An experimental and theoretical study of the vibrationally mediated photodissociation of hydroxylamine

Abstract

We present a detailed investigation of the photodissociation of hydroxylamine following direct single-photon and vibrationally mediated two-photon excitation below 42 000 cm−1. In all cases the lowest dissociation channel [NH2(X̃ 2B1)+OH(X̃ 2Π)] dominates. Single-photon dissociation at 240 nm releases most of the excess energy (20 550 cm−1) into relative translation (53%) and NH2 internal energy (40%, mostly vibrational). OH carries little internal energy (7%), most of it in the form of rotational excitation. Torsional excitation during the dissociation step leads to rotational alignment of the OH fragments and a preferential population of the Π(A″) component of the lambda doublet. Both are lost after isoenergetic two-photon excitation via O–H stretching overtones of NH2OH, also leading to higher internal excitation of the NH2 fragments (∼50%) at the expense of relative translation. At lower total excitation energies the relative translation takes up an increasing fraction of the total excess energy (⩾80% at 5820 cm−1 of excess energy). The results are discussed in terms of ab initio calculations using complete active space second-order perturbation theory with augmented triple-ζ basis sets for the lowest excited singlet states. One- and two-dimensional potential functions explain the OH product state distributions observed in different experiments in terms of the geometry relaxation of NH2OH upon electronic excitation. Crossing between the lowest excitated A′ and A″ singlet states in the Franck–Condon region leads to a barrier of ∼0.5 eV to dissociation in S1, which dominates the photodissociation dynamics.