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In the context of this work, a thick overlayer means that the thickness is such that only the electrons generated on the film contribute to the LEYS-CFS spectrum. Since the escape depth of the electrons generated at the low energies used is nearly 40 Å, a thick overlayer is one with thickness greater than 40 A, In contrast a thin layer is one which permits the contribution of the electrons generated in both sides of a heterojunction to the LEYS-CFS spectra. In this case an optimal layer thickness is around 15 A.
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