TIME-RESOLVED OPTICAL-EMISSION AND ELECTRON-ENERGY DISTRIBUTION FUNCTION MEASUREMENTS IN RF PLASMAS

Comparisons between experimentally measured time-dependent electron energy distribution functions and optical emission intensities are reported for low-frequency (100 and 400 kHz) radio-frequency driven discharges in argon. The electron energy distribution functions were measured with a time-resolved Langmuir probe system. Time-resolved optical emissions of argon resonance lines at 687.1 and 750.4 nm were determined by photon-counting methods. Known ground-state and metastable-state excitation cross sections were used along with the measured electron energy distribution functions to calculate the time dependence of the optical emission intensity. It was found that a calculation using only the ground-state cross sections gave the best agreement with the time dependence of the measured optical emission. Time-dependent electron density, electron temperature, and plasma potential measurements are also reported.

TIME-RESOLVED ELECTRON-ENERGY DISTRIBUTION FUNCTION MEASUREMENTS IN A PULSED MAGNETIC MULTIPOLE HYDROGEN DISCHARGE

The time evolution of measured plasma parameters, including the electron energy distribution function (EEDF), in the discharge and post-discharge regime of a pulsed hydrogen magnetic multipole plasma is presented. The time necessary for the plasma to reach equilibrium has been established as 160-mu-s. The present results clarify the mechanisms which initiate the discharge. The decay rates of the charged-particle density and energy in the post-discharge have been measured. These measurements indicate that particle transport to the wall is the dominant loss mechanism for both charged-particle density and energy. The time-resolved EEDF is found to be non-Maxwellian in the discharge and Maxwellian in the late post-discharge.

Space and time resolved rotational state populations and gas temperatures in an inductively coupled hydrogen RF discharge

Gas temperature is of major importance in plasma based surface treatment, since the surface processes are strongly temperature sensitive. The spatial distribution of reactive species responsible for surface modification is also influenced by the gas temperature. Industrial applications of RF plasma reactors require a high degree of homogeneity of the plasma in contact with the substrate. Reliable measurements of spatially resolved gas temperatures are, therefore, of great importance. The gas temperature can be obtained, e.g. by optical emission spectroscopy (OES). Common methods of OES to obtain gas temperatures from analysis of rotational distributions in excited states do not include the population dynamics influenced by cascading processes from higher electronic states. A model was developed to evaluate this effect on the apparent rotational temperature that is observed. Phase resolved OES confirmed the validity of this model. It was found that cascading leads to higher apparent temperatures, but the deviation (similar or equal to 25 K) is relatively small and can be ignored in most cases. This analysis is applied to investigate axially and radially resolved temperature profiles in an inductively coupled hydrogen RF discharge.

Time-resolved resonance Raman spectroscopy

Vibrational Raman spectroscopy is now widely recognized as a useful technique for chemical analysis. It has become increasingly popular for the characterization of stable species since the technology which underpins Raman measurements has matured. Time-resolved Raman spectroscopy has also become established as an excellent method for the characterization of transient chemical species but it is not so widely applied. However, the technical advances which have reduced the cost and increased the reliability of conventional: Raman systems can also be exploited in studies of transient species. In some cases it is just as straightforward to record the Raman-spectra of a short-lived transient species as it is to monitor a more stable sample. This raises the possibility of routinely adding time-domain Raman measurements to more conventional Raman techniques, increasing the selectivity of the analysis while retaining its ability to provide spectral information which is characteristic of the species under investigation.

Time-resolved resonance Raman scattering of triplet state anthracene in supercritical CO2

The first report of time-resolved resonance Raman (TR(3)) scattering in a supercritical fluid is presented. TR(3) spectra of the lowest triplet excited state (T-1) of anthracene in supercritical (SC) CO2 have been obtained over the pressure range 90-500 bar. These data have been complemented by conventional flash photolysis measurements of the excited state lifetime, transient absorbance difference, and fluorescence spectra over a similar pressure range. The spectroscopic data show systematic changes with increasing pressure; the Delta A spectra of the TI state recorded at two different temperatures display a red shift with increasing fluid pressure, which is in agreement with earlier work carried out over a smaller range of pressures. Similar shifts in the fluorescence are also observed. The vibrational frequencies of the T-1 state of anthracene are found to be relatively insensitive to applied pressure; indeed, the transient bands are readily identified by comparison with resonance Raman (RR) spectra of the T-1 state in cyclohexane solution. Small but well-defined shifts to lower cm(-1) with increasing pressure are observed in some of the vibrational bands of SC COE. The most marked change in the excited state Raman spectra is that the intensity of the T-1 anthracene features, relative to those of CO2, increases with applied pressure. The information which each of the above spectroscopic methods gives on the question of how pressure changes affect the structure and local environment of the excited state probe molecule in the SCF is discussed. Possible explanations for the observed increase in RR band intensities in terms of increased resonance Raman enhancement arising from the spectral shifts and/or the increased solubility of anthracene in CO2 with increasing pressure are also considered.

