ROTATIONAL ENERGY-TRANSFER ON A HOT SURFACE IN A LOW-PRESSURE FLOW STUDIED BY CARS

This paper reports in situ CARS (coherent anti-Stokes Raman scattering) rotational temperature measurements in a low-pressure flow. In order to understand the gas-phase temperature distribution in front of a hot surface, thermal exchanges are studied on different gas/surface systems: H-2/SiC, H-2/C, H-2/W, or N2/W. We observe a gap between the temperature of the hot surface and that of the nearby gas. This temperature gap is predicted using an accommodation coefficient which ranges from 0.15 for H-2 on a graphite surface to 0.05 on a SiC surface. The measurements show a quite different behavior with H-2 and N2. With the latter, a non-Boltzmann rotational population distribution is observed at a distance of one mean free path from the surface. Physisorption and chemisorption mechanisms are invoked to explain the results.