Abstract
The formation of negative ions following low energy (0–14 eV) electron impact to CHF 2Cl under different phase conditions is studied including isolated gas phase molecules, free homogeneous clusters, and multilayer CHF 2Cl films condensed on a metallic substrate at cryogenic temperature. In the gas phase we observe three very weak resonances on the Cl −, F −, and H − dissociative attachment (DA) channels peaking near 1.1, 3.4, and ≈6 eV, respectively. From clusters, we find solvated fragment ions of the form Cl − · (CHF 2Cl) n and F − · (CHF 2Cl) n but no parent anions or undissociated larger cluster ions. Electron impact to condensed CHF 2Cl leads to remarkably strong resonance features in the desorption yield with peak maxima at 9.7 (Cl −, F −) and 5.5 eV (H −). The absolute cross section for ion desorption is about 2 orders of magnitude higher than the gas phase DA cross section. This observation is explained in terms of a particular medium enhanced DA reaction at the surface based on the conversion of a short lived open channel resonance into a longer lived (closed channel) Feshbach resonance. Keywords: Negative ions; Dissociative attachment; Desorption
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