X-ray spectroscopy of thermally distorted electronic states in crystals

A new type of x-ray spectroscopy is proposed which can detect the thermal-m otion-induced distortions of atomic electronic states in crystals. It is sh own that those distortions can cause extra Bragg reflections (so-called for bidden reflections) and that their intensity should grow with increasing te mperature. The reason is that the thermal displacements, which change the s ymmetry of atomic environment, can modify the tensor amplitude of x-ray res onant scattering. In the first approximation, the structure factor of extra reflections is proportional to the reflection vector H and to the mean-squ are thermal displacement <(u(j)u(k))over bar> for optical phonons. It is de monstrated that the forbidden resonant reflections, observed recently in Ge , could be caused by the thermal motion. (C) 1999 American Institute of Phy sics. [S0021-3640(99)01012-9].