Plasma Ionization in Low-Pressure Radio-Frequency Discharges. Part I: Optical Measurements

The electron dynamics in the low-pressure operation regime ($«$ 5 Pa) of a neon capacitively coupled plasma is investigated using phase-resolved optical emission spectroscopy. Plasma ionization and sustainment mechanisms are governed by the expanding and contracting sheath and complex wave–particle interactions. Electrons are energized through the advancing and retreating electric field of the RF sheath. The associated interaction of energetic sheath electrons with thermal bulk plasma electrons drives a two-stream instability also dissipating power in the plasma.

Absolute Calibration of Atomic Density Measurements by Laser-Induced Fluorescence Spectroscopy with Two-Photon Excitation (TALIF)

The two-photon resonances of atomic hydrogen (? = 2 × 205.1 nm), atomic nitrogen (? = 2 × 206.6 nm) and atomic oxygen (? = 2 × 225.6 nm) are investigated together with two selected transitions in krypton (? = 2×204.2 nm) and xenon (? = 2×225.5 nm). The natural lifetimes of the excited states, quenching coefficients for the most important collisions partners, and the relevant ratios of the two-photon excitation cross sections are measured. These data can be applied to provide a calibration for two-photon laser-induced fluorescence measurements based on comparisons with spectrally neighbouring noble gas resonances.

Laser-Induced Fluorescence Measurements of Absolute Atomic Densities: Concepts and Limitations

The potential of laser-induced fluorescence spectroscopy of atoms is reviewed with emphasis on the determination of absolute densities. Examples of experiments with single-photon and two-photon excitation are presented. Calibration methods applicable with the different schemes are discussed. A new method is presented that has the potential to allow absolute measurement in plasmas of elevated pressure where collisional depletion of the excited state is present.

Absolute Atomic Oxygen Density Measurements by Two-Photon Absorption Laser-induced Fluorescence Spectroscopy in an RF-Excited Atmospheric Pressure Plasma Jet

The atmospheric pressure plasma jet is a capacitively coupled radio frequency discharge (13.56 MHz) running with a high helium flux (2m3 h-1) between concentric electrodes. Small amounts (0.5%) of admixed molecular oxygen do not disturb the homogeneous plasma discharge. The jet effluent leaving the discharge through the ring-shaped nozzle contains high concentrations of radicals at a low gas temperature—the key property for a variety of applications aiming at treatment of thermally sensitive surfaces. We report on absolute atomic oxygen density measurements by two-photon absorption laser-induced fluorescence (TALIF) spectroscopy in the jet effluent. Calibration is performed with the aid of a comparative TALIF measurement with xenon. An excitation scheme (different from the one earlier published) providing spectral matching of both the two-photon resonances and the fluorescence transitions is applied.

Characterisation of the Interaction of Lactate Dehydrogenase With Tween-20 Using Isothermal Titration Calorimetry, Interfacial Rheometry and Surface Tension Measurements

In this study the nature of the interaction between Tween-20 and lactate dehydrogenase (LDH) was investigated using isothermal titration calorimetry (ITC). In addition the effects of the protein and surfactant on the interfacial properties were followed with interfacial rheology and surface tension measurements in order to understand the mechanism by which the surfactant prevents protein adsorption to the air– water interface. Comparisons were made with Tween-40 and Tween-80 in order to further investigate the mechanism. ITC measurements indicated a weak, probably hydrophobic, interaction between Tween-20 and LDH. Prevention of LDH adsorption to the air–water interface by the Tween surfactants was correlated with surface energy rather than surfactant CMC. While surface pressure appears to be the main driving force for the displacement of LDH from the air–water interface by Tween-20 a solubilisation mechanism may exist for other protein molecules. More generally the results of this study highlight the value of the use of ITC and interfacial measurements in characterising the surface behaviour of mixed surfactant and protein systems.

Magnetic field measurements in laser-produced plasmas via proton deflectometry

Large magnetic fields generated during laser-matter interaction at irradiances of ~ 5×1014 W?cm-2 have been measured using a deflectometry technique employing MeV laser-accelerated protons. Azimuthal magnetic fields were identified unambiguously via a characteristic proton deflection pattern and found to have an amplitude of ~ 45 T in the outer coronal region. Comparison with magnetohydrodynamic simulations confirms that in this regime the mathTe×mathne source is the main field generation mechanism, while additional terms are negligible.