Excitation processes in an r.f.-boosted, pulsed hollow cathode lamp

Abstract

Time and space resolved emission spectroscopy was used to examine the excitation processes in an r.f.-boosted, pulsed hollow cathode lamp. Sealed commercial hollow cathode lamps with copper cathodes and neon or argon buffer gases were driven with temporally spaced unidirectional current pulses and radio frequency bursts. The neon filled lamp was studied most extensively; the argon lamp was used for comparison. Three excitation periods were considered: the current pulse, the r.f. burst, and the afterglow. Each of these periods was marked by a unique set of emission characteristics that suggested different combinations of excitation processes. Excitation during the current pulse appears to be a combination electron impact and charge exchange. Charge exchange excites the 3 d 95 s levels of Cull while electron impact excites the other ion levels and the neutral levels. Charge exchange continues to excite the ion spectrum during the afterglow. Afterglow emission from the neutral spectrum apparently results from two types of recombination processes. The exact recombination mechanisms could not be unambiguously assigned. Emission results from the r.f. burst suggest that electron impact is the dominant excitation process. Excitation of the neutral and ion spectra are from their respective ground states. There appears to be little ionization by electron impact. To evaluate the lamp's suitability as a line source for analytical atomic spectroscopy the profile of the 324.8 nm neutral resonance line was examined during the current pulse and the r.f. burst. The profile during the current pulse is strongly dependent on radial position in the cathode bore. Near the edges of the cathode the line is severely self-reversed. The profile during the r.f. burst is uniform across the cathode bore and is affected only slightly by self-absorption.