Free-free experiments: The search for dressed atom effects

Experiments on free-free electron scattering, specifically the absorption or emission of 1.17 eV photons from a Nd:YAG laser field by an unbound electron when it is scattered by an atom or molecule, are reviewed. For large scattering angles such experiments are well described by a simple analytical theory that is independent of the properties of the target. At small scattering angles this theory breaks down for targets with a high dipole polarizability α, and an additional term needs to be incorporated in the scattering amplitude. This term is proportional to the dipole polarizability, and hence introduces the properties of the target into the free-free cross section - i.e., the laser field 'dresses' the atom. A progress report is given of free-free experiments designed to look for such 'dressed atom' effects during the electron-impact excitation of argon in the presence of a laser field; the lowest excited states of argon have α ≈ 300 atomic units.