Product state distributions in the dissociation of H3 (n=2,3) Rydberg states

Abstract

Dissociation of the 2s 2A1′, 2p 2A2″, 3s 2A1′, and 3d 2E″ Rydberg states of the H3 molecule is investigated using a fast neutral beam photofragment spectrometer. A beam of 3–6 keV metastable H3 2p 2A2″ (N=K=0) molecules, generated by charge transfer neutralization of H+3 ions in Cs vapor, is intersected by a laser beam to selectively populate a specific rovibrational level in each of the H3 states for study. The correlated pair of fragments H+H2, created by dissociation of the H3 state, is observed by a time- and position-sensitive detector, which specifies the fragments’ center-of-mass kinetic energy release and angle of ejection. The 3s 2A1′ (N=1, K=0) and 3d 2E″ (N=1, G=0, R=1) rotational levels are prepared either in their ground vibrational states or in vibrationally excited states by pumping transitions from the 2p 2A2″ (N=K=0) level with a tunable dye laser. All of these photoexcited levels are observed to both predissociate and to radiate into the dissociative ground state. In contrast, the 2s 2A1′ (N=1, K=0) level, produced by stimulated emission pumping from the 2p 2A2″ (N=K=0) level using a CO2 laser, is observed to only predissociate. Predissociation of the H3 produces a discrete release of kinetic energy to the H+H2 fragments that uniquely identifies the production of a specific rovibrational level in the H2. Monte Carlo simulation of the detector response, combined with observation of the corresponding dissociations in D3 (where predissociation is essentially negligible), allows quantitative separation of H+H2 predissociation fragments from a background of continuous energy release produced by radiative dissociation. Branching ratios in the radiative and the predissociative decay channels are estimated and detailed distributions for the production of the H2(v,J) final state are determined. Comparison of experimental distributions with recent theoretical calculations illuminates the important role nonlinear couplings play in determining the dissociation dynamics of a particular H3 Rydberg state. Additionally, transition energies for the vibrationally excited levels of H3 are determined with improved accuracy, the relative importance of the H+H+H dissociation channel is estimated, and spontaneous dissociation of the metastable 2p 2A2″ (N=K=0) level is discussed.