Rotational relaxation in supersonic beams of hydrogen by high resolution photoelectron spectroscopy

The rotational relaxation of n‐H2, p‐H2, HD, and n‐D2 in a free jet expansion was studied by means of rotationally resolved photoelectron spectroscopy using a collimated supersonic molecular beam. Rotational state distributions were determined from the relative intensities of the Q branch rotational components for a wide range of stagnation pressures with nozzle temperatures from 300 to 700 K. Significant deviations from a Boltzmann distribution were observed for those cases in which the degree of rotational relaxation was substantial. HD was found to relax after undergoing only one‐tenth of the number of collisions required to relax H2 or D2. The state‐to‐state rate constants method of Rabitz and Lam was used to model the relaxation of p‐H2 and was confirmed to be quite effective in accounting for the experimentally observed population distributions.