A novel calibration method for the determination of absolute species densities by laser spectroscopy with two-photon absorption is presented. The method is based on a comparative measurement with a noble gas which has a two-photon resonance spectrally close to the transition investigated. Application of this scheme for the calibration of LIF measurements to determine atomic oxygen, generated in a capacitively coupled rf reactor, and for calibration of a RIS measurement of atomic oxygen and carbon generated by sputtering is demonstrated. The relevant excitation cross section of the involved 7p[3/2]2 xenon state, which is used for calibration of atomic oxygen densities, is determined.

The density of atomic oxygen in an oxygen rf discharge in the Gaseous Electronics Conference reference cell is determined from the intensity ratios of the argon λ=750 nm line (2p1–1s2) and the atomic oxygen lines λ=777 nm (5P–5S) and λ=844 nm (3P–3S). Laser induced fluorescence spectroscopy with two-photon excitation is applied to the oxygen plasma, and the results of both methods are compared. The improved actinometry is based on the calculation of electronic collisional excitation of the upper levels of these transitions. The required information on the electron energy distribution function is obtained from a model calculation of the discharge. Good agreement of the results are obtained, if excitations via dissociative channels and also quenching rates are accounted for.

The plasma of a 13.56 MHz capacitively coupled oxygen discharge of the Gaseous Electronics Conference Reference Cell type exhibits fluctuations of the plasma potential and of light emission in the kilohertz range. This behavior can be explained in terms of an attachment-induced ionization instability in connection with a sufficiently high fraction of negative ions. A necessary condition for the appearance of this type of instability is a strong positive dependence on electron temperature of the electron attachment coefficient describing the formation of negative oxygen ions. A strong volume loss of negative ions by neutral particles—metastable molecules in the present case—is an additional condition. Calculations of the temperature dependent coefficients for ionization and attachment by solving the time-independent Boltzmann equation in the two-term approximation in combination with the fluid equations of the charge carriers show that the criteria for the development of an attachment-induced ionization instability are fulfilled in the pressure domain around 30 Pa and for medium radio frequency voltages around 300 V.