Electronic and rotational energy relaxation in molecular helium: Observation of reactive collisions by isotope-selective laser excitation

We have shown that reactive collisions play a substantial role in the rotational energy relaxation of He2(e3Pi/sub g/) molecules: about 16% of the total energy transfer from a given rotational level occurs through the exchange of helium nuclei. These observations may be relevant to problems of astrophysical interest, as well as to proposed schemes of laser isotope separation in more complex molecules where similar processes may lead to fast excitation exchange with atoms or molecules of undesirable isotopes. Radiative and collisional relaxation rates for the He2(e3Pi/sub g/) molecule are derived from the analysis of a large set of experimental data; rotational redistribution is well described by the simple Polanyi--Woodall exponential gap law. Some new results concerning the dynamics of the d3 Σ+/sub u/, f 3Σ+/sub u/, and f 3Pi/sub u/ electronic levels are also reported