Abstract
The anharmonic frequencies and the bandwidths of the lattice phonons of naphthalene at 4 K have been calculated using an intermolecular potential which includes atom-atom and quadrupole-quadrupole contributions. The calculated bandwidths agree well with the available experimental data. The contribution of the cubic and quartic terms of the crystal hamiltonian to the anharmonic shifts is discussed in terms of phonon diagrams. The calculated quartic shifts are found to be positive whereas the cubic shifts are smaller and negative. The total anharmonic shifts are then positive and it is shown that this is in agreement with the observed dependence of the phonon frequencies with temperature. The mechanism of energy transfer between the optical lattice phonons and the two-phonon manifold of the crystal is discussed in terms of phonon-phonon coupling processes. Up and down conversions are analyzed and their relative efficiency is evaluated. The temperature dependence of both the anharmonic shifts and bandwidths is studied in the range 4–68 K at constant volume. Comparison with the available experimental data at constant pressure is discussed.
Published Version
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