Abstract
The thermoreflectance spectrum of LiF between 12 and 30 eV was measured and several of the structures interpreted. The absorption-edge region is interpreted in terms of a Wannier exciton series converging to the fundamental band gap . Structure associated directly with the band gap is not manifest, so the energy is determined indirectly to be 14.2 ± 0.2 eV. The exciton state generates the first strong structure in and we suggest that the exciton-phonon interaction, along with a central-cell correction, can give a significant contribution to its binding energy. Structures at higher energy have been associated with the interband transitions and between the crystal-field-split valence band at and the lower conduction band. The strong electron-hole interaction modifies the expected line shape and a hyperbolic exciton, associated with the transitions at , may exist as an antiresonance in the continuum. A strong feature at 22.2 eV in is associated with excitonic transitions at involving the second -like conduction band. The corresponding peak at 26.4 eV in overlaps the "valence-band" plasmon at 24.6 eV. No evidence for double excitations is found around 25 eV in either or . The spectrum shows for the first time which structures in the energy-loss function are generated by longitudinal excitons and which by plasmons.
- Received 12 January 1976
DOI:https://doi.org/10.1103/PhysRevB.13.5530
©1976 American Physical Society