The plasma enhanced chemical vapor deposition of a-C:H films using methane and acetylene as precursors was studied. Noninvasive in situ techniques were used to analyze the plasma processes with respect to the self-bias voltage, the displacement currents to the grounded electrode, the neutral gas composition, the optical sheath thickness as well as current and energy of the ions hitting the powered electrode. The a-C:H films were characterized for their deposition rate, surface roughness, hardness, mass density, and hydrogen content. Ion mean free paths, suitable for low-pressure rf sheaths, have been quantified for both precursors. The film with the highest hardness of 25GPa was formed in the C2H2 discharge when the mean energy per deposited carbon atom was approximately 50eV. The hardness obtained with the CH4 discharge was lower at 17GPa and less sensitive to changes in the process parameters. It was found that the creation of hard (hardness >15GPa) a-C:H films from both precursors is possible if the mean energy per deposited carbon atom exceeds only ∼15eV. Further film characteristics such as surface roughness and hydrogen content show the interplay of ion flux and deposition from radicals to form the a-C:H structure and properties.

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Electron attachment to NCCCCN, dicyanoacetylene (2-butynedinitrile), has been observed. Metastable parent anions, NCCCCN , with microsecond or longer lifetimes are formed close to 0 eV electron energy with a cross Ϫ * section of ≥0.25 2. The stability of NCCCCN suggests that radiative attachment to NCCCCN and similar Ϫ * A linear carbon chain molecules may be an important mechanism for the formation of negatively charged molecular ions in astrophysical environments. CCCN and CN fragment anions are formed at ∼3 and ∼6 eV.

Fragmentation of metastable SF 6 − * ions formed in low energy electron attachment to SF 6 has been investigated. The dissociation reaction SF 6 − * → SF 5 − + F has been observed ϳ1.5-3.4 s and ϳ17-32 s after electron attachment in a time-of-flight and a double focusing two sector field mass spectrometer, respectively. Metastable dissociation is observed with maximum intensity at ϳ0.3 eV between the SF 6 − * peak at zero and the SF 5 − peak at ϳ0.4 eV. The kinetic energy released in dissociation is low, with a most probable value of 18 meV. The lifetime of SF 6 − * decreases as the electron energy increases, but it is not possible to fit this decrease with statistical Rice-RamspergerKassel/quasiequilibrium theory. Metastable dissociation of SF 6 − * appears to compete with autodetachment of the electron at all electron energies.