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 local probe of excited atoms, which can provide direct information on lattice distortion, relaxation and atomic rearrangements associated with electronic excitations. In situ XAFS experiments during optical excitation are reported. Utilizing a grazing-incidence fluorescence excitation, which minimizes the mismatch between the probing depths of X-ray excitation and optical pumping, it is found that the dominant photoinduced defect state in amorphous selenium at low temperature is a pair of threefold neutral $C^0_3$ states: $2C^*_2 \rightarrow (C^0_3-C^0_3)$, where $C^*_2$ denotes the twofold excited lone-pair state. The results indicate that optical pumping of chalcogen atoms at low temperature leads to locally over-coordinated defect pairs similar to those in the liquid state. The origin of reversible photostructural changes, such as photodarkening or photoinduced fluidity, is attributed to the structural disorder caused by the formation and annihilation of dynamical interchain bonds during optical melting.