The pressure-induced infrared absorption of hydrogen was studied in pure hydrogen and in hydrogen–helium, hydrogen–argon, and hydrogen–nitrogen mixtures at pressures up to 5000 atm. at room temperature. The integrated absorption coefficient can be expressed in the form α1ρaρp + α2ρaρp2 over the whole range of densities (ρa = density of H2, ρp = density of the perturbing gas, [Formula: see text] in the mixture experiments). The coefficient α2 is much smaller than predicted from the effect of finite molecular volumes; this is interpreted as a partial cancellation of the induced moments in ternary collisions. The splitting of the Q branch of the fundamental, which is due to the participation of the relative kinetic energies of the colliding molecules in the absorption process, increases linearly with the density because of ternary collisions; a more rapid increase observed at very high densities is not yet explained. The components of the overtone and double vibrational transition, like the QQ and S components of the fundamental, show no splitting or broadening with increasing density; these absorptions are believed to be due to quadrupole interactions while the QP and QR components of the fundamental are due to overlap interactions.
Infrared absorption of hydrogen-related defects in ammonothermal GaN