Unveiling Excited-State Dynamics of Nitrobenzene through Time-Resolved X-ray Absorption Spectroscopy

Following ultraviolet (UV) excitation, nitroaromatic molecules can undergo complex excited-state relaxation and photofragmentation processes that may generate reactive nitrogen oxide species (NO_x). However, the excited-state dynamics associated with NO_x formation remain incompletely understood. Here, we investigate the photochemistry of nitrobenzene using time-resolved X-ray absorption spectroscopy at the oxygen K-edge, combined with high-level electronic structure calculations. Upon 267 nm excitation in the gas phase, nitrobenzene undergoes rapid relaxation from the initially populated $S_3$¹(L_Bππ*) state to the $S_1$¹(n_Aπ*)/$T_1$³(n_Aπ*) states, where a significant fraction of the population remains trapped for at least 100 ps rather than undergoing rapid internal conversion to the S₀ state. Signatures of NO or NO₂ formation are not observed within this time window at the current signal-to-noise ratio, indicating that photofragmentation remains a minor channel up to 100 ps. These results provide element-specific insight into nitroaromatic photochemistry and support a mechanism in which photofragmentation is delayed by transient excited-state trapping.