Experimental determination of rotational relaxation in molecular hydrogen and deuterium

In a newly constructed cryogenic shock tube made of stainless steel, the density in the relaxation field behind the incident shock has been studied interferometrically. The shock tube consists of a high-pressure section kept at room temperature and a thermally isolated low-pressure section at temperatures between 90 K and 300 K. The rotational relaxation time is quantitatively determined by means of a laser interferometer with a spatial and time resolution of 0.2 mm and 30 nsec, respectively. The interferometer is completely set up within the vacuum isolation tube surrounding the low-pressure section. The results are compared with those given by other authors. This comparison shows that for a critical interpretation of the here obtained results and those given in the literature the influence of shock curvature, boundary layer, and laser beam diameter has to be taken into account