Optogalvanism in a neon plasma

The optogalvanic effect (OGE) in low‐power, low‐temperature, radio‐frequency (RF) plasma in neon has been studied by pulse excitation of various 2 p 5 3 s →2 p 5 3 p (i.e., 1 s j →2 p k in Paschen notation) neon transitions and subsequent observation of the return to equilibrium of the separate ionization and acoustic OGE components. Of the four 1 s j states, the 1 s 3 and 1 s 5 metastables are responsible for plasma maintenance and the 1 s 2 and 1 s 4 states are radiatively trapped. The 10 2 p k states merely serve as doorways for a laser‐induced redistribution of the 1 s j populations. The ionization component, apart from signs, is identical for all excitations and is conditioned by two sets of kinetic events: the return to equilibrium of the 1 s 3 and 1 s 5 metastable populations and the radiative trapping of the 1 s 2 state. The acoustic components, while conditioned by the same events as well as by trapping of the 1 s 4 state, vary considerably from one 1 s j →2 p k excitation to another and from the ionization component. All differences are explicable on the basis of the following model: (1) All nonradiative 2 p k →1 s j and 1 s 3, 5 → 1 S 0 deexcitations launch an acoustic wave at the site of laser excitation, but this wave must travel at the speed of sound to regions of high ion densities where, by a physical movement of the charge carriers, it registers an OGE effect. (2) The 1 s 2, 4 → 1 S 0 nonradiative deexcitations also launch an acoustic wave, but this wave produces an instantaneous OGE effect because no travel requirement are imposed on it. A random walk of trapped states at the speed of light ensures that these states can dump their energy uniformly throughout the plasma and, specifically, that they can do so where the ion densities are high. That is, they do not have to travel at the speed of sound from the site of laser excitation to the region of high carrier densities. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 369–377, 1999