ChemMatCARS Nuggets - Materials Science

Electron Dynamics of LiF Imaged in Real Time,  Confirming Decades-Old Theoretical Prediction

Accomplishment
A new method involving inelastic x-ray scattering makes it possible to image the dynamics and lifetime of excitons in real time with resolution of ~1 Å and ~20 attoseconds. An exciton is created when an electron is excited from the valance band to the conduction band in a material and subsequently binds to the hole it left behind. The resulting electron-hole pair is typically described as either a Frenkel exciton, which is strongly bound, or a Wannier exciton, which is loosely bound and delocalized.

For these measurements, lithium fluoride (LiF) was chosen because of long-unresolved questions about exciton expansion in this and other alkali halides. In LiF, our data show that an exciton forms within 50 attoseconds of excitation and persists for about 5,000 attoseconds (5 femtoseconds). It expands (delocalizes) and, unexpectedly, oscillates in space with a spatial period of about 1/3 of the lattice parameter.

Impact
These results confirm predictions made in the 1940s that excitons in alkali halides would be the strongly bound Frenkel type but also exhibit a non-local character. The results also show the applicability of the new inelastic X-ray methods to studies of very fast atomic phenomena in general.

Measured disturbance in the probability of finding a valence electron of LiF at a certain location after excitation. A flat plane would represent the steady state. The disturbance dies out with time (to back of image).

Principal Investigator: Peter Abbamonte, University of Illinois at Urbana-Champaign

October 2007

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