Collisional effects on the internal and translational energy distributions of laser-excited and thermally heated SF6 supersonic beams

Abstract

Collision induced multiple-photon absorption by SF6 of cw CO2 laser radition [P(16) 10.6 μm, 8.5 W] has been carried out inside a transparent, capillary, nozzle beam source. Resonant laser excitation of the gas just upstream of the nozzle exit enhances the internal energy by 0.38 eV molecule−1 over its value at Tnz=290 K. In comparison, simple thermal heating of the nozzle to 544 K, which provides a similar velocity distribution in the supersonic beam to that found with laser irradiation, produces a beam enhanced in internal energy by only 0.03 eV molecule−1. Energies were determined using bolometer, mass filter (or flux meter), and time-of-flight measurements. Laser excitation in the nozzle induces high vibrational temperatures in the beam which cannot be achieved by simple thermal heating of the nozzle. For an assumed Boltzmann distribution of vibrational energy, the ‘‘vibrational temperature’’ of laser excited SF6 increases (from Tnz=290 K) to Tvib=680 K. Tvib for the thermally heated SF6 is always less than the nozzle temperature. The internal energies measured are lower than those predicted using a free-jet isenthalpic expansion equation.