Competition between reactivity and relaxation in free electron attachment to molecules. A case study in C6F5X (X=F, Cl, Br, +I) under different stages of aggregation

The reactivity of the C6F5X (X=F, Cl, Br, I) molecules following low energy (0-15 eV) electron attachment is studied in the gas phase under single collision conditions, free molecular clusters and condensed molecules by means of crossed beams and surface experiments. All four molecules exhibit a very prominent resonance for low energy electron attachment (< 1 eV, attachment cross section > 10-14 cm2). Under collision free conditions thermal electron capture generates long lived molecular parent anions C6F5X-*. Along the line Cl, Br, I dissociation into X-+C6F5 and X+C6F5- increasingly competes until for X=1 only chemical fragmentation is observed on the mass spectrometric time scale. In free molecular clusters chemical fragmentation is quantitatively quenched at low energies in favour of associative attachment yielding undissociated, relaxed ions (C6F5X)-n, n≥1. A further dissociative resonance at 6.5 eV in C6F5Cl is considerably enhanced in clusters. If these molecules are finally condensed on a solid surface, one observes a prominent Cl- desorption resonance at 6.5 eV. While the quantitative quenching of the chemical reactivity at low energies is due to the additional possibilities of energy dissipation under aggregation, the enhanced reactivity at 6.5 eV is interpreted by the conversion of a core excited open channel resonance in single molecules into a closed channel (Feshbach) resonance when it is coupled to environmental molecules. (author)