Photodissociation dynamics of D2O via the (B)over-tilde(1A1) electronic state

Photodissociation dynamics of D2O in the (B) over tilde((1)A(1)) state at different photolysis wavelengths have been investigated using the D-atom Rydberg tagging time-of-flight (TOF) technique, in combination with a tunable vacuum ultraviolet photolysis light source. TOF spectra of the D-atom product from the D2O photodissociation in both parallel and perpendicular polarizations have been measured. Product kinetic energy distributions and angular distributions have been derived from these TOF spectra. From these distributions, internal state distributions of the OD product as well as the OD quantum state specific angular anisotropy parameters have been derived. Two product channels governed by distinct dissociation dynamics have been clearly observed in the (B) over tilde((1)A(1)) state photodissociation: ground electronic state radical product OD(X (2) Pi) + D and excited electronic state OD(A (2) Sigma(+)) + D. The OD(A) + D channel proceeds via adiabatic pathway on the (B) over tilde((1)A(1)) state surface, producing rovibrational excitation in the OD(A) product, while the OD(X) + D channel is generated through nonadiabatic pathway mainly via conical intersections between the (B) over tilde((1)A(1)) and the (X) over tilde((1)A(1)) state surfaces. Due to strong angular force induced by the conical intersections, the OD(X) product is extremely hot in the rotational excitation close to the energy limit (N similar to 50 for v = 0). However, the vibrational excitation is cold in the OD(X) product with dominant population in the ground vibrational state v = 0. Detailed experimental results at different photolysis wavelengths show that at higher energy the unstable periodic orbit, from which dissociation starts, on the (B) over tilde state has stronger excitation degree of the OD internal state. The negative angular anisotropy parameters of the OD(A) products suggest that the angular forces in this adiabatic dissociation pathway from these periodic orbits have changed the original angular distribution of the D2O molecule excited by the (B) over tilde((1)A(1)) <- (X) over tilde((1)A(1)) parallel transition. (C) 2011 American Institute of Physics. [doi:10.1063/1.3555589]