A low-pressure (1-10 hPa) pulsed microwave discharge (2.45 GHz) in oxygen of power 100-1000 W operated in a cylindrical vessel is investigated by miscellaneous diagnostic techniques to determine space- and time-resolved concentrations of the most important species. The electron density is measured by monitoring the detuning of a Fabry-Perot resonator, which is in resonance at lambda = 337 mu m (HCN-laser). The same technique is used to detect those electrons generated when a sufficiently strong laser beam crosses laterally the intracavity HCN laser mode and destroys O- and O-2(-) ions in the discharge by photodetachment. During the release of photodetached electrons the microwave field in the discharge chamber changes. This field alteration is utilized to determine the n(O-)/n(O2-) ratio, which is about 7 over the entire pulse. The concentration of rovibrational states of O-2 and vibrational temperatures are measured by coherent anti-Stokes-Raman scattering (CARS) as well as O concentrations by two-photon-allowed laser induced fluorescence (TALIF).A global kinetic model has been developed to describe the production of various species such as oxygen atoms, metastable molecules, ozone and charged particles. The experimentally observed and theoretically predicted temporal behaviour of thr oxygen atom density are in a good agreement, if wall losses of type O --> 1/2O(2) with a probability of 0.004 are taken into account. Both experiment and modelling show a sharp enhancement of the electron density at the instant of the pulse switch-off. The effect has been explained to result mainly from the processes of collisional detachment of O- ions by collisions with metastable molecules O-2(a) and oxygen atoms after switch-off.

Investigations on the breakdown and the maintenance phase in a pulsed N2 microwave plasma at 2.45 GHz under low pressure (1-10 mbar) have been carried out. The discharge chamber has a cylindrical geometry with a length of 80 mm and a radius of 45 mm. Microwave pulses with a duration of 50-200 microseconds and a repetition rate of 10-500 Hz were typical for the experiments. The behaviour of the electric field in the duct has been measured and calculated. The electron number density has been determined using an HCN laser interferometer and a Langmuir probe. The microwave power absorbed in the plasma was obtained measuring the forwardly directed and the reflected time resolved power flux. The vibrational excitation of the N2 molecules in the plasma was studied with spatial and temporal resolution on the basis of the coherent anti-Stokes Raman scattering (CARS) technique.

A pulsed microwave discharge burning close to the centre of a cylindrical chamber (/2 = 2.45 GHz) in pure nitrogen at low pressure (5 mbar) has been investigated theoretically. Boltzmann equation analysis is applied for the description of the breakdown and the consecutive maintenance phase of the discharge. The breakdown electric field is estimated using the calculated ionization frequency and the losses by the free electron diffusion. A self-consistent model for electron and heavy-particle kinetics coupling the rate master equations for the vibrational levels N2(X,v) of the electronic ground state of the nitrogen molecule to the Boltzmann equation of electrons has been developed. Besides the momentum transfer, inelastic as well as superelastic e-V collisions, ionization and free diffusion of electrons have been taken into account in the Boltzmann equation. The balance of the populations of the vibrational levels includes e-V, V-V and V-T exchanges, dissociation, atom re-association, vibrational deactivation on the walls and the effect of molecular diffusion. A further relation used is the global energy balance of electrons. The calculated breakdown electric field strength and the temporal course of the population of vibrational levels have been compared with experimental breakdown data and CARS measurements. Calculations and measurements are found to agree well.

Coherent anti-Stokes Raman scattering (CARS) carried out using the BOXCARS technique is applied to quantify the density and temperature of hydrogen, methane, and methyl in a process plasma generated by a surfatron in the pressure range from 30 to 3000 Pa. Below approximately 500 Pa all species and their temperatures have a flat distribution dominated by diffusive processes. At higher pressure the plasma localizes near the inner surface of the surfatron tube. A strong depletion of the methane ground state density is observed, where the gas temperature remains at 400 K on a moderate level. On the other side the methyl density is high at the position of high methane depletion. The measured methyl density is a factor of 200 lower than the methane density, in contrast to the factor 10–20 often reported in the literature. The measured data can be interpreted by proper modeling by means of the Boltzmann equation of electrons, balancing of chemical reactions and diffusive processes. The calculated methane and methyl densities agree with the measured ones only if the electron temperature is assumed to be near 1 eV and thus much lower than predicted in previous papers.

Resonance enhanced CARS and LIF have been applied to the CH radicals in a microwave excited Ar/H\(_2\)/CH\(_4\) plasma (\(p = 21\) Pa, \(P_{\rm Mikro} = 2.7\) kW). Both techniques yield similar nonthermal rotational population distributions of CH(X\(^2\Pi_r\)) in its vibrational ground state (\(v=0\)), which can be described by two rotational temperatures, \(T_{\rm rot,1} \approx 600\) K being in the order of the gas temperature for rotational states with \(N \le 7\), and a considerably higher \(T_{\rm rot,2}\) for the higher rotational states. This result is in goodagreement with previous resonance CARS and LIF measurements in similar plasmas. With resonance CARS additional measurements on CH in the \(v=1\) state could be performed yielding a vibrational temperature of 2440 K, the total CH density was about \(1.6 \times 10^{18}\) m\(^{-3}\). The detection limits of both techniques are determined, in our case about \(2 \times 10^8\) CH radicals per quantum state in the detection volume, and their advantages and disadvantages are discussed.

Methyl (CH3) is generated by photodissociation of methyliodide (CH3I) applying an excimer laser operating at 248 nm. The CH3 is detected using coherent anti-Stokes Raman spectroscopy (CARS). The CH3 density is deduced by monitoring simultaneously the reduction of the CARS signal of the parent molecule CH3I. CARS spectra of CH3 (v = 3005 cm−1) are recorded and the rotational temperature is evaluated. The CARS signal intensities are compared with those of deuterium and methane in order to obtain an absolute calibration of the Raman cross section. The measurements yield a Raman cross section of dσ/dΩ = 7 × 10−31cm2/sr at λP = 532 nm.

Resonance enhanced coherent anti-Stokes Raman scattering (RECARS) is applied to the CH radical produced in a microwave (2.45 GHz) excited ArCH4 plasma. The electronic A2Δ − X2Πr transition of the CH radical is used to obtain a resonance enhancement of the rotational CARS lines. With this technique rotational lines up to N = 15 and up to v = 2 are measured. In good agreement with earlier LIF measurements in similar plasmas we obtain two rotational temperatures for CH(X2Πr), Trot,1 = 671 ± 32 K for N < 8, and Trot,2 = 1400 ± 91 K for N ≥ 8. The vibrational temperature is about 2800 K and the CH density is estimated to be in the range of 1010 to 1011 cm−3.

Coherent anti-Stokes Raman scattering (CARS) is applied to a microwave (2.45 GHz) excited plasma used for plasma enhanced chemical vapor deposition. The applicability of CARS to low pressure (2 Pa) plasmas is verified, and the absence of saturation is carefully checked. Ground state concentration profiles as well as rotational temperatures of CH4 are presented with high spatial and temporal resolution. The measurements show a decrease of the CH4 ground state density under the influence of the plasma down to ≊25% (nCH4=1.0×1020 m−3) of its initial value. The rotational temperature is nearby room temperature across the total discharge volume. A straightforward modeling of the plasma explains the decrease of CH4 ground state densities as an effect of electron collisions and delivers an approximate value of the electron temperature of about 3 eV.