Raman spectroscopy of hypersonic shock waves

Raman spectroscopy is shown to be an efficient diagnostic methodology for t he study of hypersonic shock waves. As a test, absolute density and rotatio nal population profiles have been measured across five representative norma l shock waves of N-2 generated in a free jet, spanning the Mach number rang e 7.7<M<15.3. The interconversion of three differentiated populations (cold , scattered, and rethermalized molecules) across these shock waves shows a largely bimodal rotational distribution function with additional contributi on of scattered molecules, in close analogy with the velocity distribution function known from helium shock waves [G. Pham-Van-Diep er at, Science 245 , 624 (1989)]. Quantitative data on invariance trends of density profiles a nd properties of the wake beyond the shock waves are reported.