Nascent vibrational populations in He*(2 1,3S)+H2, HD, D2 Penning ionization from electron spectroscopy in crossed supersonic molecular beams

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Abstract
Penning ionization electron spectroscopy (PIES) in crossed supersonic molecular beams has been used to measure nascent H+2, HD+, and D+2 vibrational populations, line shifts, and their collision energy dependence in the case of H2 (E=1.6, 2.1, and 2.6 kcal/mol), in the title systems. The use of optical spin–state selection and beam modulation enabled estimates of vibrational populations to be obtained out to the dissociation limit, although the low electron energy resolution (90 meV) employed and low counting rates allowed typically only 12 vibrational states for 1S, 9 for 3S, to be resolved. With a few mild exceptions, the following properties were found. The vibrational populations, which are characteristic of the Penning ionization transition state rather than the asymptotic final state, follow Franck–Condon behavior quite closely, more so than do He i photoionization populations, independent of spin and isotope, and nearly independent of E in the range examined. As expected from the E independence, the H2 results compare well with and extend earlier Maxwellian beam PIES populations; isotopomeric populations have not been reported previously. All vibrational peaks are blueshifted (to higher electron energy) by a roughly constant amount for a given E, and the blueshift grows with increasing E. The results suggest that ionization occurs largely on the repulsive part of the incoming potential energy surface, but with little coupling between the translational and vibrational coordinates. A spherical‐potential model based on previous experimental estimates of incoming potential surfaces, combined with a Franck–Condon distribution of the reaction flux into product vibration, produces simulated spectra that compare well with experiment both as to intensity envelope and blueshift.