Ultrafast dynamics of aniline following 269–238 nm excitation and the role of the S2(π3s/πσ*) state

Abstract

Femtosecond time-resolved photoelectron imaging is employed to investigate ultrafast electronic relaxation in aniline, a prototypical aromatic amine. The molecule is excited at wavelengths between 269 and 238 nm. We observe that the S₂(π3s/πσ^*) state is populated directly during the excitation process at all wavelengths and that the population bifurcates to two decay pathways. One of these involves ultrafast relaxation from the Rydberg component of S₂(π3s/πσ^*) to the S₁(ππ)^* state, from which it relaxes back to the electronic ground state on a much longer timescale. The other appears to involve motion along the πσ^* dissociative potential energy surface. At higher excitation energies, the dominant excitation is to the S₃(ππ^*) state, which undergoes extremely efficient electronic relaxation back to the ground state. Our study supports some conclusions reached from H-atom photofragment translational spectroscopy measurements and pump–probe photoionization measurements and contradicts some others.