Origin of the Raman active 490 cm^-1 mode in binary As-S and Ge-S and ternary Ge-As-S glasses from compositional studies and Ab-initio Cluster calculations.

Raman scattering results on binary As-S and Ge-S and ternary Ge-As-S provide evidence of a mode near 490 cm^-1. The fractional scattering strength of the mode in question normalized to the sum of the S₈ (472cm^-1) and S_n chain (461cm^-1) mode reveals evidence of a local maximum near x = 25% in binary As_xS_100-x glasses and near y = 20% in binary Ge_yS_100-y glasses. These results are most suggestive that the mode in question has a frequency which is tied to the S-S stretch between a pair of AsS₃ pyramids or between a pair of GeS₄ tetrahedra. We have now carried forward NRLMOL calculations on select clusters and results of these will be compared to experiments. Preliminary results place the symmetric stretch mode frequency at 450 cm^-1 for a 14 atom cluster of (H-S-As)₂ -S-S- (As-S-H)₂.