Effect of the Diffraction Condition on the Mean Free Path of 50 kV Electron for Plasmon Excitation in Aluminium

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Abstract
The mean free path of high energy electrons for the plasmon excitation has been measured in a single crystal of aluminium by a Möllenstedt type energy analyser combined with an electron microscope. The mean free path of 50 kV electron is 1000±100Å when the crystal is far away from the exact Bragg condition to the incident electrons. When the crystal is near the exact Bragg condition to the incident electrons, an anomaly in the mean free path was observed. The mean free path was 900±100Å and 1100±100Å when W >0 and W W is a parameter showing the deviation from the exact Bragg condition. This result indicates that there are two mean free paths corresponding to the two Bloch waves, i. e. The Bloch wave on the branch-1 has the shorter mean free path than that on the branch-2.