Raman Scattering Measurements of Vibrational Relaxation in Expanding Nitrogen

Abstract

Spontaneous Raman scattering spectroscopy is used to study vibrational relaxation behavior in a nonequilibrium expansion. A sample of vibrationally excited, undissociated N₂ is produced in a reflected shock tunnel and rapidly expanded through a two-dimensional nozzle. The flow characteristics of the short-lived steady expansion are confirmed by comparing holographic interferometry measurements directly to 2-D flowfleld calculations. The vibrationally relaxing gas is probed on the axis of the nozzle by a single 250 mJ pulse from a 248 nm KrF excimer laser. The Raman scattered light is dispersed and detected by an intensified CCD array. Analysis of the Raman spectra shows that the relative populations of the first few vibrational levels do not deviate significantly from a Boltzmann distribution. Pointwise measurements of the vibrational energy are reported for corrected area expansion ratios of up to 5.5 for stagnation conditions of 102 atm, 2800 K and 102 atm, 5600 K. Good agreement is found between the measured vibrational state distributions and detailed numerical simulations based on the Landau-Teller model and on an anharmonic oscillator model. Relaxation times are deduced by comparison to the Landau-Teller solutions and found to be within the uncertainty of Millikan and White’s correlation for vibrational relaxation under shock heating conditions.