Temperature dependence of the vibrational relaxation processes in natural and isotopically pure S-32(8): Effect of the isotopic impurities on infrared phonon lifetimes

We have measured the temperature dependence of the infrared bandwidth of several lattice phonons and internal vibrons in natural and S-32 isotopically pure orthorhombic sulfur crystals, using a high resolution FTIR interferometer. The experimental data were analyzed in terms of anharmonic phonon-phonon coupling processes and of contributions to the bandwidth due to the presence of randomly distributed isotopic impurities. The same three-phonon processes that contribute to the lifetime of the Raman bands were found to be active also for the infrared ones in the isotopically pure crystal. In parallel to the increase of the two-phonon density of states, the third-order anharmonic coupling coefficients for phonon decay processes were found to increase from the lowest to the highest lattice mode. The effect of the isotopic impurities was explained in terms of two contributions, the first, which is temperature independent, due to pure harmonic scattering processes and the second due to the combined effect of anharmonicity and scattering processes, which is instead dependent on the temperature. The contribution due to the presence of impurities was found to dominate the width of the lattice modes in the natural crystal at low temperatures. This purely harmonic contribution can be in some cases almost one order of magnitude larger than that due to the normal anharmonic decay processes in the isotopically pure crystal. Only the width of the factor group component of infrared and Raman vibrons close to the frequency of the isolated molecule was found to be affected by the presence of impurities through a resonant scattering mechanism. (C) 1997 American Institute of Physics.