Mark Gealy's Current Research:
I spent the 1998-99 academic year on sabbatical doing research at the J.R. Macdonald Laboratory at Kansas State University. I chose this location primarily because it houses a target apparatus for atomic collisions that I designed several years ago at the University of Nebraska. The chamber was moved to KSU in 1993 so that collisions involving faster and more highly charged ion projectiles could be studied. Given that I had one academic year to work there, it made sense to work with a relatively familiar piece of equipment.
Doubly Differential Cross Sections in Ion-Atom Collisions
Ionization is a prominent process in ion-atom collisions at moderate (keV/u) to high (MeV/u) impact energies.In addition to moderately high cross sections (probabilities) for ionization events, there is typically a very large energy loss per collision compared to charge transfer and excitation processes.Consequently, ionization processes are of considerable importance to a wide variety of investigators studying energy deposition in matter by fast projectiles.Examples of fields in which data on collisional ionization are needed include plasma fusion, auroral and other aeronomical investigations, astrophysics and studies of radiation effects in biological and other materials. Doubly differential measurements reveal the angular and energy distributions of the scattered particles and provide particularly stringent tests of theoretical models predicting the outcomes of the experiments.
To extend a considerable body of earlier work using the 'familiar' scattering chamber, I conducted a series of measurements of cross sections, differential in electron energy and scattering angle, for ionization in collisions of fast carbon ions with helium atoms. Specifically, we studied the reactions: , where n = 4 and 5.The collision velocity was 10au and scattered electrons were detected over an angular range from 15o to 160o with respect to the initial projectile direction, and from 1.0 eV to a maximum of 6.0 keV in energy.
Centennial Scholars, 2000:Jon Schauer, Mandy Kerstein and Dr. Gealy
During the summer of 1999, a Centennial Scholars grant allowed me to engage Concordia students Jon Schauer and Mandy Kerstein in the task of obtaining the cross sections from the measured raw data.We also took a trip down to the Macdonald lab to see the apparatus and meet with collaborating scientists on the project.A public lecture was presented at Concordia in November 2000.Progress on the work was reported at two conferences:APS Division of Atomic, Molecular and Optical Physics, Storrs, CT, June 2000 and the XXII International Conference on Photonic, Electronic and Atomic Collisions, Santa Fe, New Mexico, July, 2001 A detailed report of the completed work may be seen here (must have Adobe Acrobat reader).Papers are currently in preparation for publication in the Physical Review.
Zero-Degree Auger Electron Spectroscopy (ZAPS)
Much of my time at KSU was spent designing a new differentially pumped target cell for the chamber as part of a research project that is expected to result in a Ph.D. dissertation for graduate student Habib Aliabadi. Habib designed a new two-stage parallel plate electron spectrometer for studying energetic electrons emitted in the forward (projectile) direction in collisions of fast heavy one-electron ions with H2 and He gas targets. Projectiles will include F8+, Mg11+ and Si13+ at laboratory speeds on the order of 10% the speed of light.
The products to be detected are electrons liberated in the same direction as the projectiles in the collisions. The spectrometer counts the electrons as a function of their energy (speed). We are especially interested in those coming from doubly excited states of the projectiles formed in a process called Resonant Transfer Excitation (RTE). This process involves the simultaneous capture of a target electron into an excited state of the ion and the excitation of the projectile electron.
The subsequent decay of these highly unstable doubly excited ions often occurs by a so-called Auger process in which one electron drops to the ground state of the one-electron ion and the other electron emerges from the ion with the remainder of the doubly excited ion's excess energy. The energies of these Auger electrons are quantized and the measurement of their energies reveals the excitation energy of the doubly excited state from which they came. The relative numbers of electrons at each quantized energy reveals the relative probability for populating a particular state.
This work is an extension of similar measurements that have been made previously with slower, lighter ion projectiles. There are several reasons why the experiments get rapidly more difficult with heavier and faster projectiles.
From left to right: Habib Aliabadi, Ridvan Unal, Manolis Benis, Pat Richard (ri-SHARD), Peter Zavodszky and Mahendrajit Singh. Okay everyone say, "Resonant Transfer Excitation!".