The reactivity of slow electrons with molecules at different degrees of aggregation: Gas phase, clusters and condensed phase

The interaction of free electrons in the energy range between 0 and 15 eV with molecules is reviewed. The studies include beam experiments with gas phase molecules under collision-free conditions and at higher pressures, homogeneous and heterogeneous clusters generated in supersonic beams, and molecules adsorbed and condensed on a cold metallic surface. In single molecules under collision-free conditions the usual relaxation process of a transient negative ion formed by free electron attachment is unimolecular decomposition into stable anionic and neutral fragments (dissociative attachment) with cross-sections exceeding 10^-14 cm². A remarkable exception is C₆₀ which forms metastable anions C^-₆₀ up to 14 eV electron energy. In clusters, intramolecular stabilization processes lead to the formation of stabilized molecular anions. In addition, intramolecular electron transfer processes can strongly contribute to anion formation in homogeneous and heterogeneous clusters. In condensed and adsorbed molecules, finally, effective desorption of negative fragment ions is observed when repulsive precursor ions are involved. The reactivity of transient anions formed by low energy attachment is generally strongly quenched with the degree of aggregation in favour of associative processes. In the case of core excited resonances, however, the reactivity can be enhanced by the surrounding medium. This effect is explained by the conversion of an open channel resonance in isolated molecules into a Feshbach resonance in clusters and condensed molecules.