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 unde r single collision conditions, free molecular clusters and condensed m olecules by means of crossed beams and surface experiments. All four m olecules exhibit a very prominent resonance for low energy electron at tachment (<1 eV, attachment cross section >10(-14) cm(2)). Under colli sion free conditions thermal electron capture generates long lived mol ecular parent anions C6F5X-. Along the line Cl, Br, I dissociation in to X-+C6F5 and X+C(6)F(5)(-)increasingly competes until for X=I only c hemical 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 greater than or equal to 1 A further dissociative resonance at 6.5 eV in C6F5Cl is considerably enhanched 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 l ow energies is due to the additional possibilities of energy dissipati on under aggregation, the enhanched reactivity at 6.5 eV is interprete d by the conversion of a core excited open channel resonance in single molecules into a closed channel (Feshbach) resonance when it is coupl ed to environmental molecules.