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[combining 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 (2)B. 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 npi*-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)pipi*-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)A. The second channel leads to pyrrolidine in the electronic ground state 1 (2)B, presumably after crossing to the S(1) state via a conical intersection.