New free-electron laser and high-harmonic generation X-ray light sources are capable of supplying pulses short and intense enough to perform resonant nonlinear time-resolved experiments in molecules. Valence-electron motions can be triggered impulsively by core excitations and monitored with high temporal and spatial resolution. We discuss possible experiments that employ attosecond X-ray pulses to probe the quantum coherence and correlations of valence electrons and holes, rather than the charge density alone, building on the analogy with existing studies of vibrational motions using femtosecond techniques in the visible regime.
We report simulations of X-ray absorption near edge structure (XANES), resonant inelastic X-ray scattering (RIXS) and 1D stimulated X-ray Raman spectroscopy (SXRS) signals of cysteine at the oxygen, nitrogen, and sulfur K and L 2,3 edges. Comparison of the simulated XANES signals with experiment shows that the restricted window time-dependent density functional theory is more accurate and computationally less expensive than the static exchange method. Simulated RIXS and 1D SXRS signals give some insights into the correlation of different excitations in the molecule.
Expressions for the two-dimensional stimulated x-ray Raman spectroscopy (2D-SXRS) signal obtained using attosecond x-ray pulses are derived. The 1D-and 2D-SXRS signals are calculated for trans-N-methyl acetamide (NMA) with broad bandwidth (181 as, 14.2 eV FWHM) pulses tuned to the oxygen and nitrogen K-edges. Crosspeaks in 2D signals reveal electronic Franck-Condon overlaps between valence orbitals and relaxed orbitals in the presence of the core-hole.
Understanding the excitation energy transfer mechanism in multiporphyrin arrays is key for designing artificial light-harvesting devices and other molecular electronics applications. Simulations of the stimulated X-ray Raman spectroscopy signals of a Zn/Ni porphyrin heterodimer induced by attosecond X-ray pulses show that these signals can directly reveal electron-hole pair motions. These dynamics are visualized by a natural orbital decomposition of the valence electron wavepackets.chromophore aggregates | core transitions | REW-TDDFT | nonlinear P orphyrin rings are pyrole-based cyclic conjugated systems that serve as the main building blocks in many devices that depend on their high excitation energy transfer (EET) efficiency (1-4). Because of their stability and interesting structural, electronic, and optical properties, porphyrin compounds have a wide range of uses as chemical sensors (5), photosensitizers in photodynamic therapy for cancer (6), nonlinear optical materials (7-9), and molecular electronic (10-12) and spintronic devices (13,14).Porphyrin-based molecules hold a pivotal position in the chemistry of engineered photoactive organic compounds, and extensive electronic structure calculations of monomeric (15) and oligomeric (16, 17) porphyrin molecules, porphyrin structures in biomacromolecules (18,19), and quasi-1D and -2D porphyrin systems with infinite sizes have been carried out (13,14,(20)(21)(22)(23). Most applications involve multiporphyrin arrays, either in linear or in cyclic shape, or dendrimers (24). Porphyrin dimers, which are still small enough to be treated with relatively high-level modern quantum chemistry methods, can offer basic clues to track down the more complicated EET dynamics in multiporphyrin arrays.The kinetics of EET in multiporphyrin systems have long been studied by time-resolved fluorescence anisotropy decay (25) and pump-probe techniques, using visible light (26).Here we present a simulation study that shows how recently developed attosecond sources of X-ray pulses may be used to probe the energy transfer dynamics in a porphyrin dimer. Intense attosecond X-ray pulses, recently made available by new X-ray free electron laser (XFEL) (27, 28) and higher harmonic generation (29, 30) sources, have bandwidths covering multiple electron volts and can prepare coherent superpositions of valence electronically excited states through an impulsive Raman process (31). The short durations of these pulses make them ideal for tracing valence electronic dynamics that evolve with extremely short periods. X-ray pulses can also exploit the spectrally isolated coreexcitation frequencies to create valence excitations in the neighborhood of a selected atom, a type of localized excitation not generally accessible using visible or UV pulses. An experimental realization of a two-color pump-probe X-ray source from XFEL radiation was recently reported (32). These new sources have also been used in time-dependent X-ray diffraction studies (33), to monitor ultrafast changes in the conductivity of se...
We show that broadband x-ray pulses can create wavepackets of valence electrons and holes localized in the vicinity of a selected atom (nitrogen, oxygen or sulfur in cysteine) by stimulated resonant Raman scattering. The subsequent dynamics reveals highly correlated motions of entangled electrons and hole quasiparticles. This information goes beyond the time-dependent total charge density derived from x-ray diffraction.
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