Do Hollow Atoms Exist in Front of an Insulating LiF(100) Surface?

Abstract

First measurements of ALL Auger spectra arising from collisions of N + ions on an insulating LiF(100) surface are presented. The beam energy is varied from 0.08 up to 16 keV. The incident angle is chosen such that the perpendicular velocity component is constant over the indicated energy range. Spectral features arising from above surface KLL emission, observed during interaction of N + with a conducting Si(100) surface, are absent for LiF, implying pronounced differences in the dynamics governing the above-surface neutralization and deexcitation of the highly charged projectiles. PACS numbers: 31.50.+w, 36.20.Kd One of the main goals for the study of the interaction of highly charged ions with surfaces is to find out which electronic processes are induced by the highly charged projectiles, and to what extent this can give rise to sputtering or surface modification [1 -5]. For metallic targets reasonable agreement has been obtained regarding the first steps in the ion surface interaction. First the ion is attracted towards the surface by its own image charge; at a reasonably well-defined distance in front of the surface, electrons are transferred from the solid into excited orbits of the projectile, leading to a neutralized, multiply excited "hollow atom" with empty or sparsely filled inner orbits.However, a lot of discussion is still going on regarding the next steps: the decay of these hollow atoms.Clearly, one observes KLL Auger processes. But it is by no means clear whether this represents the last step of an Auger cascade from higher projectile shells or whether electrons are directly captured into the L shell from inner shell target orbitals. Also it is still a point of discussion to what extent the KLL decay takes place above or below the surface. In view of possible implications for surface modifications it is important to localize where electrons are emitted, i.e. , where the potential energy carried by the projectiles is deposited.For an insulator, already the first steps of ion surface interaction might be quite different, as compared to a conducting surface. Delocalized electrons are not as abundantly available as in a metal, and therefore it is questionable whether hollow atoms will be formed in the same way as in front of a metal surface. This in turn may have a large inhuence on the impact energy of the originally slow projectiles: Without hollow atom formation the projectile's image charge would not be "switched off" at relatively large distances as for metal surfaces, and therefore the ion would be accelerated by its image charge up to very close distances from the surface. This would impose a lower limit to the kinetic impact energy significantly higher than for metal surfaces. The absence of hollow atom formation would also imply that a considerably smaller amount of the projectile's potential energy is released above the surface, leading to a much more violent interaction.In order to investigate highly charged ion interaction with an insulating surface we have measured energy spectra of KLL...

Electron emission following the interaction of slow highly charged ions with solids

“…It is interesting to note that the target with the greatest electron yield is not the gold, but the HOPG. It is also interesting to note that an insulating target CaF 2 has a larger electron emission yield than the gold target in agreement with the recent results for another insulator LiF (Limburg et al 1997). It is possible that in the case of an insulator the target charges up as electrons are removed from the surface.…”

Emission of secondary particles from metals and insulators at impact of slow highly charged ions

“…We reproduce this result, measuring slightly higher yields from Si02 than from Au for incident protons and 03+ ions. It has been suggested that hollow atom formation is strongly inhibited for impact of highly charged ions like~and Ne* [7] on insulators like LiF. Winter et al [4] found that less efficient above surface electron emission was more than compensated by more efficient below sutiace emission from LiF, resulting in higher electron yields from LiF than from Au targets for impact of slow HCI with q<lO+.…”

“…Whh most electrons populating high n Rydberg states, a highly excited, but neutral "hollow atom" finally hits the surface. It has been suggested that hollow atom formation in front of insulator surfaces is relatively inhibited in comparison to impact on metals [7]. At impact, electrons in Rydberg states with radii in excess of a characteristic surface screening length are -peeled off, and a fraction of eleotrons escapes into the vacuum.…”

Diagnostics of warm dense matter by high-resolution X-ray spectroscopy of hollow ions

“…1; for more details, see Skobelev et al, 2012). Transitions of this kind were also observed in experiments on the interaction of the ion beams (Briand et al, 1996;Bailey et al, 1990;Limburg et al, 1995;Ninomiya et al, 1997;Winter et al, 1999;Schenkel et al, 1999;McMahon et al, 2011;Rzadkiewicz et al, 2010), synchrotron radiation (Diamant et al, 2000b;2000a;, or X-ray free electron laser beams (Vinko et al, 2012) with a solid target. The first observation of the similar emission pattern for multi-charged ions was done at the Naval Research Laboratory's NIKE KrF laser facility using ns laser pulses (Aglitsky et al, 1996).…”