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

Abstract

The photodissociation reaction of N-nitrosopyrrolidine isolated and cooled in a supersonic jet has been studied following excitation to the S₁ and S₂ electronic states. The nascent NO ( ²Π_½,, v, j) radicals were ionized by state-selective (1 + 1)-REMPI via the A²Σ⁺ state. The angularly resolved velocity distribution of these ions was measured with the velocity-map imaging (VMI) technique. Photodissociation from S₁ produces NO in the vibrational ground state and the pyrrolidine radical in the electronic ground state 1 ²B. About 73% of the excess energy is converted into kinetic energy of the fragments. The velocity distribution shows a strong negative anisotropy (β = −0.9) in accordance with the nπ*-character of the S₀ → S₁ transition. An upper limit for the N–NO dissociation energy of (14 640 ± 340) cm⁻¹ is determined. We conclude that photodissociation from S₁ occurs very fast on a completely repulsive potential energy surface. Excitation into the S₂ ππ*-state leads to a bimodal velocity distribution. Two dissociation channels can be distinguished which show both positive anisotropy (β = 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 S₂ potential energy surface, leading to pyrrolidine radicals in the excited electronic state 1 ²A. The second channel leads to pyrrolidine in the electronic ground state 1 ²B, presumably after crossing to the S₁ state via a conical intersection.