The application of CW laser collisionally induced fluorescence modelling to determine neon excited-state electron collisional rate coefficients

Abstract

We describe a method for the determination of rate coefficients for collisional excitation out of the 1s states of a noble gas using continuous-wave laser collisionally induced fluorescence (CW LCIF, i.e. fluorescence from an upper level which is not that of the laser transition). The intensity of CW LCIF lines from an upper 2p level that is strongly coupled to an influential 1s level is dominated by the electron collisional rate out of that level. By an appropriate choice of LCIF line we fit our recently published theory to experimental observations to determine a set of electron collisional direct 1s mixing and 1s-2p excitation rate coefficients. The values of coefficients determined in this way show very good agreement with those which have been calculated using cross section data from the literature with the measured electron temperature. For cases where accurate cross section data are available for the 1s-2p transitions CW LCIF diagnostics should, via these rate coefficients, provide reliable values of the bulk electron temperature.

Investigation of the neon 2p collisional excitation transfer processes via CW laser collisionally induced fluorescence

“…In general, as we detailed previously [3,5], CW LCIF is governed by the effect of the 1s density perturbations. This often has the effect of overshadowing the role of the 2p excitation transfer, although in some instances a relatively even balance exists.…”

“…We have developed a new diagnostic technique based on laser collisionally induced fluorescence (LCIF), which employs CW laser excitation in noble gases. Previously, using our CW LCIF model [3], supported by data from tuneable laser optogalvanic spectroscopy [4], we have made accurate determinations of the very important 1s coupling and 1s-2p excitation coefficients [5]. In this paper we extend our CW LCIF application even further to show that the technique can also be used to determine the noble gas 2p collisional excitation transfer coefficients.…”

“…The model therefore allows each contribution to the observed LCIF signal to be quantified for any 2p-1s transition, so we can determine specific collisional coefficients between the 1s and 2p states, as demonstrated in [5]. Here we utilize this quantitative description of the role of the 1s perturbations and excite a range of 1s-2p transitions (table 1) aimed at isolating the collisional 2p coupling processes by identifying those 2p levels which are strongly populated by excitation transfer from the laser-excited 2p level.…”

“…where M ij and M ji are the coefficients for the ground-state atom collisional transfer from 2p i to 2p j and 2p j to 2p i respectively, g i and g j represent the statistical weights of the 2p i and 2p j states, E ij is the energy separation between the two states, k is the Boltzmann constant and T g is the gas temperature. 2p 3 -2p 4 0.17 ± 0.02 0.15 0.15 ± 0.01 0.12 ± 0.04 0.17 ± 0.07 2p 4 -2p 5 1.…”

“…In these calculations we applied the same method as described previously [5] to determine values of the rate coefficients for 1s-2p excitation by the low-energy electrons in the bulk of the energy distribution as functions of the reduced electric field.…”