  Lead sulfide is an important semiconductor that has found technological
applications for over a century. Raman spectroscopy, a standard tool for the
investigation and characterization of semiconductors, has limited application
to this material because of the forbidden nature of its first order scattering
and its opacity to visible lasers. Nevertheless, useful vibrational spectra
from two-phonon processes are obtained with red lasers, probably because of a
resonance in the concomitant electronic transitions. Here we report temperature
dependent spectra, covering the 10-300 K range, for two samples with different
sulfur isotopic compositions. The results are analyzed by comparison with ab
initio calculations of the lattice dynamics of PbS and the corresponding
densities of one and two-phonon states. Emphasis is placed on the analysis of
the two phonon band centered at ~430 cm-1.
