Phonon dynamics and relaxation processes in isotopically pure 35Cl2 and natural crystalline chlorine

We have measured the Raman spectrum of natural and 99% isotope 35 chlorine crystal, in the lattice and in the internal mode regions, using a high resolution Raman spectrometer. We report the temperature dependence of the linewidth for both the lattice and the internal vibrational modes. The data were analyzed in terms of anharmonic interactions and contributions from the isotopic disorder. The lattice phonons are little affected by the isotopic disorder, while the internal vibrons are extremely sensitive to the presence of isotopic impurities. In the latter case the linewidths of the A g and B 3g internal vibrons are about one and two orders of magnitude, respectively, larger in the natural than in the isotopically pure sample. The Raman spectrum of natural chlorine in the internal region is correctly reproduced by an harmonic lattice dynamics calculation weighted over the statistical distribution of the isotopic species in the lattice.