Enhanced formation of negative ions by electron attachment to highly excited molecules in a flowing afterglow plasma

Preliminary evidence for efficient negative-ion formation using a plasma mixing scheme was reported in a recent letter [L. A. Pinnaduwage, W. Ding, and D. L. McCorkle, Appl. Phys. Lett. 71, 3634 (1997)]. In the present article we confirm the negative ion formation using a probe-assisted photodetachment technique and estimate rate constants for electron attachment to electronically excited CH4 and NO in a flowing afterglow plasma. It is shown that enhanced electron attachment to molecules in highly excited states populated via excitation transfer from rare gas metastables is responsible for the observed negative ion formation. Implications for plasma processing and plasma remediation discharges are also discussed.