Free-free scattering is the scattering of a free electron by an atom in the presence of, for example, a laser beam — the free electron may either absorb or emit one or more photons from the beam. (A free electron cannot absorb or emit photons without the presence of an atom because both energy and momentum cannot be conserved; the photon is a "relativistic" particle traveling at the speed of light!) The term free-free scattering is used since the electron is free (not bound to the atom) both before and after the event. The absorption or emission of radiation by charged particles during collisions with atoms and molecules has long been known to be important in astrophysical and electrical plasma phenomena. Free-free transitions are also known to dominate the radiation transport in certain types of air plasmas, such as cascade arcs and shock tubes. Free-free experiments carried out under controlled laboratory conditions, using lasers and well-defined electron beams, can therefore provide detailed information on these important processes.
Simple theories of free-free scattering assume the atom's only role is to balance energy and momentum; the type of atom and its structure do not enter the theory. Such simple theories have been extremely successful in describing the majority of experiments, but more sophisticated theories predict that the electric field of the laser beam (which, of course, is a special type of electromagnetic radiation) can affect the free-free scattering process for very small scattering angles. This is known as the dressed-atom effect since the atom is sitting in the electric field when the scattering takes place. Simple theories predict that the free-free cross section decreases as the scattering angle becomes small, whereas the dressed atom picture results in a rapidly increasing free-free cross section at small scattering angles. Although this process was predicted more than 40 years ago, to date there has been only one experiment reported to see such an effect.
Dressed-atom effects may be qualitatively thought of as due to mixing of atomic target states by the electric field of the laser beam. This mixing is proportional to the dipole polarizability, alpha, of the target atom. (Alpha is a measure of the distortion of the electron orbitals in the target by an electric field.) Therefore, a large alpha results in a target that is a strong mixture of different states, which has a profound effect on the free-free cross section. In particular, at smallish angles, where the free-free cross section for elastic scattering becomes small, dressed-atom theory predicts a rapid increase in the free-free cross section as the scattering angle tends to zero.
Our research looks for dressed-atom effects in free-free scattering for the electron-impact excitation of the lowest excited states of the rare gas atoms argon and helium. These excited states have very high dielectric polarizabilities, alpha, some 10 to 100 times those of the ground states. We have carried out exploratory free-free experiments on both these systems and have seen indications that there are dressed-atom effects for 350 eV electrons scattered through small angles of about 2 degrees. However, the experiments take at least a week of continuous data-taking to acquire statistics at the 4 sigma level, and they are difficult to reproduce. The experiments continue.
(Funding by National Science Foundation)