Quadrupolar S₁(J) + S₀(J)- and hexadecapolar U-transitions in H₂ at 77 K induced by intermolecular collisions

Abstract

Double transitions S₁(J) + S₀(J) corresponding to the vibration-rotation transition in one molecule and a rotational transition in the other molecule, occurring simultaneously in a colliding pair, have been observed in the infrared fundamental band of normal H₂ at 77 K for gas densities in the range 100-320 amagat with a 2 m absorption cell. These transitions arise because of the contribution to the intermolecular interaction by the anisotropic component of the polarizability of one molecule in the quadrupole field of the other and occur in the high-wavenumber tail of the S₁(J) and Q₁(J) + S₀(J) components of the band. The experimental profiles were analyzed by assuming appropriate line-shape functions and using the available matrix elements of the quadrupole moment, isotropic polarizability and anisotropy of the polarizability of the H₂ molecule. From this analysis the characteristic half-with parameter δq and the binary and ternary absorption coefficients of the S₁(J) + S₀(J) transitions have been obtained. -- U-branch transitions corresponding to ΔJ = +4 in the fundamental band of normal H₂ at 77 K for gas densities up to 520 amagat have also been investigated. These transitions arise on account of the hexadecapolar induction mechanism in the colliding pairs of molecules and occur on the high-wavenumber wing of the S₁(l) + S₀(l) transition. Although a preliminary report on the observation of the U-transitions was made by previous researchers, no analysis of the profiles was attempted prior to the present work. We have now carried out the profile analysis of these transitions by assuming a line shape similar to that of the quadrupolar lines of the band and using the recently available matrix elements of the hexadecapole moment of the H₂ molecule The characteristic half-width parameter δu and the binary and ternary absorption coefficients of the U branch transitions have been obtained from the analysis.