TEMPERATURE-DEPENDENCE OF VIBRATIONAL-RELAXATION PROCESSES IN SULFUR CRYSTALS - 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 S-32. 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% S-34 . 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.