Abstract
In a thin film, intraband electronic transitions are infrared-active. This is because the electrons exchange momentum with the surfaces. The selection rules for the transitions are given, and the matrix elements calculated using a band model. These matrix elements are shown to be the same as those obtained from the effectively-free-electron model, i.e. the model using free-electron wavefunctions with the band dispersion relation. The matrix elements are also the same for a realistic surface potential step as for an infinite one.

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