PerspectiveImaging Techniques

Low-energy electron diffraction at ultrafast speeds

Science  11 Jul 2014:
Vol. 345, Issue 6193, pp. 137-138
DOI: 10.1126/science.1256199

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Summary

Low-energy electron diffraction (LEED) has been used to determine the surface structure of crystalline materials because the diffracted electrons only probe the top atomic layers. First reported by Davisson and Germer in 1927 (1), the LEED technique became widely used when ultrahigh vacuum techniques introduced in the 1960s allowed surfaces to remain relatively free from adsorbed background gases during a typical experiment. However, the information LEED provides about the relative ordering of atoms on a surface, whether at atomic resolution or on a larger mesoscopic scale, can only be understood in a time-averaged, quasi-stationary manner. Dynamical aspects, such as changes with temperature, have been only grasped indirectly with well-developed theoretical models to describe the averaged measured quantities. Time-resolved monitoring of LEED patterns would enable direct visualization of lattice motions or light-induced ultrafast phase transitions. On page 200 of this issue, Gulde et al. (2) report an ultrafast implementation of LEED and have resolved the ultrafast melting of a poly(methylmethacrylate) (PMMA) layer adsorbed to a graphene substrate.