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Tracking the relaxation of 2,5-dimethylpyrrole by femtosecond time-resolved photoelectron and photoion detection.

The relaxation of 2,5-dimethylpyrrole after excitation in the 290-239 nm range, which covers the weak absorption of the S1 (1)A2 πσ* state, dissociative along the N-H bond, and the stronger band mostly attributed to the (1)B2 ππ* state, has been investigated by time-resolved ion and photoelectron techniques. The measurements yield an invariant lifetime of ∼55 fs for the (1)πσ* 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. Faraday Discuss. 2013, 163, 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 (1)B2 ππ* ← S0 transition. The relaxation of this state by coupling to a very short living S1 (1)πσ* state, or by alternative routes, is discussed in the light of the collected photoelectron measurements.