  We present a study of the interaction of a relativistically planar channeled
electron with an intense electromagnetic field. Using a S-Matrix approach in
the Strong Field Approximation, it is shown that the crystal periodicity
affects drastically the excitation process, suppressing the possibility of
multiphoton absorption except for some particular cases. This selective
excitation opens the possibility to control the dynamics of the channeling
process by means of an external field. Explicit expressions for the S-matrix
N-photon excitation rates together with the corresponding conservation laws are
obtained from the relativistic quantum mechanical Dirac equation.
