Electron-Atom Scattering in the Presence of a 1.17eV Laser Field

University Of Kentucky Research Foundation, Lexington KY

Investigators

Abstract

This project examines the so-called free-free transitions of electrons exposed to laser light. In a gas of neutral atoms, bound atomic electrons may readily emit or absorb photons (particles of light, or electromagnetic radiation). However, in a partially ionized gas (a plasma), in addition to neutral atoms, ions and unbound (free) electrons are present. Because of the laws of the conservation of energy and momentum, these free electrons can only absorb (or emit) a photon during a collision with an atom. This process is known as a free-free transition, i.e., the electron is free before the collision and free after it, but has gained or lost an amount of energy equal to that of one (or more) photons that are present. It has been shown that free-free processes have a significant effect on the properties of astrophysical plasmas, such as those in the stellar (and solar) atmosphere and interstellar space, and also on the plasmas encountered in fusion research (fusion reactors) and the lighting industry (florescent tubes). It is therefore important to have a detailed knowledge of free-free processes, and these may be studied in the laboratory by scattering a beam of electrons from a gas jet in the presence of a laser beam. All experiments to date, except one, found that free-free transitions are independent of which atom (or molecule) is used. Very recently, an experiment carried out in Japan found the first experimental evidence that the type of atom can affect the transition. The experiments were carried out with Xenon atoms for which the effects were observable, but very small. This project will carry out similar experiments in Potassium, where a simple theory predicts that the effects will be ten times larger. The project will also develop what is known as a multipass system, where the laser beam (which is fired 30 times a second to produce laser pulses) will be bounced back and forth between mirrors at least ten times, and will therefore enable experiments to be carried out in one tenth of the time. The system will use a special "optical door" which will allow the laser pulse to be "injected" into the space between the two mirrors, before it is trapped. The target independence of free-free transitions is a requirement of the simple semi-classical Kroll-Watson model, which to date has been in agreement with (practically) all experiments. This implies that the atom's role is simply to balance momentum and is not "dressed" by the laser field. A simple model by Zon shows that any such dressing is dependent on the electric-dipole polarizability alpha of the target; for helium (alpha=1.4) dressing effects my be safely ignored. The first experiment to observe dressing effects was for xenon (alpha=28) by Morimoto et al. They observed effects that were qualitatively, but not quantitatively, in agreement with Zon's model. This disagreement may be due to experimental effects at the very small scattering angles used. The experiments to be carried out in the present project will be carried out in Potassium (alpha=290) for which Zon's model predicts large dressing effects at scattering angles readily accessible experimentally. The multipass laser system will use a Pockels cell to rotate the polarization of the laser pulse once it is injected into the system, thereby trapping it in a repetitive optical path that contains a polarizing beam-splitting cube.