Identification of an ultrafast internal conversion pathway of pyrazine by time-resolved vacuum ultraviolet photoelectron spectrum simulations

Abstract

The internal conversion from the optically bright S${}_2$ (${}^1$B${}_{2\mathrm{u}}$, $\pi {\pi }^{\ast }$) state to the dark S${}_1$ (${}^1$B${}_{3\mathrm{u}}$, n${\pi }^{\ast }$) state in pyrazine is a standard benchmark for experimental and theoretical studies on ultrafast radiationless decay. Since 2008 a few theoretical groups have suggested significant contributions of other dark states S${}_3$ (${}^1$A${}_{\mathrm{u}}$, n${\pi }^{\ast }$) and S${}_4$ (${}^1$B${}_{2\mathrm{g}}$, n${\pi }^{\ast }$) to the decay of S${}_2$. We have previously reported the results of nuclear wave packet simulations [Phys. Chem. Chem. Phys. 17, 2012 (2015)] and photoelectron spectrum calculations [Chem. Phys. 515, 704 (2018)] that support the conventional two-state picture. In this article, the two different approaches, i.e., wave packet simulation and photoelectron spectrum calculation are combined: We computed the time-resolved vacuum ultraviolet photoelectron spectrum and photoelectron angular distribution for the ionization of the wave packet transferred from S${}_2$ to S${}_1$. The present results reproduce almost all the characteristic features of the corresponding experimental time-resolved spectrum [T. Horio et al., J. Chem. Phys. 145, 044306 (2016)] such as a rapid change from a three-band to two-band structure. This further supports the existence and character of the widely accepted pathway (S${}_2$ $\to$ S${}_1$) of ultrafast internal conversion in pyrazine.

30 pages, 8 figures. This article has been submitted to The Journal of Chemical Physics