Time-Resolved Optical Pump-Resonant X-ray Probe Spectroscopy of 4-Thiouracil: A Simulation Study

Abstract

We theoretically monitor the photoinduced ππ* → nπ* internal conversion process in 4-thiouracil (4TU), triggered by an optical pump. The elementsensitive spectroscopic signatures are recorded by a resonant X-ray probe tuned to the sulfur, oxygen, or nitrogen K-edge. We employ high-level electronic structure methods optimized for core-excited electronic structure calculation combined with quantum nuclear wavepacket dynamics computed on two relevant nuclear modes, fully accounting for their quantum nature of nuclear motions. We critically discuss the capabilities and limitations of the resonant technique. For sulfur and nitrogen, we document a pre-edge spectral window free from ground-state background and rich with ππ* and nπ* absorption features. The lowest sulfur K-edge shows strong absorption for both ππ* and nπ*. In the lowest nitrogen K-edge window, we resolve a state-specific fingerprint of the ππ* and an approximate timing of the conical intersection via its depletion. A spectral signature of the nπ* transition, not accessible by UV−vis spectroscopy, is identified. The oxygen K-edge is not sensitive to molecular deformations and gives steady transient absorption features without spectral dynamics. The ππ*/nπ* coherence information is masked by more intense contributions from populations. Altogether, element-specific time-resolved resonant X-ray spectroscopy provides a detailed picture of the electronic excited-state dynamics and therefore a sensitive window into the photophysics of thiobases.

Monitoring vibronic coherences and molecular aromaticity in photoexcited cyclooctatetraene with an X-ray probe: a simulation study

“…The central frequency was chosen to maximize the signal strength by maximizing the polarizability cross section while still staying offresonant (pre-resonant) 53 . Otherwise the populations will contribute and dominate the coherences 54,55 . The signal is finite only when there is an overlap of nuclear wavepackets in different electronic states, making this technique free from population background.…”

“…53 Otherwise the populations will contribute and dominate the coherences. 54,55 The signal is nite only when there is an overlap of nuclear wavepackets in different electronic states, making this technique free from population background. A single broadband pulse with the same central frequency and bandwidth (Fig.…”

Attosecond Monitoring of Nonadiabatic Molecular Dynamics by Transient X-ray Transmission Spectroscopy

“…ATAS, in contrast, is dominated by population terms. This allows one to track the evolution of the electronic states with the unique advantages of XUV and X-ray sources, such as excellent temporal resolution and sensitivity to optically dark valence states due to different selection rules of core-excited states. , At the same time, we have shown that the weaker modulations in the signal due to the coherences can be highlighted in ATAS, e.g., by computing a FROG spectrogram.…”

Time-resolved X-ray and XUV based spectroscopic methods for nonadiabatic processes in photochemistry

“…Multiple off-resonant stimulated Raman methods have been proposed to study coherence present in a molecule. [81][82][83][84] In a stimulated Raman process, two temporally overlapping pulses are used as a probe that creates a Raman transition between two electronic states. One of these stimulated Raman spectroscopic methods is TRUECARS, 51 which detects the occurrence of CI by relying on the generated electronic coherence near a CI.…”