Rotational relaxation of molecular hydrogen

The rotational relaxation rate in molecular hydrogen has been determined experimentally between 300 and 1900°K by time of flight analysis of molecular beams from supersonic free jets. Over their common temperature range from 295 to 1073°K our results are in reasonable agreement with the ultrasonic measurements of Winter and Hill which show a rotational collision number Zr between 300 and 400. In terms of momentum transfer collisions Zr is relatively insensitive to temperature. This observation suggests that rotation-rotation transfers play an important role.