Interaction of low energy electrons with gaseous and condensed perfluoro compounds

Formation of anions induced by collisions of low energy electrons (0–14 eV) with the perfluorinated compounds CF4, C2F6, C2F4, and C6F6 is studied (a) in the gas phase under collision-free conditions in a beam experiment, and (b) in the condensed phase where the electron beam interacts with the molecules deposited under ultrahigh vacuum conditions in definite amounts on a cold (20 K) metallic substrate. In the gas phase different fragment anions [F−, (M–F)−, CF−3] are formed via dissociative attachment (DA). From condensed molecules, desorption of anions is virtually restricted to F−. This can be explained by the polarization interaction of the excess charge and the desorption kinematics preferring desorption of light fragment ions. Below 10 eV F− desorption is induced by DA at or near the surface while above 10 eV a nonresonant signal from dipolar dissociation is observed. The present results indicate that the desorption cross section is essentially controlled by the amount of translational energy released to F− in the dissociation of the precursor anion (M−). In CF4 the precursor ion CF−4 is assigned as the T2 shape resonance with its pronounced σCF* antibonding nature. This results in F− with high kinetic energy release, and, consequently, a high desorption cross section. In C2F6 the decomposition of the transitory anion is less direct resulting in a comparatively low desorption yield. In C2F4 and C6F6, on the other hand, electron capture proceeds via the π* system associated with indirect (statistical) decomposition processes. This results in a very low desorption cross section for both compounds.