Transient CARS spectroscopy of rotational transitions in H2: the statistical dependences of the Doppler and collision dephasing

The S₀(0) and S₀(1) rotational transitions in the H₂ molecules are studied by the method of transient CARS spectroscopy at temperatures 296 and 80 K in a broad range of gas densities – from the density at which Doppler dephasing occurs to the density corresponding to collision dephasing. The slowest decay of the responses of the scattered probe pulse at the molar gas density ~0.2 amagat corresponds to the Dicke line narrowing. It is shown that the dephasing of the S₀(1) transition in the natural H₂ mixture leads to a substantial discrepancy between the experimental results and the model of statistically independent perturbations of the rotational and translational motion of molecules at densities within a few tenths of amagat. This discrepancy becomes quite significant at liquid nitrogen temperature. At the same time, the dephasing of the S₀(0) transition better corresponds to this model, as well as the dephasing of the S₀(1) transition in the buffer He gas. The deviation from the model of statistically independent perturbations is described by introducing the correlation parameter. It is assumed that the features of the dephasing of the S₀(1) transition observed in the natural H₂ mixture are mainly caused by resonance dephasing collisions.