Pump and probe X-ray absorption fine structure using high-brilliance photon sources.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid-State Chemistry

Background:

• X-ray absorption fine structure (XAFS) probes local atomic environments.
• Electronic excitations can induce structural changes in materials.
• Amorphous selenium exhibits reversible photostructural phenomena.

Purpose of the Study:

• To investigate the nature of photoinduced defect states in amorphous selenium.
• To elucidate the mechanisms behind reversible photostructural changes.
• To correlate electronic excitations with atomic rearrangements.

Main Methods:

• In situ pump and probe X-ray absorption fine structure (XAFS) experiments.
• Grazing-incidence fluorescence excitation technique.
• Optical excitation and X-ray probing during experiments.

Main Results:

• Identified the dominant photoinduced defect state as a pair of threefold neutral $C^0_3$ states ($2C^*_2 \rightarrow (C^0_3-C^0_3)$).
• Observed that optical pumping at low temperatures leads to locally over-coordinated defect pairs.
• Linked reversible photostructural changes to structural disorder from dynamical interchain bond formation/annihilation.

Conclusions:

• The formation of specific defect pairs drives photostructural changes in amorphous selenium.
• Optical melting and the creation of dynamical interchain bonds are key to reversible transformations.
• Findings provide insights into the local atomic rearrangements following electronic excitation in chalcogenides.