Raman scattering measurements of molecular hydrogen in a low-density, arc-heated plasma

Raman scattering measurements of molecular hydrogen density and temperature were made in a low-density, are-heated plasma flow. Pulsed-laser excitatio n was used to improve the signal-to-noise ratio in this high-background env ironment. Quantum-limited detection was achieved through the use of gated p hoton counting and a high-power Nd:YAG laser. Radial profiles of rotational temperature and density at the exit plane were measured for five power lev els. In all cases the profiles were asymmetric about the centerline. The ro tational temperatures were compared with the translational temperatures of atomic hydrogen from LIF studies and found to be slightly lower. This resul t suggests that this flow, like the cold flow, is not in translational-rota tional equilibrium.