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 ${\Gamma }_{15}\to {\Gamma }_1$. Structure associated directly with the band gap is not manifest, so the ${\Gamma }_{15}-{\Gamma }_1$ energy is determined indirectly to be 14.2 ± 0.2 eV. The $n=1\mathrm{}$ exciton state generates the first strong structure in $\Delta \tilde{\epsilon }$ 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 $L_3^{}{}_{}{}^{\prime }\to L_1$ and $L_2^{}{}_{}{}^{\prime }\to L_1$ 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 $\Delta \tilde{\epsilon }$ is associated with excitonic transitions at $X$ involving the second $d$-like conduction band. The corresponding peak at 26.4 eV in $\Delta \left[\mathrm{Im}\right(-\frac{1}{\tilde{\epsilon }}\left)\right]$ overlaps the "valence-band" plasmon at 24.6 eV. No evidence for double excitations is found around 25 eV in either $\Delta \tilde{\epsilon }$ or $\Delta \left[\mathrm{Im}\right(-\frac{1}{\tilde{\epsilon }}\left)\right]$. The $\Delta \left[\mathrm{Im}\right(-\frac{1}{\tilde{\epsilon }}\left)\right]$ spectrum shows for the first time which structures in the energy-loss function are generated by longitudinal excitons and which by plasmons.