A calculation of the anharmonic phonon frequencies in solid deuterated naphthalene-d8

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Abstract
A detailed procedure leading to the evaluation of the explicit anharmonic contributions to the phonon frequency shift and the linewidth to the lowest order is described for a molecular crystal. An application is then made to obtain numerical results for phonon frequency shifts and the linewidths in deuterated naphthalene. The assumptions are made that the molecules behave as rigid units and the intermolecular potential is a pairwise sum of the atom-atom potentials. Using a 6-exp potential, and the potential parameters provided by Kitaigorodski (1973), the results for all the external modes at the Gamma points (q=0) are presented and compared with the available experimental data. Keeping in view the inadequacy of many of the measured results, and the fact that there is only a fair agreement between the measured and calculated harmonic phonon frequencies with the chosen potential, the results from the calculation presented here for phonon shifts and linewidths are very encouraging. An attempt made to compare the results from the three sets of potential parameters-those of Kitaigorodski, Mackenzie et al. (1977) and Williams (1966, 1967)-reveals the superiority of Williams's parameters.