Vibrational excitation of HF molecules in a nozzle beam by a cw HF laser in an antiparallel coaxial arrangement

Abstract

We have demonstrated that a significant fraction (≳14%) of HF molecules in a given rotational state (j = 2) in a nozzle beam can be excited by a cw HF laser beam directed antiparallel to the molecular beam. The excitation, which is thought to occur near the nozzle orifice, was detected indirectly by observing the decrease in the signal on the mass 21 (H2F+) peak due to (HF)2 dimers in a mass spectrometer viewing the beam. The observed dependence of this effect on the laser lines and the measured dimer spectrum reported in this paper both indicate that the excitation of the monomer is the primary process. An indirect heating of the gas by contact with the laser-heated orifice walls could be ruled out by experiments using the v = 2→1 laser lines, which showed a much smaller effect. Thus, the results are consistent with a three-step mechanism: (1) vibrational laser excitation of the monomers, (2) V–V transfer to the dimers, and (3) predissociation of the metastable dimers. Calculations based on this model and a simplified description of the nozzle beam when fitted to the data provide information on the fraction excited as well as on the efficiency of V–V transfer.