Mechanism of photoinduced changes in the structure and optical properties of amorphous As2S3

We propose a mechanism of photostructural changes in amorphous As2S3 ( a-As2S3) on the basis of ab initio molecular orbital calculations on clusters of atoms modeling the local structure of the amorphous system. We have found that trigonal AsS3 pyramidal units can be transformed into a fivefold coordinated As site having four As-S bonds and one As-As bond via a photoionization process. This photoinduced coordination defect center exhibits a lower photoabsorption energy as compared with the usual pyramidal structure, explaining the observed photodarkening effect of a-As2S3.