Dissociation of the 2s 2A1´ , 2p 2A
2¨, 3s 2A
1´, and 3d 2E¨
Rydberg states of the H3 molecule is investigated using a fast neutral beam photofragment spectrometer.
A beam of 36 keV metastable H3 2p 2A
2¨ (N=K=0) molecules, generated by charge transfer neutralization of
H3+ 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 2A
1´ (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 2A
2¨ (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 2A
1´ (N=1, K=0) level, produced by stimulated emission pumping from the
2p 2A 2¨ (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 2A
2¨ (N=K=0) level is discussed. ©1996 American Institute of Physics.