Tracking the Relaxation of 2,5-Dimethylpyrrole by Femtosecond Time-Resolved Photoelectron and Photoion Detection

Abstract

The relaxation of 2,5-dimethylpyrrole after excitation in the 290–239 nm range, which covers the weak absorption of the S₁ ¹A₂ πσ* state, dissociative along the N–H bond, and the stronger band mostly attributed to the ¹B₂ ππ* state, has been investigated by time-resolved ion and photoelectron techniques. The measurements yield an invariant lifetime of ∼55 fs for the ¹πσ* state, after preparation in its Franck–Condon region with increasing vibrational content. This ultrafast rate indicates that, contrary to the observations made in pyrrole (Roberts et al.<i> Faraday Discuss.</i> <b>2013</b>, <i>163</i>, 95–116), the molecule reaches the dissociative part of the potential without any barrier effect, although calculations predict the latter to be higher than in the pyrrole case. The results are rationalized in terms of a barrier free multidimensional pathway that very likely involves out-of-plane vibrations. Additionally, a lifetime of ∼100 fs is found after excitation along the higher ¹B₂ ππ* ← S₀ transition. The relaxation of this state by coupling to a very short living S₁ ¹πσ* state, or by alternative routes, is discussed in the light of the collected photoelectron measurements