CALCULATION OF THE STRUCTURE OF THE S1(0) AND S1(0)+S1(0) LINES IN THE INFRARED SPECTRUM OF SOLID HYDROGEN

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Abstract
At low ortho concentrations, the satellites appearing in the S1(0) infrared absorption feature of solid hydrogen arise from transitions in para molecules possessing one nearest-neighboring ortho molecule. The 15-fold degeneracy of the combination of the v = 1, J = 2 excited state of the para molecule and the v = 0, J = 1 ground state of the ortho molecule is partially lifted by the quadrupole–quadrupole interaction between the two molecules. The calculated structure of the S1(0) line is in good over-all agreement with the observed profile. The agreement is improved by taking into account the quadrupole–hexadecapole interaction. The best fit is obtained for a positive value of the hexadecapole moment of about 1.2 atomic units. The fine structure of the S1(0)+S1(0) line in the overtone region of pure parahydrogen, resulting from the splitting of the 25-fold degeneracy of the upper state by the combined quadrupole–quadrupole and quadrupole–hexadecapole interaction, is also calculated. The splitting and relative intensity of the two components of the resulting doublet structure are in close agreement with the experimental values. All the calculations are performed assuming a rigid close-packed hexagonal lattice structure. The discrepancy of about 20% between the theoretical values of the quadrupole moment of a hydrogen molecule in the v = 0 and v = 1 states and the empirical values deduced from the analysis of the S1(0) and S1(0)+S1(0) lines may be due to the neglect of the splittings arising from the coupling between the rotational motions of the molecules and the lattice vibrations.