Quantitative measurements of oxygen atom and negative ion densities in a low pressure oxygen plasma by cavity ringdown spectroscopy

Abstract

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.

Quantitative measurements of singlet molecular oxygen in a low pressure ICP

“…At 300 W power and 100 mTorr pressure the total proportion of the gas that is O 2 (a 1 Δ g ) is likely to be ca 5%; as a qualitative comparison, this is significantly lower than, for example, the 12% reported in the case of O 2 /He mixtures [43], but similar to VUV absorption measurements in pure oxygen (4%) [16]. The translational and rotational temperatures measured for O 2 (a 1 Δ g ) are equilibrated and in good agreement with the translational temperature previously measured for O( 3 P) in the same chamber [22]. However, we note that care should be taken interpreting these line-of-sight temperatures, lest they are overly influenced by spatial inhomogeneity.…”

“…The laser source is a fibre coupled distributed feedback diode laser (Eblana, power ∼ 3 mW), operating at wavelengths around 1908 nm at room temperature and probing individual rotational lines within the O 2 (b 1 Σ g + ) ← O 2 (a 1 Δ g ) (0, 0) band. The CRDS arrangement is similar to that presented in [22,23], and uses an acousto-optic modulator to interrupt the probe laser, thus generating the requisite ringdown signals.…”

Quenching of O2(b1Σg + ) by O(3P) atoms. Effect of gas temperature

“…The density of O( 3 P) atoms was determined using sub-Doppler CRDS of the forbidden O( 3 P 2 ) → O( 1 D 2 ) transition around 630 nm, using a scheme similar to that presented by Peverall et al [31]. Time-resolved measurements in modulated discharges were also made, by recording the phase of each CRDS event relative to the current modulation, and sorting the measurements into time-bins [27].…”

“…where A low up is the Einstein coefficient for the O( 3 P 2 ) → O( 1 D 2 ) transition, g up is the degeneracy degree for the upper 1 D 2 state (g up = 5), g low is the degeneracy degree for the lower 3 P 2 state (g low = 5), g J = 5, 3 and 1 for J = 2, 1 and 0 respectively. The populations of the O( 3 P J ) states are close to equilibrium with the gas translational temperature [31]. The total number density of O( 3 P) atoms was then calculated from O( 3 P 2 ) density using the relation:…”

Mechanism for the Efficient Homogeneous Nucleation of Ice in a Weakly Ionized, Ultracold Plasma

“…This suggests that the conclusion by Baumann (2016) that all negative ions would be depleted by atomic oxygen does not occur in the presence of water vapor as the situation becomes more complicated than having only the O + O − → O 2 + e reaction. Peverall et al (2020) observed O − in the presence of a much larger atomic oxygen density in an O 2 discharge. The O − density was as much as five times larger than the free electron density; this observation further counters the assertion by Baumann (2016) that negative ions cannot exist in the presence of atomic oxygen.…”