ANION YIELDS PRODUCED BY LOW-ENERGY ELECTRON-IMPACT ON CONDENSED HYDROCARBON FILMS

Low-energy (0-20 eV) electron impact on thin condensed hydrocarbon films is observed to produce, via dissociative attachment and dipolar dissociation, significant yields of H- and much lower yields of CH(n)- (n = 1-3) fragment anions. These yields have been measured as a function of incident electron energy and film thickness for the saturated n-C(n)H2n+2 (n = 2, 4-9) and for the unsaturated 1-C(n)H2n (n = 2-4) hydrocarbons. Each type of anion desorbing from the saturated hydrocarbon films exhibits a single dissociative attachment resonance at approximately 10 eV, the energy of which is found to vary only slightly with hydrocarbon chain length, in contrast with the trends observed in the gas phase for H- formation from these systems. The hydrogen anion yields produced from the unsaturated species are similar to those from the saturated species, while the larger fragment anions typically demonstrate asymmetric or bimodal yield distributions. The thickness dependence of the anion yields suggests that the observed H- anions are formed in the uppermost 3-5 layers of the film, while the larger desorbing anion fragments arise from the first 1-2 layers.