Electronically induced desorption; A specific localized radiation damage effect on surfaces

A survey is given of the effects of radiation (electrons, photons; to some extent ions) on surface layers which show up through liberation of particles (ions and neutrals) as a consequence of primary electronic excitations. It is shown that the observed characteristics of these effects (thresholds at valence as well as at core ionization energies, wide variations of cross sections with small changes of bonding to the surface, changing ion/neutral ratios, narrowly focussed angular distributions of ions and even of neutrals, energy distributions ranging up to 10 eV for ions and to 2 eV for neutrals, strong isotope effects) can be understood from rather general mechanisms with some common properties: A primary electronic excitation is followed by a rapid electronic rearrangement which can lead to transfer of energy from the electronic to the nuclear motion and which competes with delocalization of the energy. The total cross-sections and branching ratios can be determined by each of these factors. The relevance to radiation damage in ultramicroscopy may lie in the fact that here, too, transfer of electronic to nuclear motion is necessary for damage to occur, that again the competition of this transfer with delocalization of the primary excitation determines the overall efficiency of the damage process, and that therefore the resulting damage probabilities can be very different from the primary excitation cross-sections.