Negative ion formation from low energy (0-15 eV) electron impact to CF2Cl2under different phase conditions

Negative ion formation following low energy (0-10 eV) electron attachment t o free and bound CF2Cl2 molecules is studied in (1) a molecular beam experi ment (single molecules, homogeneous clusters, and mixed CF2Cl2/NH3 clusters ) and (2) a UHV surface experiment where desorption of negative ions from c ondensed CF2Cl2 is observed. From single gas phase CF2Cl2 molecules we obse rve Cl- and F- generated via dissociative electron attachment from a resona nce near 0 eV and 3 eV, respectively, as the most abundant ions. From homog eneous clusters (CF2Cl2)(n), we additionally detect undissociated complexes of the form (M)(n)(-)(M=CF2Cl2) including the stabilized monomer CF2Cl2- a nd also "solvated fragment ions" of the form M-n.X-(X=Cl, F). Their relativ e abundance vs size (n) of the final product varies in a significant differ ent way between (M)(n)(-) and M-n.X- reflecting the different relaxation pr obabilities in the initial cluster. In the desorption spectra, the dominant low energy Cl- gas phase resonance is strongly suppressed in favor of a si gnificant resonant feature appearing near 8 eV. These last results are disc ussed in light of previously reported giant enhancements of electron induce d desorption of Cl- and F- from CF2Cl2 on Ru coadsorbed with water or ammon ia ices under 250 eV electron impact [Q. B. Lu and T. E. Madey, Phys. Rev. Lett. 82, 4122 (1999); J. Chem. Phys. 111, 2861 (1999)]. (C) 2000 American Institute of Physics. [S0021-9606(00)01548-8].