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

TL;DR

This study combines wave packet simulations and photoelectron spectrum calculations to elucidate the ultrafast internal conversion pathway in pyrazine, successfully reproducing experimental spectra and confirming the S2 to S1 decay mechanism.

Contribution

It introduces a combined approach of wave packet and photoelectron spectrum simulations to analyze pyrazine's internal conversion pathway, providing detailed spectral insights.

Findings

Reproduces experimental time-resolved spectra features

Supports the S2 to S1 ultrafast decay pathway

Clarifies the role of dark states in pyrazine decay

Abstract

The internal conversion from the optically bright S$_2$ ($^1$B$_{2\mathrm{u}}$, $\pi\pi^*$) state to the dark S$_1$ ($^1$B$_{3\mathrm{u}}$, n$\pi^*$) 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$\pi^*$) and S$_4$ ($^1$B$_{2\mathrm{g}}$, n$\pi^*$) 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$_2$ $\rightarrow$ S$_1$) of ultrafast internal conversion in pyrazine.