Recombination front

As the star expands, the photosphere becomes deeper as the recombination front proceeds through the hydrogen-rich envelope deeper in mass. At the same time a heat wave is propagating out from the interior. At a certain stage, energy flux due to radioactive decays exceeds that from shock heating. The dates when the radioactivity starts to dominate and when the luminosity reaches its peak depend on the above three factors as follows. 

In addition to excess carrier effects, van Roosbroeck and Casey (1972) have noted that for these relaxation semiconductors, the recombination front is established at the anode, which thus is where the voltage drop, and therefore heating, occurs. The kinetics are locally enhanced because of this local, additional, thermal effect. 

A diagram of a typical interferometer (Michelson type) is shown in Figure 7.8. It consists of fixed and moving front-surface plane mirrors (A and B) and a beamsplitter. Collimated infrared radiation from the source incident on the beamsplitter is divided into two beams of equal intensity that pass to the fixed and moving mirrors respectively. Each is reflected back on itself, recombining at the beamsplitter from where they are directed through the sample compartment and onto the detector. Small... 

This is the regime of cathodic currents. The silicon atoms of the electrode do not participate in the chemical reaction in this regime. An n-type electrode is under forward bias and the current is caused by majority carriers (electrons). The fact that photogenerated minority carriers (holes) are detectable at the collector indicates that the front is under flat band or accumulation. A decrease of IBC with cathodization time is observed. As Fig. 3.2 shows, the minority carrier current at the collector after switching to a cathodic potential is identical to that at VQcp in the first moment, but then it decreases within seconds to lower values, as indicated by arrows in Fig. 3.2. This can be interpreted as an increase of the surface recombination velocity with time under cathodic potential. It can be speculated that protons, which rapidly diffuse into the bulk of the electrode, are responsible for the change of the electronic properties of the surface layer [A17]. However, any other effect sufficient to produce a surface recombination velocity in excess of 100 cm s 1 would produce similar results. 

A current of photogenerated holes observed in regime 4 at the collector at low VeB of about 1 V for an illuminated n-type substrate indicates that no significant SCR is present at the front side the n-type electrode is in inversion. If the bias is increased this current disappears indicating an SCR or an increased surface recombination velocity at the emitter. In regime 4a all photogenerated holes are consumed by Feb- Breakdown of the junction in regime 4 does not lead to pore formation. 

For not too low doped samples (D W), however, the contribution of 1SCR is usually negligible. If the surface recombination velocity at the illuminated front is low, IBPC then only depends on sample thickness D, illumination intensity eP, and minority charge carrier diffusion length ID. 

The two examples shown demonstrate how an interference effect can be produced either by amplitude division of an incident beam, followed by retardation (achieved in this case by multiple reflection between the partially reflective parallel plates of a Fabry-Perot resonator) and recombination, or by division of the wave front at the multiple equally spaced slits of a diffraction grating, again followed by recombination. 

It should be noted that not all flames have the behaviors discussed above. For example, the equilibrium species distribution in some H2-N20-Ar flames has essentially the same mole number as the reactants. As a result the adiabatic flame temperature is achieved directly in the flame front with no long recombination tail. Ammonia-oxygen flames exhibit a slow approach to chemical equilibrium, albeit with a long dissociation, not recombination, tail [279], Here the temperature in the flame front overshoots the adiabatic flame temperature, with the equilibrium temperature being approached from above as the dissociation reactions proceed. In certain highly strained, rich, hydrocarbon flames (e.g., C2H2-H2-O2), such as those used for flame-based diamond growth, the temperature can also overshoot the adiabatic flame temperature in the flame front. Here the overshoot is caused by the relatively slow dissociation of the excess acetylene [270]. 

The third factor that is important in determining the detection limit is the conversion efficiency of the kinetics. A conversion efficiency of 1.0 requires that the airstream have a velocity substantially less than 200 m/s because uniform mixing of NO is very difficult. At the same time, collisions of the sample airstream with wall surfaces in slower inlet systems may cause a chemical loss of CIO and BrO, because they are both reactive with wall surfaces. The solution to this problem was suggested by Soderman (83). Soderman s novel design consists of two nested ducts in which the air speed is decreased from 200 m/s to 60 m/s in a 14-cm-diameter outer duct that protrudes 60 cm in front of the left wing pod and is reduced to 20 m/s inside a smaller 5-cm-square duct in which the measurements are made. The entrance to the smaller measurement duct is 60 cm downstream of the entrance to the outer duct, and the NO injector tubes, the two CIO detection axes, and the one BrO axis are 25 cm, 37.5 cm, 55 cm, and 72.5 cm downstream of the entrance of the measurement duct. Ninety percent of the air that enters the outer duct bypasses the measurement duct through additional duct work, and only the center 10% of the airstream is captured and sampled by the measurement duct. These two flows are recombined downstream of the instrument and are vented out the side of the wing pod that houses the instrument. 

The quantum yield of the photocurrent for an electrode illuminated from the front side can be calculated from a simple model described by Gartner (j+) provided some simplifying assumptions are applicable. This model is shown in Figure 1. If surface recombination can be neglected, the quantum yield 4> is obtained as... 

Tsai TH, Chen SL, Xiao X, Chiang YH, et al. 2006. Gene therapy of focal cerebral ischemia using defective recombinant adeno-associated virus vectors. Front Biosci. 11 2061-2070. 

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