Rotranslational state-to-state rates and spectral representation of inelastic collisions in low-temperature molecular hydrogen

Inelastic collisions in natural H-2 are studied from the experimental and theoretical points of view between 10 and 140 K. Rotational populations and number densities measured by Raman spectroscopy along supersonic expansions of H-2 provide the link between experimental and theoretical rotranslational state-to-state rate coefficients of H-2 in the vibrational ground state. These rates are calculated in the close- scattering approach with the MOLSCAT code employing a recent ab initio H-2-H-2 potential. The calculated rates are assessed by means of a master equation describing the time evolution of the experimental rotational populations. The feasibility for obtaining the rates on the sole basis of the experiment is discussed. The dominant processes j(1)j(2)-> j(1)(')j(2)(') in the investigated thermal range are found to be 21 -> 01 > 30 -> 12 > 31 -> 11, proving the importance of double processes such as 30 -> 12. Good agreement is found between theory and experiment, as well as with earlier ultrasonic measurements of relaxation times. A spectral representation is proposed in order to visualize quantitatively the collisional contributions in any nonequilibrium time evolving process. (c) 2006 American Institute of Physics.