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

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

Automated summary

In this study, we theoretically monitor the photoinduced pi pi*-> n pi* internal conversion process in 4-thiouracil (4TU) using a resonant X-ray probe tuned to the sulfur, oxygen, or nitrogen K-edge. We discuss the capabilities and limitations of the resonant technique and observe specific spectral features for sulfur and nitrogen, as well as an inaccessible transition for oxygen. Overall, this study provides a detailed understanding of the electronic excited-state dynamics and offers insights into the photophysics of thiobases.