Correlation of low-energy excitations with photodarkening in a − As₂S₃

Low-temperature sound velocity and absorption of differently treated amorphous As₂S₃ films have been measured around 150 MHz using surface acoustic waves. The data show the typical behaviour of glasses governed by low-energy excitations. Illumination of the films at 5 K using photon energies equal to and higher than the band gap increases the absorption and temperature dependence of the sound velocity by creation of defects. Between 325 nm and 488 nm the change does not depend on the wavelength but on the number of absorbed photons. The initial state is only reset after annealing at the glass transition temperature. The photoinduced change of the sound velocity data suggests that upon illumination quasiharmonic oscillators are transformed into two-level systems on average. The strong correlation of the photoinduced changes with photodarkening favours the twisting of adjacent AsS₃ pyramids as microscopic model for the low-energy excitations.