Exploring ultrafast dynamics of pyrazine by time-resolved photoelectron imaging

We present the simulation of time-resolved photoelectron imaging spectra of pyrazine in the gas phase. The approach we have adopted is based on the combination of the ab initio nonadiabatic molecular dynamics "on the fly" with an approximate treatment of the photoionization process using Dyson orbitals and Coulomb functions to describe the bound and ionized states of the photoelectron. The method has been implemented (Humeniuk, A.; et al. J. Chem. Phys 2013, 139, 134104) in the framework of the time-dependent density functional theory and has been applied here to interrogate the ultrafast internal conversion between the S2 and S1 states in pyrazine. Conventional time-resolved photoelectron spectra without angular resolution fail to locate the S2 → S1 internal conversion, because the ionization potentials relevant for the photoionization channels S2 → D1 (π(-1)) and S1 → D0 (n(-1)) are almost identical. Introducing the angular resolution in the photoelectron spectra reveals evidence of such ultrafast internal conversion and provides a more detailed picture of the overall dynamics. The simulated time- and energy-dependent anisotropy map obtained within the Dyson/time-dependent density functional theory approach is in good agreement with its experimental counterpart provided by Horio et al. (Horio, T.; et al. J. Am. Chem. Soc. 2009, 131, 10932). Our theoretical approach represents a general tool for mapping the time- and angle-resolved photoelectron spectra in complex systems and thus can be used to investigate the ultrafast relaxation processes occurring in isolated molecules.