Abstract
The internal conversion of photo-excited pyrazine, which occurs rapidly on a time scale of about 20 fs, has long been considered to proceed via a conical intersection between the optically bright S2 (1B2u, ππ∗) and dark S1 (1B3u, nπ∗) states. Since 2008, several theoretical studies have raised the possibility that other dark states S3 (1Au, nπ∗) and S4 (1B2g, nπ∗) may participate dominantly in the early stage of the nonradiative decay of S2. To clarify this issue, being motivated by the recent pump–probe experiment by Horio et al. [J. Chem. Phys. 145 (2016) 044306], we calculated vacuum ultraviolet photoelectron spectra for ionization from each of the four excited states. Comparison was made with the measured time-resolved photoelectron spectrum exhibiting a temporally varying multi-band structure. We confirmed no contribution of S3 or S4 and thus the validity of the conventional two-state (S2 → S1) picture for ultrafast nonradiative transition in pyrazine.
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