Low energy (0-14 eV) electron impact to CHF2Cl at different phase conditions:: medium enhanced desorption of anions

The formation of negative ions following low energy (0-14 eV) electron impact to CHF2Cl under different phase conditions is studied including isolated gas phase molecules, free homogeneous clusters, and multilayer CHF2Cl 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 approximate to 6 eV, respectively. From clusters, we find solvated fragment ions of the form Cl-.(CHF2Cl)(n) and F-.((CHFCl)-Cl-2)(n) but no parent anions or undissociated larger cluster ions. Electron impact to condensed CHF2Cl 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. (C) 2000 Elsevier Science B.V.