Using in situ synchrotron x-ray photoelectron spectroscopy, we obtained high-resolution $\text{As}3d$ and $\text{Se}3d$ spectra of Se-rich ${\text{As}}_{36}{\text{Se}}_{64}$ glassy films during illumination with band-gap light. We report direct experimental evidence of the chemical origin of the light-induced scalar permanent change in the structure and, more interestingly, an indication of the chemical origin of the light-induced polarization-dependent (vector) effects. Our data show that the As atom plays a key role in both the permanent scalar effect and the vector effect. The latter is explained by the polarization-dependent interaction of homopolar -As-As- bonds in ${\text{As}}_4{\text{Se}}_4$ clusters with the band-gap light. The chemical reaction resulting from illumination explains anisotropic properties such as permanent light-induced mass transport.

• Received 23 July 2008

DOI:https://doi.org/10.1103/PhysRevB.79.054204