Molecular beams effusive

Fig. 4. Schematic of a high vacuum molecular beam epitaxy (MBE) chamber containing four effusion (Knudsen) cells. Also shown is a high energy electron...

Gopinath, C. S. and Zaera, F. (2000) Transient kinetics during the isothermal reduction of NO by CO on Rh(lll) as studied with effusive collimated molecular beams , J. Phys. Chem. B, 104, 3194. 

In conventional gas electron diffraction experiments, an effusive beam is used in which vibrational levels of molecules are thermally populated and the width of a peak in a radial distribution curve is determined by thermally averaged mean amplitudes. When a molecular beam or a free jet is used, mean amplitudes could become small, since the contribution from the vibrationally excited levels is reduced significantly. As a consequence, sharper peaks are expected in the radial distribution curve, and the spatial resolution of the snapshot could be improved. However, it seems that the observed peaks in the radial distribution curve are considerably broad even though a molecular beam is used. There could be some reasons to have such broadened peaks in the radial distribution curve. 

Other than the earlier work reviewed by Ashfold et al. (3), only three studies on the photodissociation dynamics have been reported for this molecule (153,154,158). The first study reported the quantum state distribution of the CN radical obtained in an effusive molecular beam and in a static gas cell, while the second study reported the observations in a pulsed molecular beam. The dynamics remains the same despite the fact that the initial internal state distribution of the C1CN molecule changes. This of course shows that hot bands are not important in the photodissociation of this molecule at this wavelength. 

In molecular beam epitaxy (MBE) [317], molecular beams are used to deposit epitaxial layers onto the surface of a heated crystalline substrate (typically at 500-600° C). Epitaxial means that the crystal structure of the grown layer matches the crystal structure of the substrate. This is possible only if the two materials are the same (homoepitaxy) or if the crystalline structure of the two materials is very similar (heteroepitaxy). In MBE, a high purity of the substrates and the ion beams must be ensured. Effusion cells are used as beam sources and fast shutters allow one to quickly disrupt the deposition process and create layers with very sharply defined interfaces. Molecular beam epitaxy is of high technical importance in the production of III-V semiconductor compounds for sophisticated electronic and optoelectronic devices. Overviews are Refs. [318,319],... 

The rate at which molecules pour out of a small hole into a vacuum (see Figure 7.6) is proportional to the velocity of the molecules in the direction of the hole (or, equivalently, if the hole were filled with a plug, it is proportional to the rate at which molecules hit the plug). This process, which creates a molecular beam into the vacuum, is called effusion. A mixture of two gases with two different masses will have different mean velocities in the direction of the wall, and the lighter gas will leave the box more rapidly. 

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 ... 

Kaiser s studies employed a conventional spectrometer with A and B electric quadrupole fields, and by passing the HC1 gas through a microwave discharge situated prior to the molecular beam source, populations in the ratios 21 3 1 for the v = 0, I and 2 vibrational levels were obtained. An effusion source was operated at 170 K and line widths close to 1 kHz were obtained similar studies of DC1 were described, except that in this case the gas was preheated to 1440 K to produce increased vibrational excitation. Kaiser was able to observe spectra of H35C1 in J = 1, v = 0, 1,... 

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. 

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 ... 

Blythe, Grosser, and Bernstein [151 ] have used crossed molecular beams to observe the J = 2 - 0 rotational deexcitation process in D2. A velocity-selected atomic beam of potassium was made to impinge on a modulated Da beam from an effusive (T = I8PK) source. The scattered K atoms were detected by surface ionization on a hot Pt-W ribbon, from which the ions were drawn into an electron multiplier equipped with lock-in amplification. 

The essential elements.of the experiment are a) an effusive molecular beam source, b) inhomogeneous deflecting electric polefaces, c) surface ionization detector, capable of translation in order to obtain the deflected beam pattern. 1, 2, are the distance from the source to the front of the polefaces, the length of the polefaces and the distance from the back of the polefaces to the detector, respectively. A general review of deflection methods for determining polarizabilities is given by Miller and Bederson (8). 

If molecules are allowed to escape from an enclosure into an evacuated space through a small hole under the conditions described for the effusion method in Section 3.3, they may be collimated into a molecular beam by the use of slits. If a pulse of this molecular beam is allowed to fall on a moving receiving plate, the condensed... 

The other point to be considered in experiments of this nature is that as in the effusion method, the molecular beam must be in thermal dissociative equilibrium with the enclosure. This could fairly readily be checked by showing that the results are independent of the ratio of the hole to the surface of the enclosure, but such a test has apparently not been made in the experiments mentioned above. There is of course good a posteriori evidence that this was so in the work on the alkali metals, but it may not liave held in the work on bismuth. 

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