Charge-exchange mechanisms at the threshold for inelasticity in Ne+ collisions with surfaces

Abstract

We present a study on scattering of 100–1400 eV Ne+ ions off Mg, Al, Si, and P surfaces. Exit energy distributions and yields of single-scattered Ne+ and Ne2+ 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 Rmin (distance of closest approach) is reached, three phenomena occur simultaneously: Ne2+ generation, reversal of the Ne+ yield trend, and inelastic losses in Ne+ and Ne2+. Rmin values for the Ne2+ turn-on agree very well with the L-shell overlap distances of the colliding partners, suggesting that electron transfer involving the highly promoted 4fsigma molecular orbital (correlated to the Ne 2p) at close internuclear distance (~0.5 Å) is responsible. For the Ne+ yield, a clear transition from nonlocal neutralization to Rmin-dependent collision induced neutralization was observed. Binary collision inelasticities (Qbin) were evaluated for Ne+ and Ne2+ off Al and Si by taking into account electron straggling. Saturation-like behavior at Rmin<0.5 Å was seen for Ne+ (Qbin~40–45 eV) and Ne2+ (68–75 eV). These losses fit well with double promotion of Ne0-->Ne** (2p43s2, 41–45 eV) and Ne+-->Ne+** (2p33s2/3s3p, 69–72 eV), followed by autoionization as the projectile leaves the surface region to give Ne+ and Ne2+. In contrast, Qbin values for Ne2+ 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 Ne2+ is only generated in double excitation of surviving Ne+. 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+[Single Bond]P system shows striking similarities with the other target cases from the perspective of a well-defined Ne2+ turn-on, continually increasing Ne2+ yield with impact energy, and inelasticity values which point to the same 4fsigma excitation pathway. The decreasing Rmin requirement for higher target Z in terms of Ne2+ production has been confirmed for the Mg through P series, where hard collision excitation is governed by L-shell orbital overlaps.

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