The infrared fundamental band of liquid and solid hydrogen was investigated over a range of para-concentrations from 25% to 100% with a prism spectrometer and, in part, with a grating spectrometer at a resolution of ~0.2 cm−1. The spectrum of the solid shows (a) comparatively sharp Q, S(0), and S(1) lines due to quadrupolar interaction, (b) broad bands interpreted as combination tones of the molecular frequencies with the lattice frequencies (phonon spectra), and (c) weak double transitions of the type S1(0) + S0(0). At high resolution the quadrupolar S(0) and S(1) groups show weak single transitions, S1(0) and S1(1), and much stronger double transitions of the type Q1(J) + S0(J), J = 0,1. In solid parahydrogen the lines become very sharp and the double transition, Q1(0) + S0(0), shows a complex structure; the observations are in good agreement with the theory of the rotational and vibrational levels of solid parahydrogen by Van Kranendonk. The quadrupolar Q branch shows a structure which is interpreted as double transitions of the type Q ± δi where the δi are the small changes in energy due to the orientational transitions of two ortho-molecules. The phonon spectra show a maximum at the Debye temperature of the solid, and, at higher resolution, a structure indicative of the various branches of the lattice frequencies. A long extension of the phonon spectrum towards high frequencies is probably due to multiple phonon creation. Double transitions, for which the cancellation principle in induced absorption does not apply, account for at least 98% of the intensity of the spectrum of the solid.

The Raman spectra of liquid (~18° K) and solid (~2° K) n-H2, p-H2, n-D2, o-D2(80%), and HD were photographed with a reciprocal linear dispersion of 3 to 6 cm−1 per mm. The S0 rotational lines show broadening of a few cm−1 but the Q1 vibrational lines are very sharp. The S0(0) transition of p-H2 and o-D2 is a triplet of sharp lines, but the corresponding transition in HD is not split. The vibrational frequencies in the liquid are lowered by 7 to 9 cm−1 and in the solid by 8 to 11 cm−1 from the gas values. The Raman spectrum of p-H2 has been discussed in detail by Van Kranendonk. In the present communication the vibrational shifts in the various solids are correlated by representing them as the sums of shifts due to dispersion forces, overlap forces, and vibrational coupling.

The Raman spectrum of hydrogen as a pure gas and in mixtures with helium and argon was studied in the pressure range 100 to 2000 atmospheres at room temperature. Frequency shifts and half-widths of the vibrational Q1 lines and the rotational S0(0) and S0(1) lines were measured at a series of gas densities with a high-dispersion grating spectrograph. The perturbations in ωe and B0 are represented as power series in the density with linear and quadratic terms. The linear term for Δω0 is positive for H2–He, slightly negative for H2, and more negative for H2–A. These results are discussed in terms of an anharmonic oscillator model for the H2 molecule, a polarizability model for the dispersion forces, and the classical pair distribution function. The rotational lines show additional perturbations which are not accounted for in this treatment.

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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.