Stochastic electron heating in a capacitive RF discharge withnon-Maxwellian and time-varying distributions

In capacitively coupled radio frequency discharges, the electrons gain and lose energy by reflection from oscillating, high voltage sheaths. When time-averaged, this results in stochastic heating, which at low pressure is responsible for most of the electron heating in these discharges. Previous derivations of stochastic heating rates have generally assumed that the electron distribution is a time-invariant, single-temperature Maxwellian, and that the sheath motion is slow compared to the average electron velocity, so that electrons gain or lose a small amount of energy in each sheath reflection. Here we solve for the stochastic heating rates in the opposite limit of fast sheath motion and consider the applicability of the slow and fast sheath equations in the intermediate region. We also consider the effect of a two-temperature Maxwellian distribution on particle balance and the effect of a time-varying temperature on the heating rates and densities     

Feasibility studies for the detection of volatile organic compounds in the gas phase using microelectrode sensors

The use of microelectrode sensors to detect volatile organic compounds (VOCs) in air is demonstrated. In general, VOCs that oxidize easily to form protons gave a larger electrochemical response. The use of voltammetry for speciation and the effect of electrode size on the electrochemical response are discussed. We demonstrate that surface enhanced Raman spectroscopy (SERS) can be used to monitor the electrochemical reactions in situ and discuss its applicability in identifying the electroactive species.     

Fermi acceleration revisited

Mappings that have been used to describe the Fermi acceleration mechanism are examined. It is shown that results which appear to be contradictory are due to differences in the mapping equations. For those mappings that can be locally approximated by the standard mapping, the value of the nonlinear parameter of the standard mapping, for which the last isolating KAM surface exists, can be used to predict the loss of KAM stability with action for the more general mappings. Previous results of the variation in the density distribution in the stochastic region of the phase space, averaged over phases, is shown to be consistent with the ergodic hypothesis. Fine scale structure of the mappings is found to be model dependent. The standard mapping is a member of a class of mappings which retains some KAM trajectories at arbitrarily large nonlinearity. This feature is not generic to a wider class of mappings discussed in this paper. The stability of two-iteration fixed points are discussed in detail, including the bifurcation sequence for one type of mapping.     

Detonation interaction with a diffuse interface and subsequent chemical reaction

We have investigated the interaction of a detonation with an interface separating a combustible from an oxidizing mixture. The ethylene-oxygen combustible mixture had a fuel-rich composition to promote secondary combustion with the oxidizer in the turbulent mixing zone that resulted from the interaction. Diffuse interfaces were created by the formation of a gravity current using a sliding valve that initially separated the test gas and combustible mixture. Opening the valve allowed a gravity current to develop before the detonation was initiated. By varying the delay between opening the valve and initiating the detonation it was possible to achieve a wide range of interface conditions. The interface orientation and thickness with respect to the detonation wave have a profound effect on the outcome of the interaction. Diffuse interfaces result in curved detonation waves with a transmitted shock and following turbulent mixing zone. The impulse was measured to quantify the degree of secondary combustion, which accounted for 1–5% of the total impulse. A model was developed that estimated the volume expansion of a fluid element due to combustion in the turbulent mixing zone and predicted the resulting impulse increment.      

Pressurized membrane indicator system for fluorogenic-based fiber-optic chemical sensors

A new kind of fiber-optic-based chemical sensor for the detection of chemical species in solution is described. The sensor uses a fluorogenic indicator that is forced through an ultrafiltration membrane into the analyte solution. Complex formation between the indicator molecule and target ion occurs at the membrane/solution interface, where light from a bifurcated fiber-optic cable stimulates fluorescence. Fluorescence emission from the sample is transmitted back up the cable to a photodiode detector. Characterization of the sensor was performed using the response of the indicator, 8-hydroxyquinoline-5-sulfonic acid, to magnesium ion. The sensor responds linearly to increasing and decreasing concentration changes on time scales of 1 s. Chemical modification of the indicator solution to enhance sensitivity is demonstrated. The use of an internal standard to correct for variations in indicator flow-rate and pressure is discussed.     

Porphyrins XXXV . Exciton coupling in μ-oxo Scandum dimers

The absorption and emission spectra at room temperature and at 77 K are reported for the monomers and μ-oxo dimers of (OEP)Sc(III) and (TPP)Sc(III). [Here (OEP) is octaethylporphin and (TPP) is tetraphenylporphin.] Exciton coupling effects are strongest in the B(Soret) band of [(OEP)Sc]₂O dimer: (i) The peak is blue shifted by 11 nm; (ii) the Soret band has a long red tail out to 480 mn; (iii) the fluorescence polarization shows a broad negativ band ≈ 440 nm. A vibronic exciton coupling model can roughly interpret the data if there is substantial and variable tilting of the ring planes. Exciton effects are weaker in the B(Soret) band of [(TPP)SC]₂O, presumably because there is less tilting. The effect of dimer formation on the Q band of [(OEP)Scl₂O is to red shift the band ≈ 420 cm^?1 and to nearly double the Q(0,0) halfwidth; there is no change in fluorescence yield with dimerization. Presumably for Q bands exciton coupling is weaker than inhomogeneous broadening. Both the phosphorescence yield and triplet lifetime at 77 K drop by $\text{case2}\text{3}$ in the dimer, showing faster radiationless decay.