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The depth sensitivity of REELS can be varied by changing the primarybeam energy [1] or the incidence and collection angles.[2] The sampling depth probed in REELS depends on the primary electron beam energy, can be described based on the firstorder term in the Poisson distribution, and is given by the probability of the occurrence of one inelastic collision,
 [2301a]
where,
d  The depth;
λ  The inelastic mean
free path, depending on the primary electron beam energy;
p_{1}  The probability that one inelastic collision
occurs in depth d.
The EELS sampling depth is given by,
 [2301b]
where,
α  The incidence angle;
β  The collection angle.
In REELS measurements, d is normally defined as the depth from which 95% of the inelastically scattered electrons are collected. For many materials, the d is often in the range of less than 5 nm at the beam energies of less than 1 keV.
[1] D. F. Cox and G. B.Hoflund, Surf. Sci. 151,202 (1985).
[2] P. Sen, M. S. Hegde, and C. N. R. Rao, Appl. Surf. Sci. 10, 63 (1982).
