Laser pumping of SF6 in the collisional region of a nozzle beam: Bolometric detection of internal excitation

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Abstract
The influence of low-power CO2 laser radiation upon the internal and translational energy of a supersonic SF6 molecular beam has been investigated. A line-tunable cw (chopped) laser beam, approximately 7 W, is used to irradiate the gas in the expansion region at the exit of the nozzle. Time-of-flight analysis of the skimmed SF6 beam has been carried out using an electron bombardment ionizer–mass filter for monitoring number density. A pumped, liquid He-cooled (1.8 K) bolometer was used to measure laser-induced changes in total energy flux (power) of the incident beam. Vibration–translation/rotation relaxation converts some of the absorbed infrared (IR) energy into translational kinetic energy, as manifested by an increase in both transverse and axial velocity of the SF6 molecules. A significant loss of forward beam flux due to the enhanced radial velocity component is observed. From data on the number density of the SF6 beam, its velocity distribution and the power delivered to the bolometer, the laser-induced enhancement in internal energy per SF6 molecule can be determined. A significant fraction (90%) of the absorbed energy is found to be ’’frozen’’ in the internal degrees of freedom of SF6. Under typical conditions, there is a 15% laser-induced increase in total energy content of the transmitted beam molecules, corresponding to a net energy deposition of 0.22 IR photons per SF6 molecule in the beam.