Rigorous theoretical analysis of the continuous wave optogalvanic effect in the neon positive column

Abstract

A rate-equation model with greatly improved quantitative rigour is detailed for the CW optogalvanic effect on the 1s-2p transitions of neon atoms in the positive column of a dc normal glow discharge. This work constitutes one part of a wider complementary programme which also includes CW laser collisionally-induced fluorescence, optical emission spectroscopy and optical absorption spectroscopy for the excited-state populations, all employing the same atomic and discharge data set. Our aim has been to produce a theoretical model and test it with stringently collected data, to demonstrate that tunable laser CW optogalvanic spectroscopy (OGS) can provide a truly quantitative diagnostic of the excited-state kinetics in low-temperature discharges. The model is deliberately restricted to just six essential perturbed rate equations, four for the 1s states, one for the charged particles and one for the discharge current. Our formulation, via the 1s and 2p CW-induced pump rate perturbations, allows very direct identification of the important excited-state kinetics producing the OGE. The principles and scope of our theory are demonstrated for a neon filling pressure of 2.0 Torr and currents of 1-10 mA, based on fitting the model for three carefully selected transitions, the 1s5-2p4 (594.5 nm), 1s4-2p4 (609.6 nm) and 1s5-2p9 (640.2 nm). Results show that the magnitudes of the CW optogalvanic effect on the 1s-2p transitions are strongly dependent on the discharge pumping rates of the 1s states and their coupling, and confirms that cascade effects must be included in the 1s excitation rate coefficients.