Beam equivalent pressure

The structures with self-organized GaN/AlN QDs were grown by molecular beam epitaxy (MBE) on (0001) sapphire substrates. Ammonia was used as the source of active nitrogen. A single layer of GaN QDs was formed on the AIN buffer surface by a particular MBE growth mode at relatively low substrate temperatures (Ts 540°C). A beam equivalent pressure (BEP) of gallium flux was 5.4T0 Torr and BEP of ammonia flux was 10" Torr. To obtain GaN QDs... 

A more precise group of methods measure the beam equivalent pressure in molecular beams near the substrate or the atom fraction of interest in the gas phase. There are several ways of doing this including electron impact emission spectroscopy (EIES), conventional ionization gauges, mass spectrometers, glow-discharge optical spectroscopy, and other methods. We will briefly consider these four in turn. 

Figure 18 CO2 transients on 4nm Pd particles supported on MgO(l 00) as a function of sample temperature with an isotropic pressure of oxygen (5 x 10 8 Torr) and a pulsed molecular beam of CO (3.4 x 107 Torr equivalent pressure) (from Ref. [146]).

The micrographs of the different samples are represented on Fig. 21. D mean diameter, n cluster number density, T sample temperature, P02 oxygen pressure, /idjp/Vss amplitude of the dip divided by the steady state production rate of CO2 (see text). The equivalent pressure of the CO beam is 3.4 x 10-7 Torr (from Refs [88,145] and unpublished results). 

Figure 25a shows the steady state production rate of C02 calculated for various CO pressures. We recognize the typical volcano shape observed in the experiments. The temperature corresponding to the maximum rate is very close to the experimental values. Figure 25b displays the variation of the coverage of the adsorbed species as a function of temperature for a NO pressure of 5 x 10-8 Torr and the same equivalent pressure in the CO beam. The nitrogen coverage is not represented because it remains very low due to the small barrier for associative desorption of N2. At low temperature, the concentration... 

The aerosol produced by a laboratory pulverized coal combustor was size classified in the range 0.03 to 4 ym Stokes equivalent diameter using a low-pressure cascade impactor. The samples thus collected were analyzed using a focussed beam particle induced X-ray emission technique. This combination of techniques was shown to be capable of resolving much of the structure of the submicron coal ash aerosol. Two distinct modes in the mass distribution were observed. The break between these modes was at a particle size of about 0.1... 

Two UV detectors are also available from Laboratory Data Control, the UV Monitor and the Duo Monitor. The UV Monitor (Fig.3.45) consists of an optical unit anda control unit. The optical unit contains the UV source (low-pressure mercury lamp), sample, reference cells and photodetector. The control unit is connected by cable to the optical unit and may be located at a distance of up to 25 ft. The dual quartz flow cells (path-length, 10 mm diameter, 1 mm) each have a capacity of 8 (i 1. Double-beam linear-absorbance measurements may be made at either 254 nm or 280 nm. The absorbance ranges vary from 0.01 to 0.64 optical density units full scale (ODFS). The minimum detectable absorbance (equivalent to the noise) is 0.001 optical density units (OD). The drift of the photometer is usually less than 0.002 OD/h. With this system, it is possible to monitor continuously and quantitatively the absorbance at 254 or 280 nm of one liquid stream or the differential absorbance between two streams. The absorbance readout is linear and is directly related to the concentration in accordance with Beer s law. In the 280 nm mode, the 254-nm light is converted by a phosphor into a band with a maximum at 280 nm. This light is then passed to a photodetector which is sensitized for a response at 280 nm. The Duo Monitor (Fig.3.46) is a dual-wavelength continuous-flow detector with which effluents can be monitored simultaneously at 254 nm and 280 nm. The system consists of two modules, and the principle of operation is based on a modification of the 280-nm conversion kit for the UV Monitor. Light of 254-nm wavelength from a low-pressure mercury lamp is partially converted by the phosphor into a band at 280 nm. 

Transport phenomena modeling. This type of modeling is applicable when the process is well understood and quantification is possible using physical laws such as the heat, momentum, or diffusion transport equations or others. These cases can be analyzed with principles of transport phenomena and the laws governing the physicochemical changes of matter. Transport phenomena models apply to many cases of heat conduction or mass diffusion or to the flow of fluids under laminar flow conditions. Equivalent principles can be used for other problems, such as the mathematical theory of elasticity for the analysis of mechanical, thermal, or pressure stress and strain in beams, plates, or solids. 

By consideration of the relationship between the ion beam currents and the equivalent partial pressures, pj of the species at the catalyst surface, estimating the latter from the geometry and effusion characteristics of the molecular beam inlet and sampling system, it followed that the rate of production of nitric oxide at the catalyst surface, r, was given by ... 

Equivalent Lithium Pressure in the Chamber. In order to compare the effects of the two liquid metals on heavy ion beam transport, a method of converting the pressures of each element to effective pressures of a single element is needed. According to Gillespie ( ), Pb vapor pressure (and pressures of other elements) can be converted to equivalent Li vapor pressure if it is assumed that stripping of +1 ions in a heavy ion beam to a higher ionization state is the main beam degradation effect. 

Figure 26. Equivalent Li pressures for heavy ion beam stripping above Li,oPbi...

Following the TPR experiments discussed earlier, Judai et al. examined the reaction between CO and NO on size-selected Pd clusters using a pulsed molecular beam [73]. In these experiments, CO and N formation are monitored when a constant pressure of 5 x 10 mbar of CO is maintained in chamber while delivering 100 ms pulses of NO (equivalent to an effective pressure of 1 x 10 mbar) to the surface of the sample held at a constant temperature. Although TPR experiments indicated a... 

Figure 17 CO oxidation on Pd/alumina model catalysts. Top CO2 production versus time from a pulsed CO beam and a constant O2 beam. Xco is the equivalent CO pressure divided by the sum of the CO and O2 pressures. Down Simulation of the experimental data (on top) with a kinetic model. The inset is an enlargement of part of the figure showing the presence of a small dip after closing of the CO beam. (From J. Libuda et al. [203].)... 

Fig. 3.19. Steady-state CO oxidation on 1.5-nm An clusters supported on MgO(lOO) at RT studied by molecular beam reactive scattering. The equivalent in the CO beam is 5 x 10 mb, the isotropic O2 pressure is 1 x 10 mb (from [84])...

Fig. 4.35. Particle size-dependent bistability and hysteresis. On model system I (500-nm EBL-fabricated particles), the CO oxidation shows a perfectly stable bistability behavior. On the time scale accessible by the experiment (>10 s), we can arbitrarily switch between the two states by pulsing either pure CO or O2 (a and d). For the model system II (6-nm particles), a very slow transition toward a single global state is observed in the transition region between the CO- and O-rich reaction regimes (b and e). This behavior is assigned to fluctuation-induced transitions, which are accelerated by the presence of defect sites. For the smallest particles of the model system III (1.8 nm), a globally monostable kinetics is rapidly established under all conditions (c and f). For all experiments, the total flux of CO and O2 beams at the sample position was equivalent to a local pressure of 10" Pa. The surface temperature in (a-c) was 400 K and in (d-f) 415 K (from [147])...

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