Influence of the surface electric field in ion-beam-surface interaction at grazing incidence
We have measured the polarization of light emitted after ion surface scattering at small angle of incidence. The measurements are carried out with H+-, H2+- and He+-ions under UHV-conditions with mono- and polycrystalline targets. We explain the typical variation of the polarization as “post collision Stark interaction” (PCSI) in the surface electric field, which can force transitions between nearly degenerate terms. The electric field is composed of two different contributions, a strong but short range surface field which is “seen” by atomsand ions and a long range but weak field due to the image charge which is “seen” to first orderonly by ions. The influence of the electric field on H-Balmer radiation is negligible at typical survival distances rs≧0.35nm. But in contrast to H-atoms He+-ions feel the additional influence of the image field leading to a strong alteration of the polarization of the emitted light. The polarization of the Balmer-radiation stemming from Coulomb exploding H2+-beams is observed to be modified by the electric field of the “spectator proton”.