AVS 52nd International Symposium
    Surface Science Thursday Sessions
       Session SS2-ThA

Paper SS2-ThA9
Formation of Multiply Charged Ions by Direct Recoil

Thursday, November 3, 2005, 4:40 pm, Room 203

Session: Gas-Surface Reaction Dynamics
Presenter: X. Chen, University of California, Riverside
Authors: X. Chen, University of California, Riverside
Z. Sroubek, University of California, Riverside
J.A. Yarmoff, University of California, Riverside
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Low energy ions that impact a solid surface can remove material by sputtering, which produces low kinetic energy particles through a collision cascade, or direct recoil (DR), which involves the emission of a relatively high-energy particle following a hard collision between the incident ion and a surface atom. An inner-shell promotion that places one of the atoms into an excited state is a possible consequence of the hard collision. In these experiments, we find a unique process in which the DR particle is emitted with two holes, i.e., as a doubly charged ion. Si@super +@ ions were incident on atomically clean Al(100). The absolute ionization probability of scattered Si and recoiled Al were measured with time-of-flight, while detailed spectra of the ion yield were collected with an electrostatic analyzer. All of the scattered Si was neutralized, as expected due to its large ionization potential. Spectra collected of the DR Al show, however, a combination of Al@super 1+@, Al@super 2+@ and Al@super 3+@ ions. The multiply charged ions are attributed to a charge promotion of the Al 2p during the hard collision. The Al 2p level is promoted above the Fermi energy by interaction with the Si 2p level that lies just below, such that both electrons are transferred to the solid forming Al ions with two holes. Subsequently, some of the Al@super 2+@ may decay to Al@super 3+@ via autoionization, or pick up an electron to form Al@super 1+@. This mechanism is verified by the observation of an energy threshold for the process, and by spectra of the electrons emitted as the excited states decay. This process of producing multiply charged DR ions is highly effective because of the small separation between the Al and Si 2p levels. Thus, it is expected that similar behavior would occur for other closely matched systems that will be investigated, such as P ions incident on solid Si.