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.

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.

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(a1Δg, 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(b1Σg +); as a consequence, the number density of O2(b1Σg +) is (at least) one order of magnitude less than O2(a1Δg).