Infrared laser action on the spatial, velocity, and cluster-size distributions in an SF6 free jet

Abstract

SF6 in a free jet was vibrationally excited by a continuous-wave CO2 laser. The subsequent energy transfer processes, in which vibrational energy is degraded, lead to changes in the beam's spatial, velocity and cluster-size distributions. These changes were obsd. by mass-spectrometric techniques. The laser-induced perturbations of the free jet depend strongly on the location at which the laser beam is focused on the SF6 free jet. The largest perturbations were obsd. on irradn. close to the nozzle exit. Measurements were made for different stagnation pressures and temps., laser wavelengths and intensities. The main laser effects upon irradn. close to the nozzle are: (1) an increase in the mean velocity of the beam along the beam centroid, (2) a decrease in beam intensity measured near the center of the beam, and (3) an increase in the width of the velocity distribution (local temp.) of the beam. Effect 1 increases and effect 3 decrease the beam's Mach no., with the latter dominating and thus a net decrease results. These effects are enhanced with increasing stagnation pressure in the range 0.5-2 bar and with decreasing stagnation temp. in the range 198 <T0 <358 K. Comparison of the time-of-flight spectra of the SF6 monomer and dimer shows both a higher mean velocity and a lower local temp. for the latter. Measurements on the mol. beam at a wavelength where only 34SF6 absorbs indicate fast scrambling of vibrational energy between the 2 isotopomers. Cluster formation could be inhibited by irradiating the SF6 monomer in the collisional region of the beam with laser power densities in the order of a few kW-cm-2. The wavelength dependence of the laser action changes drastically with the position at which the 2 beams intersect and with the stagnation conditions. This is due to the IR absorption of the van der Waals clusters.