Cross sections for translational and rotational relaxation in Li/Li 2 and Na/Na 2 molecular beams

Abstract

Translational and rotational relaxation processes in alkali monomer-dimer free-jet expansions are investigated both theoretically and experimentally. Based on a sudden-freeze model, simple formulas are derived for the extraction of effective translational relaxation cross sections from terminal Mach numbers and effective rotational relaxation cross sections from measured terminal rotational populations. These formulas are verified in a state-resolved laser-molecular beam study. The magnitude of the rotational cross section is found to be strongly state dependent and to be essentially the same for Na 2 and Li 2. The terminal dimer velocity exceeds that for the monomers in Na/Na 2 but not in Li/Li 2. An earlier sudden-freeze analysis for rare gases is used to correlate terminal dimer mole fractions measured for Li/Li 2 beams with dimensionless parameters. From this correlation and applying energy conservation to the measured terminal velocities we found that at least 60% of the binding energy released in dimer formation in Li/Li 2 beams resides terminally in internal degrees of freedom.