemistry. The results of such an investigation point to an isomerization-blocking electronic effect as the direct cause of instantaneous (sub-picosecond) fluorescence enhancement.
关键词： fluorescence enhancement,multi-configurational quantum chemistry,transient absorption spectroscopy,optogenetics,electronic reprogramming,microbial rhodopsins
更新于2025-09-04 15:30:14
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State-Resolved Probing of Attosecond Timescale Molecular Dipoles
摘要： We report an experimental study of iodomethane attosecond transient absorption spectroscopy (ATAS) in the region of iodine 4d core-to-valence/Rydberg excitation. Similar to previous atomic experiments, XUV-NIR delay-dependent absorbance changes reflect a light-induced phase due to an NIR-field driven AC Stark shift of the excited states, as well as pathway interferences arising from couplings between neighboring states. As a novel aspect of molecular ATAS, we observe pronounced differences between the ATAS signatures of valence and Rydberg states. While the core-to-valence transitions carry the majority of the XUV oscillator strength, the core-to-Rydberg transitions are dominantly affected by a moderately strong, non-ionizing NIR field. Our experimental findings are corroborated by ab-initio calculations and ATAS simulations.
关键词： core-to-valence,attosecond transient absorption spectroscopy,iodomethane,core-to-Rydberg,AC Stark shift,ab-initio calculations,XUV-NIR
更新于2025-09-04 15:30:14
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Soft X-ray Absorption Spectroscopy of Aqueous Solutions Using a Table-Top Femtosecond Soft X-ray Source
摘要： We demonstrate the feasibility of soft X-ray absorption spectroscopy in the water window using a table-top laser-based approach with organic molecules and inorganic salts in aqueous solution. A high-order harmonic source delivers femtosecond pulses of short wavelength radiation in the photon energy range from 220 eV to 450 eV. We report static soft X-ray absorption measurements in transmission on the solvated compounds CO(NH2)2, CaCl2, and NaNO3, using flatjet technology. We monitor the absorption of the molecular samples between the carbon (≈280 eV) and nitrogen (≈400 eV) K-edges and compare our results with previous measurements performed at the BESSYII facility. We discuss the roles of pulse stability and photon flux in the outcome of our experiments. Our work paves the way towards table-top femtosecond, solution phase soft X-ray absorption spectroscopy in the water window.
关键词： aqueous solution,high-order harmonic source,flatjet technology,soft X-ray absorption spectroscopy,table-top laser-based approach,femtosecond pulses,water window
更新于2025-09-04 15:30:14