Temperature Dependence of the Raman Spectrum and the Depolarization Spectrum of AmorphousAs2S3

The polarized Stokes and anti-Stokes Raman spectrum of amorphous ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ has been measured at several temperatures in the range 20-448 °K with a He-Ne laser, and at room temperature with a c.w. dye laser. Using the Shuker-Gammon data-reduction method, the approximate density of vibrational states has been determined from the Raman spectra and has been found to be temperature independent. A new type of spectrum, the depolarization spectrum, has been measured for amorphous ${\mathrm{As}}_2$ ${\mathrm{S}}_3$. The depolarization spectrum has been defined to be the dispersion of the Raman depolarization ratio. The measured Raman and depolarization spectra have been used to test the validity of the two structural models of vitreous ${\mathrm{As}}_2$ ${\mathrm{S}}_3$ proposed to date, namely: the planar-random-network model of Bermudez and the molecular model of Lucovsky and Martin. It has been shown that both models can be used to calculate Raman spectra in reasonable agreement with experiment while only the molecular model is compatible with the observed depolarization spectrum. Further, it has been demonstrated that the depolarization spectrum, not the Raman spectrum, provides a test of the applicability of the Shuker—Gammon data-reduction technique to a given amorphous solid. The low-temperature ($\sim 20$ °K) Raman and room-temperature infrared transmission spectra of orpiment, the crystalline form of ${\mathrm{As}}_2$ ${\mathrm{S}}_3$, have been measured. Several lines unresolved in previously reported room-temperature Raman spectra of orpiment have been resolved in the low-temperature spectra. The envelope of the approximate density of vibrational states of orpiment has been found to resemble remarkably the approximate density of vibrational states of amorphous ${\mathrm{As}}_2$ ${\mathrm{S}}_3$. This resemblance has been explained using the molecular model.