Thermoreflectance of LiF between 12 and 30 eV
Is Version Of
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 Γ15→Γ1. Structure associated directly with the band gap is not manifest, so the Γ15−Γ1 energy is determined indirectly to be 14.2 ± 0.2 eV. The n=1 exciton state generates the first strong structure in Δε̃ 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 L3′→L1 and L2′→L1 between the crystal-field-split valence band at L and the lower conduction band. The strong electron-hole interaction modifies the expected line shape and a hyperbolic exciton, associated with the transitions at L, may exist as an antiresonance in the continuum. A strong feature at 22.2 eV in Δε̃ is associated with excitonic transitions at X involving the second d-like conduction band. The corresponding peak at 26.4 eV in Δ[Im(−1ε̃)] overlaps the "valence-band" plasmon at 24.6 eV. No evidence for double excitations is found around 25 eV in either Δε̃ or Δ[Im(−1ε̃)]. The Δ[Im(−1ε̃)] spectrum shows for the first time which structures in the energy-loss function are generated by longitudinal excitons and which by plasmons.
This article is from Physical Review B 13 (1976): 5530, doi:10.1103/PhysRevB.13.5530. Posted with permission.