Laser-Assisted Inelastic Electron-Atom Scattering

Funding is requested to carry out fundamental research on laser-assisted electron scattering. The work will be carried out on two apparatuses. The first, which is already in use for such experiments, will be used to measure the effects of the "dressing" of the lowest excited states of He and Ar atoms by 1.17 eV laser radiation during inelastic electron scattering. The electron gun of this apparatus will be fitted with a monochrometer in order to resolve the S and P levels; only the former is expected to show dressing effects. The apparatus will also be interfaced with an in-house developed multipass laser system to increase the free-free count rate. The second apparatus has recently been fitted with such a multipass laser system. This apparatus will be used to investigate laser-assisted electron-impact autoionization of helium. Intellectual Merit The absorption or emission of radiation during the collision of charged particles with atoms and molecules has long been known to be important in astrophysical and plasma modelling. An in-depth knowledge of these processes is therefore required which may be gained by laboratory experiments. Up to now almost all such experiments have been in agreement with a simple semi-classical theory (the Kroll-Watson Approximation) which assumes that the interaction of the radiation with the atom itself has no effect on the scattering process: the radiation only interacts with the free projectile. Recently the first experiments which observe the unambiguous breakdown of this assumption during the elastic scattering of electrons by xenon have been carried out by other workers. An estimate of the dressing of the target by the radiation's electric field may be made in terms of the electric dipole polarizability of the target. The effects in Xe were extremely difficult to measure because they occur at very small scattering angles. The proposed work will carry out experiments on the lowest excited states of He and Ar which have polarizabilities an order of magnitude larger than the ground state of Xe. The dressing effects in these systems are predicted to be observable at scattering angles easily accessible to experiments, and without the need for complicated corrections. The experiments will extend preliminary data that is encouraging. These experiments will provide benchmark data on the dressing of atoms by laser radiation during electron scattering. The proposed experiments on the helium resonances are motivated by the fact that electron-impact autoionization in the presence of a laser field has never been investigated before; previous laser assisted experiments have looked at the excitation of bound states below the ionization potential or the direct ionization into the continuum -- the proposed experiments would combine both these aspects. It is unclear whether the Kroll-Watson approximation can be used to predict the magnitude of the expected effect: the ejection of an electron from an autoionizing state corresponds to "half scattering", with a corresponding cross section one half of the full result. Also, while electron-impact excitation of the singlet D resonance is a quadrupole process, the absorption of a photon effectively converts this to a much more favored dipole process. This would be equivalent to a dressed autoionizing state resulting in a modified lineshape and Fano q-parameter. A theory/experiment collaboration on the work described above is currently being explored. Broader Impacts This is believed to be the first time that a multipass system will be used for laser-assisted electron scattering experiments; previously such systems have been implemented at storage rings, and on accelerator based experiments. It will represent a real advance in table-top laser-assisted experiments. The research will involve both undergraduate and graduate students at the University of Kentucky and includes a summer collaboration with a faculty member at Illinois Wesleyan University. The project will provide dissertation material for University of Kentucky graduate students. Students acquire skills in lasers, electronics, vacuum systems, and computer programming, networking, and interfacing with experiments. They present their work at national and international conferences.