Metastable CF and CF₂ molecules in CF₄ inductively-coupled plasmas

The radicals CF and CF₂, which are important intermediates in fluorocarbon plasma chemistry, both have low-lying metastable levels ( ⁴CF at 3.54 eV and ³CF₂ at 2.46 eV). Recent calculations (Rozum et al 2006 J. Phys. Chem. Ref. Data in press) indicate that electron-impact excitation of the ground-state radicals into these states could be fast. A recent study of inductively-coupled plasmas (ICP) in low-pressure CF₄ (Booth et al 2005 Plasma Sources Sci. Technol. 14 273) indicated the presence of a fast electron-impact induced loss process for ground-state CF and CF₂ molecules, which could be attributed to this process. In the current study ⁴CF and ³CF₂ were detected in the afterglow of ICP in pure CF₄ at pressures between 3 and 33 mTorr, from their weak forbidden optical emission back to their respective ground-states. From the lifetimes of these optical emission signals, determined as a function of gas pressure, the quenching coefficients at the chamber walls and the metastable destruction rates by gas-phase processes (giving unknown products) were estimated. Another prominent and long-lived feature of the afterglow is strong emission from the d state of C₂ molecules: the emitting C₂ molecules may be produced by chemiluminescent reactions or by excitation transfer from ³CF₂.