Exciton in ion induced electron emission

N. BAJALES; L. CRISTINA; S. MENDOZA; E.C. GOLDBERG; J. FERRÓN; R.A. BARAGIOLA

Cuernavaca

Conferencia; IV SAN LUIS PASI; 2007

The interaction of low energy ions with surfaces is very important in surface characterization. From sample treatment, as in sputter cleaning or ion depth profiling, to the excitation in the most surface sensitive techniques, like Secondary Ion Mass Spectrometry (SIMS) and Low Energy Ion Spectrometry (LEIS), ion bombardment covers an enormous range of applications. The electronic interactions among low energy ions may give place to other interesting processes like electron emission. Being this emission a very surface process, it represents a sensitive probe for the electronic structure of surfaces.    The processes leading to the secondary electron emission (SEE), induced by low energy ions, have been historically divided in two mechanisms: kinetic and potential electron emission. The release of the incoming ion potential energy can occur through Auger neutralization (AN) or resonant neutralization (RN), which may be followed by an Auger de-excitation (AD). If the incoming ion hole is bond by at least twice the work function of the solid, the electron emission occurs. Although ion induced electron emission has been a matter of an enormous amount of work, a permanent revival of its interest is prompted by new experiments promoting new theories and indeed a lot  of debate.     In this work, we report scattered ions (LEIS spectra) and electron emission of electrons (SEE spectra)coming from Highly Oriented Pyrolytic Graphite excited by low energy He+ ,Ne+ ,Ar+ and Li+ bombardment, showing a high energy structure for the electron spectra induced by the lightest noble gas ions. Through a full quantum dynamic calculation, including properly the resonant neutralization to the ground and first excited states of helium, we ascribe these high energy electrons to the electron promotion (electron hole creation) to conduction band levels close to the vacuum (exciton), followed by an Auger transition annihilation of the pair. The same calculation in the case of Li, but including only the neutralization to the ground state, is also able to explain the absence of high energy electron emission observed in this case.