Robert Peverall

This paper presents spatially-resolved measurements of the absolute ground state atomic oxygen and oxygen anion densities close to the grounded electrode in an inductively coupled oxygen plasma obtained using cavity ringdown spectroscopy. Measurements on two spin-orbit states of atomic oxygen ( ³ P ₂ , ³ P ₁ ) confirm that their populations are in thermal equilibrium and are well described by the translational temperature, such that the total atomic density can be deduced using only a single spin-orbit resolved spectroscopic transition ( ¹ D ₂ ← ³ P ₂ ). Atomic densities range from 0.4 - 2.9 × 10 ¹⁴ cm ⁻³ at a height of ≈20 mm above the electrode and over the pressure range 10 - 100 mTorr at fixed power of 300 W. Under conditions of 300 W and 100 mTorr, the atomic density shows a moderate increase of ≈45% as the electrode surface is approached, while the line-of sight average translational temperature falls from ca. 450 K towards 300 K. By contrast, the oxygen anion density falls sharply with increasing pressure and towards the electrode. The maximum anion density (ca. 1.5 × 10 ¹⁰ cm ⁻³ ) occurs in the 10 - 20 mTorr range and falls by a factor of >5 over a range of 20 mm. This change is broadly consistent with a one-dimensional model for positive ion flux in an electronegative plasma. Further laser absorption measurements on the metastable O( ⁵ S) state are used as a proxy for plasma power density and show that the small changes in geometry induced by vertically translating the ground electrode have only a minor effect.

This paper presents a unique study of the bulk plasma characteristics in a low pressure inductively coupled nitrogen plasma. Saturated cavity ringdown spectroscopy (sat-CRDS) has been used to determine the absolute number densities and translational temperatures of N₂ ⁺(X, ν = 0). The effect of saturation is readily accounted for by using an effective saturation parameter, S _eff, and determined by a simple method employing measurements at two different gain settings of the detection system. The appropriateness of this method is confirmed by comparison with fitting individual ringdown data using a time-dependent saturation parameter, S(t), within the local approximation model for sat-CRDS; the two methods are in excellent agreement in returning absolute number densities and translational temperatures. N₂ ⁺(X, ν = 0) number densities are determined across a matrix of pressure (10−100 mTorr) and radio-frequency power (200−400 W) conditions with maximum number densities of ca. 1.3 × 10¹⁰ cm^–3 while translational temperatures range from 600−1500 K.

We report the development of a novel variant of cavity ringdown polarimetry using a continuous-wave laser operating at 532 nm for highly precise chiroptical activity and magnetometry measurements. The key methodology of the apparatus relies upon the external modulation of the laser frequency at the frequency splitting between non-degenerate left- and right-circularly polarized cavity modes. The method is demonstrated by the evaluation of the Verdet constants of crystalline CeF3 and fused silica, in addition to the observation of gas- and solution-phase optical rotations of selected chiral molecules. Specifically, optical rotations of (i) vapors of α-pinene and R-(+)-limonene, (ii) mutarotating D-glucose in water, and (iii) acidified L-histidine solutions are determined. The detection sensitivities for the gas- and solution-phase chiral activity measurements are ∼30 and ∼120μdeg over a 30 s detection period per cavity round trip pass, respectively. Furthermore, the measured optical rotations for R-(+)-limonene are compared with computations performed using the TURBOMOLE quantum chemistry package. The experimentally observed optically rotatory dispersion of this cyclic monoterpene was thus rationalized via a consideration of its room temperature conformer distribution as determined by the aforementioned single-point energy calculations.

Cavity ringdown spectroscopy has been used to investigate the translational and rotational temperatures of the v = 0 and 1 vibrational levels of O₂(X) in an inductively coupled plasma at 100 mTorr (13.332 Pa) pressure. All rotational states probed display a clear increase in temperature as plasma power increases: at fixed power, the translational temperature appears largest for rotational states in v = 1 (J= 7, 16, 17, 22) and lowest in the low energy states (J = 1, 18, 19) in v = 0; highly excited rotational states (J = 28, 30, 31) of v = 0 show intermediate behaviour. The rotational temperature values behave similarly. These observations are consistent with the effects of plasma inhomogeneity and can be interpreted with a simple one-dimensional model whereby the pressure, temperature and mole fractions of the various species across the chamber (and arms) are approximated with rational profiles and the corresponding line-of-sight averaged densities and temperatures calculated. This basic model is reasonably successful at reproducing the observations for O₂(X, v = 0) and O(³P) densities. The fact that resolving several rotational transitions allows spatial variations within the plasma to be inferred from line-of-sight averaged measurements is an extremely powerful result that could be of great utility in future work.

