Effusive oven

Finally, thermal beams of atoms can be excited using one or more lasers, as described in Chapter 3. An interesting variant is the metastable beam used by Post et al,4 A Ba beam from an effusive oven is used to conduct a current from a heated... 

Utilization of both ion and neutral beams for such studies has been reported. Toennies [150] has performed measurements on the inelastic collision cross section for transitions between specified rotational states using a molecular beam apparatus. T1F molecules in the state (J, M) were separated out of a beam traversing an electrostatic four-pole field by virtue of the second-order Stark effect, and were directed into a noble-gas-filled scattering chamber. Molecules which were scattered by less than were then collected in a second four-pole field, and were analyzed for their final rotational state. The beam originated in an effusive oven source and was chopped to obtain a velocity resolution Avjv of about 7 %. The velocity change due to the inelastic encounters was about 0.3 %. Transition probabilities were calculated using time-dependent perturbation theory and the straight-line trajectory approximation. The interaction potential was taken to be purely attractive ... 

An effusion oven has constant volume, fixed temperature, and effusion hole of area A. Gas escapes through the hole, which makes the effusion rate negative,... 

As well as producing higher beam intensities, the supersonic nozzle sources have a further advantage. In the supersonic expansion through the Laval slit the gas is adiabatically cooled to very low temperatures of ca. 40°K [187] but increases its translational energy in the beam direction to the flow velocity of the gas, e.g. Mach number ca. 15 or peak velocity 70% higher than the most probable velocity of a 900°K oven [187]. Supersonic nozzles produce very narrow velocity distributions compared with the Maxwell—Boltzmann distribution obtainable from an effusive oven at the same temperature. 

As shown in Figure 21.1 these jets show little spread in their velocities and exhibit intensities of up to 10 greater than those of effusive ovens. In addition, as the gas expands reversibly and adiabati-caUy, its temperature may reach a few Kelvin. These jets are known as supersonic because their average... 

The important features of this simple effusive oven are the large (2x4 mm )... 

Equation can be applied directly to the movement of molecules escaping from a container into a vacuum. This process is effusion. Effusion is exemplified by the escape of molecules from the oven of Figure 5-7. 

The basic principle of the experimental approach is easily understood by considering the Na collisions of Eq. (14.1) and Fig. 14.2 as a concrete example. As shown by Fig. 14.7, atoms effuse from a heated oven in which the Na vapor pressure is 1 Torr.14 The Na atoms are collimated into a beam by a collimator, not shown in... 

Figure 1.2 Representation of a simple crossed-molecular-beam source [16]. The primary beam effusing from an oven source (A) is velocity selected (S) and then crosses the thermal beam issuing from a second source (B). This diagram shows the detector (D) positioned at the lab angle 0.

The problems associated with the formation and detection of molecular beams have already been referred to. They are interrelated and have largely determined which reactions have been studied with this technique. The simplest method to form a beam is to collimate the effusive flow occurring from a low-pressure source, conventionally called an oven, although its temperature may be subambient. Unfortunately, this yields low beam intensities, and the velocities in the beam are thermally distributed. As a result, even for the accurate assessment of the incident-beam intensity, a highly sensitive detector is required. Moreover, the relatively low beam temperature requires that the reaction has a small threshold energy so that an appreciable proportion of the scattering is reactive. 

A brief debate occurred in the middle 1950 s, when Miller and Kusch ( ) Interpreted the velocity profile of alkali halide molecules effusing from an oven in terms of heavier species as... 

Static gas targets such as those used by Wagenaar and de Heer (1985) are usually unsuitable for differential cross-section measurements. These days scattering experiments are carried out in a crossed-beam arrangement. A large variety of beam sources are used. These range from effusion from simple orifices or capillary arrays to supersonic nozzles, from ovens... 

The barium atom beam effuses from a molybdenum oven (orifice diam. 0.3 cm) that is resistively heated to between 1000 and 1100 K using tantalum wire windings. The oven is... 

Molecular weight from vapor pressures Molecular weight from P-V-T measurements Double oven effusion with mass spectrometer Knudsen effusion with mass spectrometer Knudsen effusion with mass spectrometer Velocity distribution analysis Velocity distribution analysis... 

Although the absolute values of the vapor pressure measurements are not used for evaluation, the ratios of the numbers of dimeric and monomeric NaF molecules that effuse from the oven at the temperatures 1115 - 1191 K determined by Eisenstadt et al. (1 ) by use of the electron-beam velocity-selector method are used. Based on the reported equilibrium data, we evaluate the enthalpy change of the reaction (A) Na F Cg) = 2 NaF(g) by the 2nd and 3rd law methods. The results are presented in the table below. 

Miller and Kusch (3 ) determined the molecular composition of KI vapor by measurement of the velocity distribution of the molecules in the beam produced as the vapor effused through a small slit in a source. The analysis was based on an assumption that the velocity distribution within the oven is Maxwellian and that the vapor effuses through the ideal slit of kinetic theory. The velocity distributions of potassium and thallium atomic beams were found to be in excellent agreement with the theoretical distributions so the determination of the molecular composition of KI beams was tried. Using the derived equilibrium constants, we calculate the enthalpy change of the dissociation reaction by the 2nd and 3rd law methods. The results are presented in the following table. 

Our experimental setup is based on this idea, and a sketch of it is shown in Fig. 5. The effusive CVo beam is produced in the same source as the Cm in Sec. 1 but this time the beam is essentially uncollimated. However, it is still vertically selected by three spatially separated height delimiters, namely the oven aperture (200 //in), a central height delimiter (between 50 and 150 /jm), and the detector laser beam with a gaussian beam waist of 8 fim, which is now horizontally oriented and fixed in position. By shifting the oven up and down... 

An effusive beam of F atoms was produced by thermally dissociating F2 at 2.0 torr and 920 K in a resistively heated nickel oven. The F beam was velocity selected with a FWHM velocity spread of 11%. The H2 beam was produced by a supersonic expansion of 80 psig through a 70 micron orifice at variable temperatures with a FWHM spread of 3%. Rotational state distributions of H2 in the beam were studied previously using molecular beam photoelectron... 

To determine the concentrations of vapor phase alkaloids detected in ETS by APCI mass spectrometry, it is necessary to have vapor phase standards which can be used for instrument calibration. Gas dilution is perhaps the best way to calibrate for compounds in the gas phase. Gas dilution requires that a standard of known concentration and a method for accurately and reproducibly diluting the standard are available. Permeation tubes and diffusion tubes, housed in a constant temperature oven, are well suited for generating gas standards with known analyte concentrations. Table 1 includes the analyte, source, and typical source effusion rates used for investigating ETS along with the ion monitored for quantitative analysis of each analyte. 

The process leading to the generation of effusive beams is relatively simple. An oven contains the... 

Let US assume molecules effusing into a vacuum tank from a small hole A in an oven that is filled with a gas or vapor at pressure p (Fig. 9.1). The molecular density behind A and the background pressure in the vacuum tank are sufficiently low to assure a large mean free path of the effusing molecules, such that collisions can be neglected. The number N 9) of molecules that travel into the cone d around the direction 6 against the symmetry axis (which we choose to be the z-axis) is proportional to cos. A slit B with width b, at a distance d from the point source A, selects a small angular in-... 

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