Time-Resolved DRIFTS, MS, and Resistance Study of SnO2 Materials: The Role of Surface Hydroxyl Groups in Formation of Donor States

Time-resolved DRIFTS, MS, and resistance measurements were used to study the interaction of undoped and Pd-doped SnO2 with H-2 in air and argon at 300 degrees C. Using first-order kinetics, we compare the time constants for the resistance drop and its partial recovery with those of the surface hydroxyl evolution and water formation in the gas phase upon exposure to hydrogen. In the case of the undoped oxide, resistance and bridging hydroxyls (BOHs) evolve similarly, manifesting a fast main drop followed by recovery at a similar rate. The rate of water formation for this material was found to be much slower than that of the main drop in both the resistance and BOHs. In contrast, the resistance change for SnO2-Pd appeared to be similar to that of water formation, and no correlation was found between the evolution of resistance and surface OHs. Isotopic exchange on both materials revealed that water formation occurs via fast and slow hydrogen transfer to surface oxygen species. While the former originates from just-adsorbed hydrogen, the latter appears to proceed from the preadsorbed OHs. Both surfaces exhibit close interaction between chemisorbed oxygen and existing bridging OH groups, indicating that the latter is an intermediate in the hydrogen oxidation and generation of donor states on the surface.

Time-resolved dosimetric verification of respiratory-gated radiotherapy exposures using a high-resolution 2D ionisation chamber array

The aim of this work was to track and verify the delivery of respiratory-gated irradiations, performed with three versions of TrueBeam linac, using a novel phantom arrangement that combined the OCTAVIUS® SRS 1000 array with a moving platform. The platform was programmed to generate sinusoidal motion of the array. This motion was tracked using the real-time position management (RPM) system and four amplitude gating options were employed to interrupt MV beam delivery when the platform was not located within set limits. Time-resolved spatial information extracted from analysis of x-ray fluences measured by the array was compared to the programmed motion of the platform and to the trace recorded by the RPM system during the delivery of the x-ray field. Temporal data recorded by the phantom and the RPM system were validated against trajectory log files, recorded by the linac during the irradiation, as well as oscilloscope waveforms recorded from the linac target signal. Gamma analysis was employed to compare time-integrated 2D x-ray dose fluences with theoretical fluences derived from the probability density function for each of the gating settings applied, where gamma criteria of 2%/2 mm, 1%/1 mm and 0.5%/0.5 mm were used to evaluate the limitations of the RPM system. Excellent agreement was observed in the analysis of spatial information extracted from the SRS 1000 array measurements. Comparisons of the average platform position with the expected position indicated absolute deviations of  <0.5 mm for all four gating settings. Differences were observed when comparing time-resolved beam-on data stored in the RPM files and trajectory logs to the true target signal waveforms. Trajectory log files underestimated the cycle time between consecutive beam-on windows by 10.0  ±  0.8 ms. All measured fluences achieved 100% pass-rates using gamma criteria of 2%/2 mm and 50% of the fluences achieved pass-rates  >90% when criteria of 0.5%/0.5 mm were used. Results using this novel phantom arrangement indicate that the RPM system is capable of accurately gating x-ray exposure during the delivery of a fixed-field treatment beam.

Measurements of helium metastable density in an atmospheric pressure glow discharge

The density of metastable helium atoms in a dielectric barrier discharge operating in helium with some impurities present has been measured using laser-collisional-induced fluorescence and absorption techniques. Time-resolved measurements indicate that helium metastables contribute to the production of impurity ions, in this case N-2(+), in the postdischarge current phase of a glow discharge. In our particular discharge environment, the helium metastable density is (1.5+/-1.4)x10(10) cm(-3), a result consistent with failure to observe absorption by metastables in a multipass absorption measurement. (C) 2004 American Institute of Physics.

Determination of quenching coefficients in a hydrogen RF discharge by time-resolved optical emission spectroscopy

In gas discharges at elevated pressure, radiation-less collisional de-excitation (quenching) has a strong influence on the population of excited states. The knowledge of quenching coefficients is therefore important for plasma diagnostics and simulations. A novel time-resolved optical emission spectroscopic (OES) technique allows the measurement of quenching coefficients for emission lines of various species, particularly of noble gases, with molecular hydrogen as collision partner. The technique exploits the short electron impact excitation during the field reversal phase within the sheath region of a hydrogen capacitively coupled RF discharge at 13.56 MHz. Quenching coefficients can be determined subsequent to this excitation from the effective lifetime of the fluorescence decay at various hydrogen pressures. The measured quenching coefficients agree very well with results obtained by means of laser excitation. The time-resolved OES technique based on electron impact excitation is not limited - in contrast to laser techniques - by optical selection rules and the energy gap between the ground state and the observed excited level.

The investigation of mechanisms in environmental catalysis using time-resolved methods

The formation of nitrogen oxides (NOx) during a combustion process is difficult to avoid because of the large exotherm and the consequent problem of avoiding local high-temperature spikes. Consequently, for many applications, such as for automotive power generation, there will be a continuing need to use catalytic after-treatment to reduce harmful emissions. The investigation of the mechanisms of the key catalytic reactions in environmental catalysis can provide an insight into the action of the catalyst, and time-resolved methods offer a powerful means to study these processes under realistic conditions. The use of Temporal Analysis of Products (TAP) and Steady State Isotopic Transient Kinetic Analysis (SSITKA) methods to investigate the reduction of NOx under various experimental conditions is described. From a detailed analysis of the SSITKA profiles, it is shown that at low temperatures the mechanism for the formation of N-2 and N2O from NO may differ from the conventional high-temperature mechanism. This is supported by density functional theory calculations, which show that the barrier to the formation of N2O from the reaction of N(ads) and NO(ads) may be too high to allow this process to occur at low temperatures. The alternative reaction of NO(ads) + NO(ads) = N2O(g) + O(ads) is shown to be much more favorable and is consistent with the SSITKA analysis. The remarkable effect of hydrogen as a reductant at low temperatures is described, and alternative interpretations of the role of hydrogen are discussed.