INTERACTION OF LOW-ENERGY ELECTRONS WITH GASEOUS AND CONDENSED PERFLUORO COMPOUNDS

Formation of anions induced by collisions of low energy electrons (0-1 4 eV) with the perfluorinated compounds CF4, C2F6, C2F4, and C6F6 is s tudied (a) in the gas phase under collision-free conditions in a beam experiment, and (b) in the condensed phase where the electron beam int eracts with the molecules deposited under ultrahigh vacuum conditions in definite amounts on a cold (20 K) metallic substrate. In the gas ph ase different fragment anions [F-, (M-F)(-), CF3-] are formed via diss ociative attachment (DA). From condensed molecules, desorption of anio ns is virtually restricted to F-. This can be explained by the polariz ation interaction of the excess charge and the desorption kinematics p referring desorption of light fragment ions. Below 10 eV F- desorption is induced by DA at or near the surface while above 10 eV a nonresona nt signal from dipolar dissociation is observed. The present results i ndicate 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 CF4- is assig ned as the T-2 shape resonance with its pronounced sigma(CF) antibond ing nature. This results in F- with high kinetic energy release, and, consequently, a high desorption cross section. In C2F6 the decompositi on of the transitory anion is less direct resulting in a comparatively low desorption yield. In C2F4 and C6F6, on the other hand, electron c apture proceeds via the pi system associated with indirect (statistic al) decomposition processes. This results in a very low desorption cro ss section for both compounds. (C) 1995 American Institute of Physics.