Thermoreflectance of LiF between 12 and 30 eV

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 Δ ̃ \~ε 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 Δ ̃ \~ε is associated with excitonic transitions at X involving the second d-like conduction band. The corresponding peak at 26.4 eV in Δ[Im(-1 ̃ \~ε)] overlaps the "valence-band" plasmon at 24.6 eV. No evidence for double excitations is found around 25 eV in either Δ ̃ \~ε or Δ[Im(-1 ̃ \~ε)]. The Δ[Im(-1 ̃ \~ε)] spectrum shows for the first time which structures in the energy-loss function are generated by longitudinal excitons and which by plasmons.