The photodissociation dynamics of N-nitrosopyrrolidine from the first and second excited singlet states studied by velocity map imaging

The photodissociation reaction of N-nitrosopyrrolidine isolated and cooled in a supersonic jet has been studied following excitation to the S-1 and S-2 electronic states. The nascent NO ((X) over tilde (2)Pi(1/2,3/2), v, j) radicals were ionized by state-selective (1 + 1)-REMPI via the A(2)Sigma(+) state. The angularly resolved velocity distribution of these ions was measured with the velocity-map imaging (VMI) technique. Photodissociation from S-1 produces NO in the vibrational ground state and the pyrrolidine radical in the electronic ground state 1 B-2. About 73% of the excess energy is converted into kinetic energy of the fragments. The velocity distribution shows a strong negative anisotropy (beta = -0.9) in accordance with the n pi*-character of the S-0 -> S-1 transition. An upper limit for the N-NO dissociation energy of (14 640 +/- 340) cm(-1) is determined. We conclude that photodissociation from S-1 occurs very fast on a completely repulsive potential energy surface. Excitation into the S-2 pi pi*-state leads to a bimodal velocity distribution. Two dissociation channels can be distinguished which show both positive anisotropy (beta = 1.3 and 1.6) but differ considerably in the total kinetic energy and the rotational energy of the NO fragment. We assign one channel to the direct dissociation on the S-2 potential energy surface, leading to pyrrolidine radicals in the excited electronic state 1 (2) . The second channel leads to pyrrolidine in the electronic ground state 1 B-2, presumably after crossing to the S-1 state via a conical intersection.