Laboratory Study of Rate Coefficients for H2:H2 Inelastic Collisions between 295 and 20K

A laboratory study of state-to-state rate coefficients (STS rates) for H-2:H-2 inelastic collisions in the v = 0 state is reported. The study, which spans the 295-20 K thermal range, is based on the use of a kinetic master equation. It describes the time-space evolution of populations of H-2 rotational levels as induced by inelastic collisions. It is applied here to a supersonic jet of natural H-2. This medium bears a large amount of relevant data that allows for the establishment of best values and confidence margins for the dominant STS rates of H-2:H-2 inelastic collisions on an experimental basis. The primary experimental data derived from the supersonic jet are the local number density, the populations of the H-2 rotational levels, and their gradients along the jet by means of high-sensitivity Raman spectroscopy with superb space resolution. First, two sets of theoretical STS rates from the literature have been tested against the experiment. The set that shows a better agreement with the experiment has then been scaled to derive an improved set of experiment-scaled STS rates (ES rates). They allow the reproduction of more than 50 experimental population gradient data within a standard deviation <1.4% along the 295-20 K thermal range. The estimated uncertainty for the ES rates ranges from similar to 3% near 300 K to similar to 6% near 20 K. ES rates and uncertainties for H-2:H-2 ground-state inelastic collisions between 300 and 20 K are presented in machine-readable format. Other (incomplete) sets of theoretical rates from the literature are discussed.