PUMP AND PROBE X-RAY-ABSORPTION FINE-STRUCTURE USING HIGH-BRILLIANCE PHOTON SOURCES

Pump and probe X-ray absorption fine structure (XAFS) is used as a loc al probe of excited atoms, which can provide direct information on lat tice distortion, relaxation and atomic rearrangements associated with electronic excitations. In situ XAFS experiments during optical excita tion are reported. Utilizing a grazing-incidence fluorescence excitati on, which minimizes the mismatch between the probing depths of X-ray e xcitation and optical pumping, it is found that the dominant photoindu ced defect state in amorphous selenium at low temperature is a pair of threefold neutral C-3(0) states: 2C(2) --> (C-3(0) - C-3(0)), where C-2 denotes the twofold excited lone-pair state. The results indicate that optical pumping of chalcogen atoms at low temperature leads to l ocally over-coordinated defect pairs similar to those in the liquid st ate. The origin of reversible photostructural changes, such as photoda rkening or photoinduced fluidity, is attributed to the structural diso rder caused by the formation and annihilation of dynamical interchain bonds during optical melting.