Temperature dependence of vibrational relaxation processes in sulfur crystal: Effect of isotopic impurities

We have studied phonon relaxation processes in orthorhombic sulfur crystals, for several lattice and internal vibrations. Bandwidths were measured as a function of temperature using a tandem Fabry-Perot spectrometer instrument with a limiting resolution of 0.001 cm-1. In order to obtain a basis for the study of the effect of isotopic impurities, we have grown single crystals of isotopically pure 32S. We have measured the bandwidths of six lattice phonons and eight internal vibrons. For most of the bands the width showed a linear variation with temperature in the classical regime, and in these cases we have interpreted the widths in terms of three-phonon decay processes. In some cases, however, a nonlinear temperature variation was observed, which we interpreted in terms of four-phonon and pure dephasing mechanisms. We have measured the widths of the same phonons and vibrons in single crystals of natural sulfur, which contains 4.2% 34S. The widths are in many instances larger when the impurity is present, and the increase is temperature-dependent. This difference is interpreted as being the sum of two contributions. One is temperature-independent, and reflects the simple scattering process, controlled by a harmonic term in the Hamiltonian, in which a phonon is dephased within its own dispersion curve. The second contribution is temperature-dependent, and is interpreted as being due to the same anharmonic processes observed in the pure crystal, scaled by a factor which takes into account the coupling of resonant scattering and anharmonic relaxation processes.