The time-resolved photoelectron spectrum of trans-azobenzene and its relationship with the photoisomerization mechanism: A surface hopping simulation with determination of Dyson orbitals

We performed a simulation of the time-resolved photoelectron spectrum (TRPES) of trans-azobenzene after ππ∗ excitation in a mixed quantum-classical framework. The electronic structure of the molecule and of its cation was obtained with a semiempirical multireference configuration interaction approach, the nonadiabatic dynamics was simulated with the surface hopping algorithm, and the TRPES was determined by computing the relevant Dyson orbitals. According to our results, the TRPES obtained with a probe photon energy of 6.0 eV shows two bands, the one at higher energy being due to the S2 electronic state of the molecule and the other one to the S1 state. For both bands, the main contribution to the spectrum originates from the D0 state of the cation. Our simulated TRPES is in very good agreement with the available experimental data, both in terms of the energetic positions of the bands and their lifetimes. Our findings confirm that the wavelength dependence of the photoisomerization quantum yields of trans-azobenzene is not due to a peculiar nonadiabatic decay process, as previously inferred from the experimental TRPES. Rather, the reason for the violation of Kasha's rule is that different regions of the nuclear phase space are explored in S1, whether this state is populated by direct excitation or by the radiationless decay of S2.