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By approximation, we can use the freeelectron model to estimate the motion of electrons interacting with materials. Based on elementary quantum mechanics, the transmission probability of secondary electrons (SEs) across the surface potential barrier from the SEM specimen, T(S/V). can be given by [1],
 [4569a]
and,  [4569b]
where E_{k} is the SE kinetic energy into the vacuum and E_{s} is the corresponding SE kinetic energy in the specimen; α is the SE angle of emission into the vacuum from the specimen.
[1] Kuhr, J.C. & Fitting, H.J. (1999) Monte Carlo simulation of electron emission
from solids. J. Electr. Spectrosc. Rel. Phen. 105, 257–273.
