Knowledge Bank

Double transitions $S1(J)+S0(J)$, corresponding to $\Delta J=+2$ and the vibrational-rotational transition in one molecule and a rotational transition in the other molecule of a colliding pair, both occuring simultaneously, and U branch transitions corresponding to $\Delta J=+4$ have been investigated in the infrared fundamental band of normal $H2$ at 77 K for gas densities in the range 100-520 amagat. The $S1(J)+S0(J)$ transitions arise because of the contribution of 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 relatively strong $S1(J)$ and $Q1(J)+S0(J)$ components of the $band.1$ The U branch transitions arise on account of the hexadecapolar induction mechanism in the colliding pairs of molecules and occur on the high-wavenumber wing of the $S1(1)+S0(1)$ transition. The experimental profiles were analyzed by assuming appropriate line-shape functions and using the available matrix elements of the quadrupole moment, isotropic polarizability, anisotropy of the polarizability and the hexadecapole moment of the $H2$ molecule. From the analysis of the profiles, the characteristic half-width parameters $\delta q$ and $\delta \ell$ of the quadrupolar and hexadecapolar components, respectively, and the binary and ternary absorption coefficients of the $S1(J)+S0(J)$ and U branch transitions have been obtained.