Dynamical reconstruction of the valence exciton in LiF*

We have used inelastic x-ray scattering, coupled with recently developed inversion techniques, to reconstruct the structure and dynamics of the valence exciton in the prototype alkali halide LiF. Our inversions, which yield resolutions $\Delta x = 0.533 \AA$ and $\Delta t = 20.67 as (2.067 \times 10^{-17} s)$, reveal that the exciton forms in less than $50 as$, oscillates with a period of $283 as$, and decays after approximately $5 fs$. It contains a pronounced $a/3$ internal periodicity, where $a = 4.027 \AA$ is the crystal lattice parameter, that changes little during the course of its life, indicating that this exciton lies very close to the Frenkel limit. Our results resolve a 70 year old contraversy about the valence exciton in alkali halides and, when compared to {\it ab initio} calculations, demonstrate a simplified theoretical approach to describing excitons in the limit of strong binding energy.