Recombination of chlorine atoms on plasma-conditioned stainless steel surfaces in the presence of adsorbed Cl₂

Abstract

We investigated the interactions of atomic and molecular chlorine with plasma-conditioned stainless steel surfaces through both experiments and modelling. The recombination of Cl during adsorption and desorption of Cl2 was characterized using a rotating-substrate technique in which portions of the cylindrical substrate surface are periodically exposed to an inductively coupled chlorine plasma and then to an Auger electron spectrometer in separate, differentially pumped chambers. After several hours of exposure to the Cl2 plasma, the stainless steel substrate became coated with a Si-oxychloride-based layer (Fe : Si : O : Cl ≈ 1 : 13 : 13 : 3) due to chlorine adsorption and the erosion of the silica discharge tube. Desorption of Cl2 from this surface was monitored through measurements of pressure rises in the Auger chamber as a function of substrate rotation frequency. Significant adsorption and desorption of Cl2 was observed with the plasma off, similar to that observed previously on plasma-conditioned anodized aluminium surfaces, but with much faster desorption rates that are most likely attributable to the smoother and non-porous stainless steel surface morphology. When the plasma was turned on, a much larger pressure rise was observed due to Langmuir–Hinshelwood recombination of Cl atoms. Recombination coefficients, γCl, ranged from 0.004 to 0.03 and increased with Cl-to-Cl2 number density ratio. This behaviour was observed previously for anodized aluminium surfaces, and was explained by the blocking of Cl recombination sites by adsorbed Cl2. Application of this variable recombination coefficient to the modelling of high-density chlorine plasmas gives a much better agreement with measured Cl2 percent dissociations compared with predictions obtained with a recombination coefficient that is independent of plasma conditions.

Global (volume-averaged) model of inductively coupled chlorine plasma: Influence of Cl wall recombination and external heating on continuous and pulse-modulated plasmas

“…It is also measured by Guha et al [5], and lower values are reported that also depend on the n Cl /n Cl 2 density ratio. Additionally, such a dependence is also observed for a stainless steel wall by Stafford et al [8]. We define these variable recombination probabilities by parametric fits [19] log 10 (γ A2 ) = −0.82 − 1.59 exp −1.81 n Cl n Cl 2 ,…”

“…This process has a major effect on the atomic and molecular chlorine densities in the plasma. This recombination rate has been measured by several groups, which have shown that it can depend on numerous parameters such as the atomic to molecular chlorine density ratio [5], the gas temperature [6], the pressure [7], the wall material [6,8], and even the surface coating history [9]. The role of the recombination probability has been studied via models by Lee et al [10] and is further discussed by Meeks et al [11] with experimental comparisons.…”

Experimental and modeling study of O and Cl atoms surface recombination reactions in O2 and Cl2 plasmas

“…At all substrate rotation frequencies, the pressure rises are higher for SS than for AA; we will come back on this point later on. One should note that when the substrate was spun with the plasma off, no increase was observed with rpm for both AA and SS substrates [28,34]. This indicates that, in our experimental conditions, O 2 (unlike Cl 2 ) [31,34] does not stick to the surface and therefore does not participate in the recombination reaction.…”

“…A "spinning-wall" technique was recently developed for studying surface reactions on plasma reactor walls [28][29][30][31][32][33][34][35][36][37]. This method couples the established ultra-high-vacuum techniques of desorption MS and AES to the high-pressure, high-charge-density plasma environment.…”

“…A small, cylindrical substrate is rapidly rotated through skimmers such that a point of the surface is periodically exposed to the plasma and is analyzed in near real-time by surface diagnostics in separate, differentially pumped chambers. This paper reviews and compares previously published spinning-wall studies of the recombination dynamics of O and Cl atoms on plasma-conditioned AA and SS surfaces [28][29][30][31][32][33][34][35]. Cl atom recombination rate was also obtained by fitting an isothermal fluid model to measured degrees of dissociation of Cl 2 obtained in low-pressure, high-density Cl 2 plasmas sustained by traveling electromagnetic surface waves.…”