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

Abstract

Vibrational and total energy dependence of the reaction rate of Na with CO2 laserexcited SF6Collision induced multiple-photon absorption by SF 6 of cw CO 2 laser radiation [PI 16) 10.6pm, 8.5 W] has been carried out inside a transparent, capillary, nozzle beam source. Resonant laser excitation ofthe gas just upstream ofthe nozzle exit enhances the internal energy by 0.38 eV molecule -lover 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 e V molecule -I . 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" oflaser excited SF 6 increases (from Tnz = 290 K) to T yib = 680 K. TVib for the thermally heated SF 6 is always less than the nozzle temperature. The internal energies measured are lower than those predicted using a free-jet isenthalpic expansion equation.1490