Andy's research focuses on the physics of atomic and molecular collisions. Andy uses laser-spectroscopic techniques, as well as atomic and molecular beam methods, to study these collisions at very low kinetic energies. Such low-energy collisions are interesting because they provide sensitive tests of quantum mechanical scattering theory. Currently, his main research efforts are divided between two distinct projects: cold collisions between electronically excited atoms, and electron-molecule scattering at threshold energies.
The first project, currently being set up on the Lafayette campus, has the goal of studying orientation, alignment, and velocity effects in associative ionization collisions of electronically excited atoms at very low temperatures. Associative ionization occurs when two free atoms collide to simultaneously form a molecular bond and eject an electron. This process for producing a molecular ion is energetically allowed when the internal excitations of the two free atoms are in excess of the ionization energy of the associated molecule. Associative ionization is of interest because it is a simple example of chemical bond formation, because it can be a common process in low temperature plasmas (important for understanding plasma etching techniques used in the manufacturing of integrated circuits), and because associative ionization cross sections at low temperatures have proven difficult to calculate.
Experimentally, associative ionization will be studied in a single effusive beam of cesium atoms. Intrabeam collisions arise from the effusive beam's inherent thermal velocity distribution. External-cavity diode lasers, (both commercial and ones built on-campus), are employed to prepare the initial excited states of cesium. The collision velocity is precisely controlled by using the diode lasers' very narrow linewidths to Doppler-tune the laser frequencies into resonance with selective components of the atomic beam's velocity distribution. This method will be used to study collisions at velocities associated with temperatures near 1K.
It is expected that the atomic hyperfine structure will be important in low temperature collisions. To better understand this structure in cesium, we have measured the hyperfine coupling constants for several 6d and 7d states. This work contributes hyperfine data with significantly improved resolution to the literature and tests our apparatus in preparation of future collision experiments.
The second project is in collaboration with the Atomic and Molecular Scattering Group at NASA's Jet Propulsion Laboratory. Under investigation is the interaction between free-electrons and molecules at energies very near attachment threshold. We have constructed a novel laser-based photoelectron source with ultrahigh energy resolution: a tunable vacuum-ultraviolet laser intersects a molecular beam containing a mixture of xenon atoms and the target molecule. The laser ionizes the xenon atoms, producing photoelectrons that are free to collide with nearby target molecules. This apparatus can easily produce electrons whose de Broglie wavelengths are an order of magnitude larger than the dimensions of a typical target molecule. At these energies, fully quantum mechanical scattering can be observed and quantum mechanical predictions can be tested.
Andy's home page is here.