Raman spectroscopy of hypersonic shock waves

Raman spectroscopy is shown to be an efficient diagnostic methodology for the study of hypersonic shock waves. As a test, absolute density and rotational population profiles have been measured across five representative normal shock waves of N2 generated in a free jet, spanning the Mach number range 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 contribution of scattered molecules, in close analogy with the velocity distribution function known from helium shock waves [G. Pham-Van-Diep et al., Science 245, 624 (1989)]. Quantitative data on invariance trends of density profiles and properties of the wake beyond the shock waves are reported.