Structural study of amorphous selenium by in situ EXAFS: Observation of photoinduced bond alternation

Photostructural change in amorphous selenium has been studied in situ by extended x-ray-absorption fine structure at 30 K. The essential features of the amorphous state are in agreement with the previous reports; the average bond length is shortened and the second-nearest-neighbor peak is smeared out, suggesting strengthening of intrachain and weakening of interchain interactions. In amorphous selenium, however, the coordination number is shown to be higher than 2 (\ensuremath{\sim}2.2) which is attributed to the presence of static threefold-coordinated sites in concentration about 20%. For these samples, we observe light-induced increase of the average coordination number (by \ensuremath{\sim}5%) and disorder while the bond length remains unchanged. This change is ascribed to the local formation of additional dynamical threefold-coordinated sites which increases structural disorder. Light-induced change of the coordination is transient: the initial coordination is restored after switching off the light, while the light-induced structural disorder remains. A microscopic mechanism of the light-induced structural change is proposed based on these experimental data.