X-Ray Transient Absorption Reveals the 1Au (Nπ*) State of Pyrazine in Electronic Relaxation

Abstract

Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. In addition to the previously characterized <sup>1</sup>B<sub>2u</sub> (ππ*) (S<sub>2</sub>) and <sup>1</sup>B<sub>3u</sub> (nπ*) (S<sub>1</sub>) states, the participation of the optically dark <sup>1</sup>A<sub>u</sub> (nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations, nonadiabatic dynamics simulations, and X-ray spectral computation. Despite <sup>1</sup>A<sub>u</sub> (nπ*) and <sup>1</sup>B<sub>3u</sub> (nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The <sup>1</sup>A<sub>u</sub> (nπ*) state is populated about 200 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.