Thermoreflectance of LiF between 12 and 30 eV

Abstract

<p>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.</p>