The hydrogen molecule as a collision partner

The properties of H2 as a collision partner are summarized in light of the predominance of H2 collisions in interstellar molecular clouds. Results of experiments on NH3-H2 collisions are discussed in terms of a model of the H2 molecule as a hard core surrounded by a soft outer shell, which is confirmed by four-level double resonance experiments in which a pump microwave radiation field is used to saturate an inversion transition which is collisionally transfered to other levels and monitored by a second radiation field. Examination of the pressure broadening of spectral lines, which can be used to distinguish collisions with a H2 molecule in the ground rotational level, which acts as if it did not have a shell, from those with rotationally excited H2 reveals that the outer shell is 60% as effective as the inner shell in causing rotation-inversion transitions, although the outer shell may dominate in causing dipole-type transitions. Astrophysical implications of molecular collisions with ground-state H2, which is likely to cause population anomalies, and with rotationally excited H2, which act to reduce such anomalies, are pointed out.