Studies of atomic oxygen in O2+CF4 rf discharges by two-photon laser-induced fluorescence and optical emission spectroscopy

Abstract

We have used two-photon laser-induced fluorescence to obtain quantitative measurements of the concentration of ground state O atoms in O2+CF4 rf discharges. Absolute calibration was achieved by generating a known concentration of atomic oxygen by UV laser photolysis of O2. Trace amounts of Ar were added to serve as an inert reference gas for concurrent optical emission measurements, in which the plasma-induced optical emission intensities from O* and Ar* lines were recorded. Emission line shapes were measured using a Fabry–Perot interfermoter to gain information on the mechanisms for formation of excited oxygen atoms in the plasma. Two excitation mechanisms were found to be important: (1) electron impact excitation of ground state atoms, e+O → O*+e, and (2) dissociative excitation of O2, e+O2 → O*+O+e. Evidence for both excitation mechanisms was obtained for O* (8446 Å) emission, with atomic excitation being dominant, whereas dissociative excitation appeared to be the dominant mechanism for O* (7774 Å) emission. Argon actinometry for the determination of ground state oxygen was directly tested. Because of the contribution from dissociative excitation, a strict proportionality, O*/Ar*∝[O]/[Ar], was not satisfied where O* (Ar*) is the intensity of an atomic oxygen (argon) emission line, and [O] ([Ar]) is the oxygen (argon) atom concentration. However, within certain limitations, the O* (8446 Å)/Ar* emission intensity ratio gives the right qualitative trends for the O atom concentration.

Use of particle-in-cell simulations to improve the actinometry technique for determination of absolute atomic oxygen density

“…Work done by previous authors has shown that application of the actinometry method does not always yield results that reflect the true atomic oxygen number density in the plasma. The validity of actinometry for atomic oxygen has been investigated for various plasma sources: RF capacitive plasma [14,15,16], ECR source [17], DC glow discharge [18], microwave source [19,20], helicon source [21], inductive source [22], and micro-scale atmospheric pressure jet source [23]. Walkrup et al [14] investigated the validity of actinometry for atomic oxygen in RF capacitive plasmas containing both pure O 2 and O 2 /CF 2 mixtures by examining line emission intensity ratios.…”

“…The validity of actinometry for atomic oxygen has been investigated for various plasma sources: RF capacitive plasma [14,15,16], ECR source [17], DC glow discharge [18], microwave source [19,20], helicon source [21], inductive source [22], and micro-scale atmospheric pressure jet source [23]. Walkrup et al [14] investigated the validity of actinometry for atomic oxygen in RF capacitive plasmas containing both pure O 2 and O 2 /CF 2 mixtures by examining line emission intensity ratios. The plasma emission for atomic oxygen at 844 nm O * (844), 777 nm O * (777) and argon at 750 nm Ar * (750) were recorded and the O * (844)/Ar * (750) and O * (777)/Ar * (750) ratios were monitored and compared with [O] results obtained using TALIF at a fixed total pressure of 400 mTorr.…”

Diagnostic based modeling for determining absolute atomic oxygen densities in atmospheric pressure helium-oxygen plasmas

“…This approach can provide a qualitative measurement of the atomic oxygen ground state density in low pressure plasmas with not too low degree of dissociation. 9 The determination of absolute O-atom densities in cold atmospheric pressure plasmas is, however, significantly more complex.…”

“…Nevertheless, dissociative excitation can become even dominant toward lower dissociation degrees. 9 The corresponding formal extension of the actinometry approach is possible. However, detailed information on the time and space dependent EEDF in the relevant energy range is required for quantitative density determination.…”

Surface oxygen exchange between yttria-stabilised zirconia and a low-temperature oxygen rf-plasma

“…The emission from an excited state of the actinometer gas into the ground state is measured. For O 2 plasmas this actinometer gas is usually Ar [28]. The emission intensities of Ar* and O* are measured simultaneously.…”