KINETIC X-RAY ABSORPTION STUDIES AND COMPUTER STRUCTURAL MODELING OF PHOTODARKENING IN AMORPHOUS ARSENIC SULFIDE

Under linearly polarized light, darkening of amorphous arsenic sulfide (a-As2S3) shows an anisotropic shift in the optical absorption edge. Kinetics and structural entities associated with these optical anisotropic effects are investigated in a polarized X-ray absorption spectroscopy study. Through a comparison of experimental observations of optical kinetics and near edge X-ray absorption, the effective structural range of photo-darkening and the local ordering around AsS3 pyramidal units involved with photo-darkening are characterized. Specific structural sites involved in photo-darkening are modelled. Structural ordering and stability are discussed in terms of these specific sites. Photo-darkening is treated as a rapid thermalization mechanism involving the structural distortions which follows optical excitation. These modelling results suggest that a helical S environment is a more likely candidated to be the anisotropic center induced by incident polarized radiation than a bridging S environment. © 1990.