Effects In Lattice Dynamics Research Articles

Lattice dynamical effects in the frequency analysis of the infrared and Raman spectra of solid parahydrogen

The frequency perturbations of the rotation–vibration levels in solid H2, due to the modulation of the anisotropic and vibrational intermolecular interaction by the lattice vibrations, are calculated. The lattice vibrations are treated in the self-consistent harmonic (SCH) approximation with short-range correlations, and the intermolecular interaction is expanded in terms of the quantum crystal phonon operators. The discrepancy between previous calculations of the effective quadrupolar coupling constant and the experimental values is resolved when the roton–phonon interaction is properly renormalized to take into account the long-range phonon correlations. The energy of the induced dipoles in the quadrupolar field of all the H2 molecules in the crystal (dielectric screening energy) is considered, and in particular the effect of this induction energy on the roton band energies is calculated. The frequencies of the infrared and Raman S0(0) lines are analyzed, taking into account the frequency shifts due to the roton–phonon interaction and the dielectric screening energy, and a value of the effective quadrupolar coupling constant which is consistent with other experimental values is derived. New values of the crystalline field constants ε2C and ε4C are given. The shift and splitting of the S1(0) Raman line is recalculated, taking into account the interaction with the lattice vibrations, and the splitting of the S1(0) + S1(0) doublet in the overtone infrared spectrum is calculated using a different perturbation scheme from the previous calculations. The limitations on the frequency analysis due to the uncertainty in the roton–phonon coupling parameters are discussed.