Laser induced fluorescence study of first part of a Na/Na 2 free jet expansion: dimer formation and excitation-energy transfer

Abstract

The very first part of a Na/Na 2 free jet expansion (0 to 1.5 nozzle diameters, D is studied by means of laser induced fluorescence. Fluorescence intensifies originating from various (υ, J) states of the Na 2 molecules are measured as functions of the distance y from the orifice along the axis of the expansion. By means of these measurements the evolution of the total number of dimers in the course of the expansion is studied. This is done for expansions with stagnation conditions ranging from p oD = 0.5 to 10 Torr mm. From the dependence of the results on p o we find that for the highest densities a radiation quenching mechanism becomes important for the interpretation of the data. It consists of a collisional transfer of the electronic excitation energy from the dimer to an atom, followed by the trapping of the subsequent atom radiation. For the energy transfer cross section we obtain a value of 80 ± 20 A 2. About the dimer formation we find that throughout the expansion the relative increase of the number of dimers is proportional to p oD . The major part of the dimerisation occurs just outside nozzle in the region 0–0.7 D. The dependence of the formation on y can be described by a universal function ϕ( y) for which an empirical form is deduced from the data. The total net increase in dimer mole fraction is found to be 4.5%/Torr mm.