Probing plasma boundary layers using saturated cavity ring-down spectroscopy Charlie Kniebe-Evans 1 , Samuel Rogers 2 , Robert Peverall 1 , Grant Ritchie 1 1 Department of Physical & Theoretical Chemistry, University of Oxford, UK 2 Wolfson Atmospheric Chemistry Laboratory, Department of Chemistry, University of York, UK This poster presents unique studies of bulk and sheath plasma characteristics in a low pressure inductively coupled nitrogen plasma. Saturated cavity ring-down spectroscopy (sat-CRDS) is utilised as an absolute, high resolution, calibration-free experimental technique to make sensitive measurements of the number densities of the influential ionic and metastable trace species N 2 + (X) and N 2 (A). The sensitivity of the cw-CRDS experiment facilitates determination of absorption coefficients with a limit of detection ≈ 1×10 -10 cm -1 , allowing ion number densities as low as 1×10 7 cm -3 to be resolved. Selective rotational lines and vibrational bands within the N 2 + (A 2 Π u ← X 2 Σ g + ) and the N 2 (B 3 Π g ← A 3 Σ u + ) electronic transitions have been studied and bulk-plasma N 2 + (X) and N 2 (A) translational temperatures observed in the ranges 650−1400 K and 550−1000 K, respectively. The bulk-plasma vibrational temperature of N 2 (A) has been determined to be ≈ 5000 K and the bulk-plasma rotational temperature ≈ 525 K. N 2 + (X) (ν = 0) number densities have been quantified and compared across a matrix of pressure (10−100 mTorr) and rf power (200−400 W) plasma conditions with maximum number densities recorded as ≈ 1×10 10 cm -3 . These trends are compared to those observed for N 2 (A) and supported by previous emission measurements. [1] Numerically verified methods have been implemented to quantify optical saturation effects and allow absolute number densities to be determined. The effects of saturation are explored further through an adjacent study on Lamb dips; a phenomena that results from optical saturation. Lamb dip widths are scrutinised and plausible broadening mechanisms considered. The contribution of hyperfine structure is considerable and verified by both measurement and simulations. The saturation spectroscopy studies are extended into the plasma sheath and presheath regions where a novel ‘wavelength switching’ experimental technique facilitates the quantification of absolute number densities as a function of height above a non-driven plasma chamber electrode. Previous work by Woodcock et al.[2] was able to resolve the acceleration of ions towards the driven electrode in a capacitively coupled plasma chamber, by the use of LIF. The ion density measurements in this work consolidate the work of Woodcock et.al. (albeit in a different sort of plasma chamber) yet have greater spatial resolution, allowing a clear decrease in absolute N 2 + (X) (ν = 0) number density to be observed as the electrode is approached, with a weaker effect observed for the N 2 (A) metastable species. CRDS has also been applied to determine the wall loss coefficients of N 2 (A) under various plasma conditions, which is vital information for plasma modelling. References 1. B Bakowski, G Hancock, R Peverall, G A D Ritchie, and L J Thornton 2004 J. Phys. D: Appl. Phys. 37 2064 2. B K Woodcock, J R Busby, T G M Freegarde, and G Hancock 1997 Journal of Applied Physics 81 5945
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