Laser-Assisted Inelastic Electron-Atom Scattering

University Of Kentucky Research Foundation, Lexington KY

Investigators

Abstract

Free-free scattering is a fundamental process that is important for the understanding of various astrophysical phenomena. The process is named for the fact that when an electron scatters from an atom in the presence of a photon field, the electron can absorb or emit one or more photons (particles of light), but the electron is free both before and after the scattering collision. Almost all experiments to date indicate that the photon interacts only with the free electron and has no effect on the target atom. However, it has been shown that a so-called "dressed atom" effect (the interaction of the photon field with the target atom) can greatly enhance the probability of the free electron absorbing or emitting a photon for very small electron scattering angles. This project investigates free-free scattering in the laboratory by using a laser to produce a well-defined photon beam, an electron gun (similar to those that used to be found in old TV sets) to produce a well-defined electron beam, with the two beams arranged to intersect each other and a beam of atoms. An important part of the project is to look for "dressed atom" effects in helium, for the special case where the scattered electron also changes the energy state of the helium atom. This work will have a broad impact on the community by providing educational opportunities for students at the high school, undergraduate, and graduate levels in the EPSCoR state of Kentucky and in collaboration with the exclusively undergraduate institution Illinois Wesleyan University. This project concerns 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 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 closely spaced He 1s2s and 1s2p singlet levels; only the former is expected to show dressing effects and it is important to eliminate the effect of the large scattering cross section of the latter. The apparatus will also be interfaced with an in-house developed multipass laser system to increase the free-free count rate; to date such systems have only been used in national labs, never in "tabletop" experiments. The multipass system uses a Pockels cell to rotate the linearly polarized laser beam 90 degrees after injection, combined with a beam splitting cube that then traps the laser beam into a repetitive path (that includes the now-deactivated Pockels cell). 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. The expected signatures of laser assisted autoionization are sidebands in the ejected-electron spectrum, one photon energy away from the autoionizing resonance positions. Such sidebands have been seen by other workers in Auger decay, but have never been observed for the autoionization process. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.