Formation and evolution of negative ion resonances at surfaces

The behaviour of molecular negative ion resonances formed by free electrons at the surfaces of multilayer molecular films is studied by means of electron stimulated desorption (ESD) of negative ions. In the low energy region ESD usually proceeds via dissociative electron attachment (DEA) to an individual molecule at or near the surface. Three prototypical cases are discussed in light of their gas phase analoga. (A) In the ESD cross-section of F− from a multilayer NF3 film features (resonances) are considerably narrower with respect to the corresponding gas phase DEA analogues. This is a mirror of the particular dynamics of the unimolecular reaction in relation to the energy constraints operative in ion desorption. (B) The absolute DEA cross-section can appreciably be enhanced by the medium due to an increase of the autodetachment lifetime of the negative ion resonance. This is demonstrated in CHF2Cl for an electronically excited resonance. (C) The possibility of specific and effective bond cleavage in DEA is used to selectively synthesize Cl2 from a chlorofluoroethane compound (1,2-C2F2Cl4). It is shown that by selecting the electron energy to values below the threshold for electronic excitation, the extend of the reaction can be pushed completely onto the product side.