Charge-exchange mechanisms at the threshold for inelasticity in Ne{sup +} collisions with surfaces
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 (United States)
We present a study on scattering of 100-1400 eV Ne{sup +} ions off Mg, Al, Si, and P surfaces. Exit energy distributions and yields of single-scattered Ne{sup +} and Ne{sup 2+} were separately measured to investigate charge exchange mechanisms occurring at the onset of inelastic losses in binary hard collision events. At low incident energies, collisions appear elastic and projectile ion survival is dominated by nonlocal Auger-type neutralization involving the target valence band. However, once a critical R{sub min} (distance of closest approach) is reached, three phenomena occur simultaneously: Ne{sup 2+} generation, reversal of the Ne{sup +} yield trend, and inelastic losses in Ne{sup +} and Ne{sup 2+}. R{sub min} values for the Ne{sup 2+} turn-on agree very well with the L-shell overlap distances of the colliding partners, suggesting that electron transfer involving the highly promoted 4f{sigma} molecular orbital (correlated to the Ne 2p) at close internuclear distance ({approx}0.5 A) is responsible. For the Ne{sup +} yield, a clear transition from nonlocal neutralization to R{sub min}-dependent collision induced neutralization was observed. Binary collision inelasticities (Q{sub bin}) were evaluated for Ne{sup +} and Ne{sup 2+} off Al and Si by taking into account electron straggling. Saturation-like behavior at R{sub min}<0.5 A was seen for Ne{sup +} (Q{sub bin}{approx}40-45 eV) and Ne{sup 2+} (68-75 eV). These losses fit well with double promotion of Ne{sup 0}{yields}Ne** (2p{sup 4}3s{sup 2}, 41-45 eV) and Ne{sup +}{yields}Ne{sup +}** (2p{sup 3}3s{sup 2}/3s3p, 69-72 eV), followed by autoionization as the projectile leaves the surface region to give Ne{sup +} and Ne{sup 2+}. In contrast, Q{sub bin} values for Ne{sup 2+} at the +2 turn-on were seen much lower (35-40 eV off Al, 55-60 eV off Si) than that required for double promotion--eliminating the possibility that Ne{sup 2+} is only generated in double excitation of surviving Ne{sup +}. Thus single-electron excitation appears to be more important in the threshold region compared to the two-electron events seen at higher collision energies. In addition, the Ne{sup +}-P system shows striking similarities with the other target cases from the perspective of a well-defined Ne{sup 2+} turn-on, continually increasing Ne{sup 2+} yield with impact energy, and inelasticity values which point to the same 4f{sigma} excitation pathway. The decreasing R{sub min} requirement for higher target Z in terms of Ne{sup 2+} production has been confirmed for the Mg through P series, where hard collision excitation is governed by L-shell orbital overlaps.
- OSTI ID:
- 20718380
- Journal Information:
- Physical Review. A, Vol. 72, Issue 1; Other Information: DOI: 10.1103/PhysRevA.72.012904; (c) 2005 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 1050-2947
- Country of Publication:
- United States
- Language:
- English
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