Theory of Radiative Electron—Atom Scattering

By radiative scattering we simply mean scattering of electrons from atomic targets in the presence of the radiation field. The relative complexity and richness of the theory of radiative scattering is due primarily to the fact that for every channel of scattering in the absence of the field, there arises a multitude of subchannels corresponding to real and virtual multiphoton emission and absorption processes. Generally, both the projectile electron and the target atom take part in the emission and absorption process during the scattering. A systematic theory of radiative scattering, comparable to the theory of ordinary electron—atom scattering, must therefore be able to account for these channels in a manageable way. Another special circumstance associated with radiative electron—atom scattering is the problem of the initial distributions of the states of the system. Unlike ordinary scattering, the projectile electron may find the target atom not just in the thermal-equilibrium state but rather in a certain superposition of field-modified states (usually far from thermal equilibrium). This situation is further complicated by the possibility that such a superposition state of the target atom may also decay due, among other things, to the incoherent influence of spontaneous emission and (multiphoton) ionization. The problem of how to relate the scattering amplitudes, as defined with respect to a given pair of “initial” and “final” states, to the observable differential scattering signals is treated subsequently.