Electron diffraction studies of laser-pumped molecules. II. Collisionally assisted absorption by SF6

Abstract

Supersonic microjets of SF6 were pumped with the cw output of a CO2 laser. Power densities ranged from 2–4 kW/cm2 at the nozzle exit (P?200 Torr) to 25 kW/cm2 downstream. Amplitudes of molecular vibration were found to increase significantly when the laser was tuned to the ν3 resonance and even a slight expansion of bond lengths was observed. Up to two photons per molecule were absorbed at the higher nozzle pressures. At such pressures the maximum response was red shifted from that of the 300 K optical absorption band. Electron diffraction patterns taken a microsecond after excitation showed virtually complete V–V relaxation. Because little V–T relaxation took place in the jet expansion, the translational temperature downstream fell to 20 K while vibrational temperatures remained hot, at perhaps 500 K. The correspondence between the pumping of supersonic jets and the irradiation of static systems is briefly discussed.