Temporal emission characteristics of millisecond pulsed radiofrequency and direct current glow discharges

Abstract

Interpretation of optical emission spectra reveals the primary excitation mechanisms for discharge gas, argon and sputtered analyte, copper, species in glow discharge plasmas operated with millisecond pulses of radiofrequency or direct current power. There is negligible difference between the two power sources. During the applied power pulse, plasma processes include ion and atom excitation through electron excitation, asymmetric charge exchange and Penning ionization. Fast ion and atom excitation processes, characterized by monitoring argon emission at 811.5 nm, occur within 2 mm of the cathode surface. Electron excitation, for both discharge gas and sputtered species, maximizes 3 mm from the cathode surface. Asymmetric charge exchange between ground state sputtered atoms and discharge gas ions, characterized by Cu II emission at 224.7 nm, occurs at 5 mm from the cathode surface. Upon power termination, the recombination of ions with thermal electrons yields excited atoms and argon metastable species. At this time, emission monitored at 811.5 nm maximizes 6–7 mm from the cathode surface, corresponding to an increase in the metastable population and, hence, Penning ionization.