Vibrational relaxation measurements in an expanding flow using spontaneous Raman scattering

Vibrational relaxation of nitrogen in a two-dimensional nozzle flow is studied with spontaneous Raman scattering. An electric arc-driven shock tube operating as a reflected shock tunnel produces stagnation conditions of 5600 K and 100 atm. A 248-nm KrF laser pulse is focused into the nozzle to produce spatially resolved spontaneous Raman spectra. Vibrational population distributions are derived from the spectra for the states v = 0 to v = 8. The experimental results are compared with two theoretical models: (1) the Landau-Teller relaxation model and (2) a numerical solution of the master equations using transition rates derived from Schwartz, Slawsky and Herzfeld (SSH) theory. We have measured a value for the Landau-Teller correction factor (phi) to be 1.0-1.5. 13 refs.