Photodarkening in glassy As2S3

Nuclear magnetic resonance (NMR) of (75)AS at 17 T has been employed to stu dy photodarkening (the shift of the optical absorption edge to lower energi es after excitation with light of energy near the optical band edge) in gla ssy As2S3. After irradiation at 514.5 nm for 230 h with 170 mW/cm(2) the av erage asymmetry parameter of the electric field gradient (EFG) tensor incre ases from about 0.09 +/- 0.01 to about 0.12 +/- 0.01. This change is revers ible on annealing at 200 degrees C for 1.75 h. An increase in the asymmetry parameter implies an increase in the departure from cylindrical symmetry i n the bonding at the arsenic pyramidal sites. (C) 2000 Elsevier Science B.V . All rights reserved.