Photodarkening in glassy As2S3

Nuclear magnetic resonance (NMR) of 75As at 17 T has been employed to study photodarkening (the shift of the optical absorption edge to lower energies after excitation with light of energy near the optical band edge) in glassy As2S3. After irradiation at 514.5 nm for 230 h with 170 mW/cm2 the average asymmetry parameter of the electric field gradient (EFG) tensor increases from about 0.09±0.01 to about 0.12±0.01. This change is reversible on annealing at 200°C for 1.75 h. An increase in the asymmetry parameter implies an increase in the departure from cylindrical symmetry in the bonding at the arsenic pyramidal sites.