Collective Effects in the Optical Absorption of Diatomic Molecules in Simple Liquids

We develop a theory of optical absorption of a diatomic molecule in a simple monatomic liquid. Using approximations appropriate to the case of a light molecule like ${\mathrm{H}}_2$, we show that the absorption spectrum near an infrared-inactive vibrational transition of the molecule can be interpreted to give information about collective effects in the fluid. We show that some features of existing experimental results on ${\mathrm{H}}_2$-Ar mixtures are consistent with the theory, although no direct evidence of collective effects is seen in these experiments.