Ultrafast nonradiative transition pathways in photo-excited pyrazine: Ab initio analysis of time-resolved vacuum ultraviolet photoelectron spectrum

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 S 2 ( 1 B 2u , ππ ∗ ) and dark S 1 ( 1 B 3u , nπ ∗ ) states. Since 2008, several theoretical studies have raised the possibility that other dark states S 3 ( 1 A u , nπ ∗ ) and S 4 ( 1 B 2g , nπ ∗ ) may participate dominantly in the early stage of the nonradiative decay of S 2 . 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 S 3 or S 4 and thus the validity of the conventional two-state (S 2  → S 1 ) picture for ultrafast nonradiative transition in pyrazine.