The absolute number density of ground state oxygen atoms, O(³P), present in a 100 mTorr oxygen plasma has been determined as a function of operating power using cavity ringdown spectroscopy (CRDS). The dissociation fraction increases by an order of magnitude from ∼0.8% at 50 W to 8% at 250 W and reflects a similar increase in the electron density over this power range. Emission spectra show that the E–H switchover is accompanied by increased rotational heating of O₂ and this behaviour is also observed in the translational temperatures determined by fitting the Doppler limited O(³P) CRDS data. The measurements are contextualised via a volume averaged kinetic model that uses the measured absolute densities of O(³P) and O₂(a ¹Δ_g, v = 0) as a function of power as its benchmarks. Despite the inherent spatial inhomogeneity of the plasma, the volume averaged model, which uses a minimal set of reactions, is able to both reproduce previous measurements on the absolute density of O⁻ and to infer physically reasonable values for both the electron temperature and number density as the E–H switch over is traversed. Time-resolved emission measurements return a value of 0.2 for the wall loss coefficient for O₂(b ¹Σ_g ⁺); as a consequence, the number density of O₂(b ¹Σ_g ⁺) is (at least) one order of magnitude less than O₂(a ¹Δ_g).

We present measurements of the densities and temperatures (rotational and translational) of the metastable a¹Δ_g (v = 0) state of O₂ 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 O₂(b¹Σ_g ⁺) ← O₂(a¹Δ_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 O₂(a¹Δ_g, v = 0) was measured as (9.5 ± 1.3) × 10¹³ 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 O₂(b¹Σ_g ⁺) and an upper limit for the vibrational temperature places the total contribution of O₂(a¹Δ_g) at between 3.6% and 5.85%. The variation of the O₂(a¹Δ_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 O₂(a¹Δ_g, v = 0) of 1.1 × 10¹² cm⁻³ over 100 ringdown events, an order of magnitude more sensitive than previously reported.

Ketone testing is an important element of the self-management of illness in type 1 diabetes. The aim of the present study was to see if a breath test for acetone could be used to predict quantitatively the levels of the ketone betahydroxybutyrate in the blood of those with type 1 diabetes, and thus be used as an alternative to capillary testing for ketones. Simultaneous capillary ketones and breath acetone were measured in 72 individuals with type 1 diabetes attending a diabetes clinic and on 9 individuals admitted to hospital with diabetic ketoacidosis. Capillary blood measurements ranged from 0.1 mmol l⁻¹ (the lower limit of the ketone monitor) to over 7 mmol l⁻¹, with breath acetone varying between 0.25 and 474 parts per million by volume. The two variables were found to be correlated and allowed modelling to be carried out which separated breath acetone levels into three categories corresponding to normal, elevated and ‘at risk’ levels of blood ketones. The results on this limited set of participants suggest that a breath acetone test could be a simple, non-invasive substitute for capillary ketone measurement in type 1 diabetes.

In the current pilot study we aimed to determine whether breath analysis could be used to help recognise intra-abdominal infection, using acute appendicitis as an exemplar condition. Our study included 53 patients (aged 18–88 years) divided into three groups: appendix group, 26 (13 male) patients suffering from acute appendicitis; control group 20 (seven male) patients undergoing elective abdominal surgery; normal group, seven patients who were clinically diagnosed with appendicitis, but whose appendix was normal on histological examination. Samples of breath were analysed using ion molecule reaction mass spectroscopy measuring the concentration of volatile compounds (VCs) with molecular masses 27–123. Intraperitoneal gas samples were collected from a subset of 23 patients (nine diagnosed with acute appendicitis). Statistically significant differences in the concentration of VCs in breath were found between the three groups. Acetone, isopropanol, propanol, butyric acid, and further unassigned VCs with molecular mass/charge ratio (m/z) 56, 61 and 87 were all identified with significant endogenous contributions. Principle component analysis was able to separate the control and appendicitis groups for seven variables: m/z = 56, 58, 59, 60, 61, 87 and 88. Comparing breath and intraperitoneal samples showed significant relationships for acetone and the VC with m/z = 61. Our data suggest that it may be possible to help diagnose acute appendicitis by breath analysis; however, factors such as length of starvation remain to be properly accounted for and the management or mitigation of background levels needs to be properly addressed, and larger studies relating breath VCs to the causative organisms may help to highlight the relative importance of individual VCs.