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We present evidence for equilibrium temporal fluctuations in a high resolution low energy electron diffraction (LEED) experiment. These fluctuations are the reciprocal space analog of current fluctuations in field emission microscopy and therefore can be used to extract surface kinetic information. We show that even when the electron beam illuminates an area larger than its correlation length, time correlated data can be extracted from LEED. To demonstrate this, we present time dependent data from a W(430) surface, which reflects thermal step fluctuations. Our results illustrate the potential of LEED as a real time, ultrafast probe. [S0031-9007(98)07297-4] Real time surface kinetic information is crucial to the understanding of many important surface phenomena including diffusion, growth, 2D phase transitions, and many others [1]. Real space probes, in particular field emission microscopy (FEM) and scanning tunneling microscopy, have been used to obtain diffusion information [2,3]. In these experiments time correlations in the emitted current from the tip can be related to the movement of atoms into and out of a small probe area on the surface. These techniques require the use of high electric fields and therefore the role of electrostatic forces on surface kinetics becomes an important issue to be evaluated [4]. If an analogous technique could be developed in reciprocal space, kinetic information could be gained for a large variety of systems near equilibrium conditions. For convenient laboratory use, low energy electron diffraction (LEED) is the experiment of choice. But because the electron beam diameter is much larger than the finite correlation length ͑z ͒ of a LEED beam, it is not clear whether the incoherent sum over 10 6 domains (where the area of a domain z 2 ) in a LEED experiment would average out any time correlations in the collected signal [5]. This has been one motivation to develop coherent x-ray sources by using small apertures to produce beams small enough to encompass a few domains [6].In this Letter, we show that beam diameter is not the limiting parameter in a temporal LEED measurement. Instead, it is the probe current density that increases the true kinetics signal relative to statistical noise. Since current densities in LEED are very large ͑ϳ10 20 electrons͞m 2 ͒, temporal measurements should be possible. To demonstrate that we have measured the temporal fluctuation from a W(430) surface that contains a high density of atomic steps using high q-resolution LEED. The motion of a step is clearly visible in the time autocorrelation function of the diffracted beam. Both temperature and wave vector changes distinguish these correlated fluctuations as being due to steps.All the data presented here were taken with a high q-resolution LEED diffractometer described elsewhere [7]. The electron energy was kept fixed at 150 eV corresponding to a wavelength of 1.00 Å. The LEED transfer width was ϳ1300 Å at the experimental geometry used in these experiments. Reciprocal space coordinates are given in ...
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