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

We propose a mechanism of photostructural changes in amorphous As2S3 (a-As2 S3) 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 coord inated As site having four As-S bonds and one As-As bond via a photoionizat ion process. This photoinduced coordination defect center exhibits a lower photoabsorption energy as compared with the usual pyramidal structure, expl aining the observed photodarkening effect of a-As2S3.