Unravelling the Electronic State of NO2 Product in Ultrafast Photodissociation of Nitromethane.

The primary photochemical reaction of nitromethane (NM) after ππ* excitation is known to be C-N bond cleavage (CH3NO2 + hν → CH3 + NO2). On the other hand, NO2 can be formed in both the ground and excited states, and identification of the electronic state of the NO2 product has been a central subject in the experimental and theoretical studies. Here we present time-resolved photoelectron spectroscopy using vacuum-ultraviolet probe pulses to observe all transient electronic states of NM and the reaction products. The result indicates that ultrafast internal conversion occurs down to S1 and S0 within 24 fs, and the dissociation proceeds on the S1 surface (τdiss ≲ 50 fs), leading to comparable product yields of NO2(A) and NO2(X). The overall dissociation quantum yield within our observation time window (<2 ps) is estimated to be 0.29.