HO 2 reaction kinetics in an atmospheric pressure plasma jet determined by cavity ring-down spectroscopyTo cite this article before publication: Michele Gianella et al 2018 Plasma Sources Sci.

In this paper we report measurements of the absolute concentration of ground state oxygen atoms produced in a low pressure (≤100 mTorr) inductively coupled oxygen plasma. These experiments have utilised cavity ringdown spectroscopy, allowing line of sight absorption to be measured on the optically forbidden 1D ← 3P transition around 630 nm. Both the translational temperature and the absolute concentrations of the two most populated spin–orbit levels (J = 1 and 2) have been determined as a function of plasma pressure at a fixed operating power of 300 W, allowing accurate determination of dissociation fraction; in all cases, the dissociation fraction is considerable, ≥10%, maximising at 15% for 20 mTorr. Time-resolved measurements of the rate of loss of the oxygen atoms when the plasma is extinguished have allowed the probability for wall-loss in the plasma chamber, γ, to be determined; in this case, for an aluminium surface, γ is determined to be ca. a few ×10−3, with the exact value depending on pressure. In addition, the O− number density is shown to be an inverse function of pressure, showing a maximum of 1.6 × 1010 cm−3 at 10 mTorr, falling to 2 × 109 cm−3 at 100 mTorr, and characteristic of a discharge operating in the detachment regime. The measured number densities are interpreted using calculated electron energy distribution functions and yield physically reasonable values for the electron number density.

Cold atmospheric pressure plasmas (CAPs) are finding an increasing number of applications in diverse fields such as sterilization, medicine and dentistry, because they induce chemical reactivity at nearambient temperature. These plasmas are usually generated in noble gas flows which propagate into air, resulting in the production of a wide variety of species which can cause primary and secondary chemistry in both the gas and liquid phases. Detailed understanding of this low temperature reactivity requires selective and sensitive measurements of radical species. In this review we focus upon several techniques from a methodological point of view that are suitable for the sensitive detection of reactive species, or show promise for this purpose. A range of traditional and contemporary spectroscopic methods for measuring across different phases is highlighted in an attempt to present a 'detection landscape' for CAP-borne radicals.

We present measurements of the densities and temperatures (rotational and translational) of the metastable a 1 Δ g (v = 0) state of O 2 in a cylindrically symmetric RF driven plasma operating in inductive mode at 100 mTorr total pressure and 300 W applied power. Line-of-sight absorption across the plasma region was determined by diode laser cavity ringdown spectroscopy on the (0, 0) vibrational band of the O 2 (b 1 Σ g + ) ← O 2 (a 1 Δ g ) transition near 1.9 μm. Four rotational quantum states were studied, with a population distribution corresponding to a rotational temperature of 346 ± 38 K. The translational temperature was determined to be 359 ± 16 K from the width of the strongest absorption line, Q(12), and in equilibrium with the rotational distribution. The absolute concentration of O 2 (a 1 Δ g , v = 0) was measured as (9.5 ± 1.3) × 10 13 cm −3 , and corresponds to an apparent (3.5 ± 0.45)% contribution to the total number density. Time-resolved CRDS measurements following plasma extinction were used to deduce a wall loss coefficient, γ, of (2.8 ± 0.3) × 10 −3 on predominantly Al surfaces. Surmising reasonable concentrations for O 2 (b 1 Σ g + ) and an upper limit for the vibrational temperature places the total contribution of O 2 (a 1 Δ g ) at between 3.6% and 5.85%. The variation of the O 2 (a 1 Δ g , v = 0) state concentration with RF power shows a clear transition from the E to H mode excitation near an applied power of 150 W. Allan variance analysis yields a minimum measurable concentration of O 2 (a 1 Δ g , v = 0) of 1.1 × 10 12 cm −3 over 100 ringdown events, an order of magnitude more sensitive than previously reported.

We present the intra-cavity Faraday modulation spectroscopy technique, whereby optical feedback cavity-enhanced spectroscopy is coupled with Faraday modulation spectroscopy to greatly enhance the interaction path length of a laser beam with a paramagnetic sample in a magnetic field. We describe a first prototype based upon a cw quantum cascade laser targeting a selection of fundamental rovibrational R-branch transitions of nitric oxide (1890 cm), consisting of a linear cavity (finesse F=6300) and a water-cooled solenoid. We demonstrate a minimum detectable Verdet constant of V=4.7×10 rad cm G Hz (at SNR = 1), corresponding to a single-pass rotation angle of 1.6×10 rad Hz and a limit of detection of 0.21 ppbv Hz NO.