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Absolute partial and total electron impact ionization cross sections for CF4 from threshold up to 180 eV

Negative ion formation following low energy (0–10 eV) electron attachment to free and bound CF2Cl2 molecules is studied in (1) a molecular beam experiment (single molecules, homogeneous clusters, and mixed CF2Cl2/NH3 clusters) and (2) a UHV surface experiment where desorption of negative ions from condensed CF2Cl2 is observed. From single gas phase CF2Cl2 molecules we observe Cl− and F− generated via dissociative electron attachment from a resonance near 0 eV and 3 eV, respectively, as the most abundant ions. From homogeneous clusters (CF2Cl2)n, we additionally detect undissociated complexes of the form (M)n−(M=CF2Cl2) including the stabilized monomer CF2Cl2− and also “solvated fragment ions” of the form Mn⋅X−(X=Cl, F). Their relative abundance vs size (n) of the final product varies in a significant different way between (M)n− and Mn⋅X− reflecting the different relaxation probabilities in the initial cluster. In the desorption spectra, the dominant low energy Cl− gas phase resonance is strongly suppressed in favor of a significant resonant feature appearing near 8 eV. These last results are discussed in light of previously reported giant enhancements of electron induced desorption of Cl− and F− from CF2Cl2 on Ru coadsorbed with water or ammonia 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)].

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The semiclassical Deutsch-Märk (DM) formalism was used to calculate absolute cross sections for the electron-impact ionization of metastable atoms such as metastable rare-gas atoms and metastable mercury and cadmium atoms from threshold to 200 eV. Systematic trends in the calculated cross section data are discussed and a comparison is made with available experimental data and with other calculations. Specifically, we calculated separately the contributions to the ionization cross sections arising from the removal of the single excited electron in the outermost subshell and the removal of the lower-lying inner-shell electrons.

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