Chemical kinetics in an atmospheric pressure helium plasma containing humidity.

Atmospheric pressure plasmas are sources of biologically active oxygen and nitrogen species, which makes them potentially suitable for the use as biomedical devices. Here, experiments and simulations are combined to investigate the formation of the key reactive oxygen species, atomic oxygen (O) and hydroxyl radicals (OH), in a radio-frequency driven atmospheric pressure plasma jet operated in humidified helium. Vacuum ultra-violet high-resolution Fourier-transform absorption spectroscopy and ultra-violet broad-band absorption spectroscopy are used to measure absolute densities of O and OH. These densities increase with increasing H2O content in the feed gas, and approach saturation values at higher admixtures on the order of 3 × 1014 cm-3 for OH and 3 × 1013 cm-3 for O. Experimental results are used to benchmark densities obtained from zero-dimensional plasma chemical kinetics simulations, which reveal the dominant formation pathways. At low humidity content, O is formed from OH+ by proton transfer to H2O, which also initiates the formation of large cluster ions. At higher humidity content, O is created by reactions between OH radicals, and lost by recombination with OH. OH is produced mainly from H2O+ by proton transfer to H2O and by electron impact dissociation of H2O. It is lost by reactions with other OH molecules to form either H2O + O or H2O2. Formation pathways change as a function of humidity content and position in the plasma channel. The understanding of the chemical kinetics of O and OH gained in this work will help in the development of plasma tailoring strategies to optimise their densities in applications.

Strong ionization asymmetry in a geometrically symmetric radio frequency capacitively coupled plasma induced by sawtooth voltage waveforms.

The ionization dynamics in geometrically symmetric parallel plate capacitively coupled plasmas driven by radio frequency tailored voltage waveforms is investigated using phase resolved optical emission spectroscopy (PROES) and particle-in-cell (PIC) simulations. Temporally asymmetric waveforms induce spatial asymmetries and offer control of the spatiotemporal dynamics of electron heating and associated ionization structures. Sawtooth waveforms with different rise and fall rates are employed using truncated Fourier series approximations of an ideal sawtooth. Experimental PROES results obtained in argon plasmas are compared with PIC simulations, showing excellent agreement. With waveforms comprising a fast voltage drop followed by a slower rise, the faster sheath expansion in front of the powered electrode causes strongly enhanced ionization in this region. The complementary waveform causes an analogous effect in front of the grounded electrode.

Use of the ultraviolet absorption spectrum of CF2 to determine the spatially resolved absolute CF2 density, rotational temperature, and vibrational distribution in a plasma etching reactor.

Broadband ultraviolet absorption spectroscopy has been used to determine CF(2) densities in a plasma etch reactor used for industrial wafer processing, using the CF(2) A (1)B(1)<--X (1)A(1) absorption spectrum. Attempts to fit the experimental spectra using previously published Franck-Condon factors gave poor results, and values for the higher vibrational levels of the A state [(0,v(2),0), with v(2) (')>6] from the ground state were missing; hence new values were calculated. These were computed for transitions between low-lying vibrational levels of CF(2) X (1)A(1) to vibrational levels of CF(2) A (1)B(1) (v(1) ('),v(2) ('),0) up to high values of the vibrational quantum numbers using high level ab initio calculations combined with an anharmonic Franck Condon factor method. The Franck Condon factors were used to determine the absorption cross sections of CF(2) at selected wavelengths, which in turn were used to calculate number densities from the experimental spectra. Number densities of CF(2) have been determined in different regions of the plasma, including the center of the plasma and outside the plasma volume, and CF(2) rotational temperatures and vibrational energy distributions were estimated. For absorption spectra obtained outside the confined plasma volume, the CF(2) density was determined as (0.39+/-0.08)x10(13) molecule cm(-3) and the vibrational and rotational temperatures were determined as 303 and 350 K, respectively. In the center of the plasma reactor, the CF(2) density is estimated as (3.0+/-0.6)x10(13) molecules cm(-3) with T(rot) approximately 500 K. The fitted vibrational distribution in the CF(2) ground state corresponds to two Boltzmann distributions with T(vib) approximately 300 and T(vib) approximately 1000 K, indicating that CF(2) molecules are initially produced highly vibrationally excited, but are partially relaxed in the plasma by collision.