Ion-beam particle accelerators spanning the kiloelectron volt (keV) to megaelectron volt (MeV) range are used in ion-atom, ion-molecule and ion-surface collision studies for a variety of basic and applied projects, providing broad experimental experience for graduate students. Collisions of atom and ion beams with atomic and molecular gas targets probe the fundamental interaction mechanisms between individual atoms and molecules. Energy-loss, x-ray and Auger-electron emission spectroscopies probe the quantum states excited in such collisions, and differential cross sections are measured where possible. One project studies common single and double-electron transfer collisions between atomic ions and small-molecule gas targets. In another, Rutherford backscattering of low- MeV energy ions is used as a tool for depth-profiling of ion-implanted and other specially prepared surfaces. These are often carried on in collaboration with local industrial laboratories.

In a new project, highly charged ion beams collide with both gases and with clean surfaces under ultrahigh vacuum. This work includes explorations of ion beam etching of diamond and other superhard materials. Slow, highly charged ions have exotic properties compared with neutral atoms. They are normally observed only in the outer atmospheres of stars, in interplanetary space or in energetic plasma fusion devices, but they can now be studied in the laboratory. For example, in collaboration with Lawrence Livermore National Laboratory, we measured the x-ray emission spectrum of Bi ions impacting on a gold surface.

Another major effort studies ionization and other reactive scattering processes that can occur in collisions of laser-excited atoms and molecules in an atomic beam at thermal energies. Tunable lasers are used to excite the atoms into known quantum states of relatively high internal energy. Diatomic processes investigated include excitation transfer and ``laser switched'' associative ionization processes (to produce molecular ions plus electrons). In this work new types of collisions with large rate coefficients have been observed, such as triatomic associative ionization between laser-excited sodium atoms and ground-state sodium molecules and selective collisional dissociation of diatomic molecules by laser-excited atoms.

These experiments are strongly complemented by related theoretical studies, e.g. ab-initio computations of cross sections for atomic collision and ionization processes in plasmas. Additional theoretical research includes characterization of singly and doubly-excited Rydberg atoms and their responses to external time-dependent electric fields (including laser fields) as well as related research in molecular physics and in ultracold collisions (see above).

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