Effusive beam source

An additional effusive beam source, operated with pure O2 as the precursor geis, is used for some studies. This source, shown schematically in Fig. 7, is a low pressure ( 100 mTorr) RF discharge tube, with a small hole (0.5 mm) in the end. The inductively coupled plasma is typically operated at a power of 50 W. The effluent (beam) from this source has been characterized by aligning the detector with the plasma source and placing... 

The construction of a special pulsed effusive beam source, which is based on a two-stage cryocooler (Sumitomo SRDK-IOIE-AIIC), is indicated schematically on the left side of Fig. 6.8. An efficient method for getting a cold gas pulse is to evaporate adsorbed gas inside a cold tube with a short voltage pulse from a thin substrate that can be cooled down... 

An effusive beam of atoms or molecules (see Ramsey, 1956 in fhe bibliography) is produced by pumping fhem fhrough a narrow slif, fypically 20 pm wide and 1 cm long, wifh a pressure of a few forr on fhe source side of fhe slif. The beam may be further collimated by suifable apertures along if. 

Those fixed-angle measurements reported to date have all used either a heated effusive inlet, or heated gas cell for sample admission [55, 56, 61, 62, 65]. Probably the higher sample number densities these sources generate, compared to a supersonic beam source, provides some compensation for the reduced collection efficiency in the fixed-angle measurement. 

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],... 

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.

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 ... 

Previous experimenters have found dimerization in all alkali halides at vapor pressures corresponding to source temperatures appropriate for effusive beams ( 5, 6, T). The percent dimerization increases as molecular weight decreases towards lighter alkali halides, with lithium fluoride possessing the largest fractional dimerization. 

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). 

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... 

Fig. 7. Schematic diagram of the interaction region, showing the orientation of the effusive plasma and hyperthermal beam sources, chopper wheel, target surface, and detector.

This conclusion was tested with a practical experiment in which the erosion yield of polystyrene under exposure to the effusive plasma source was measured with and without the addition of the pulsed hyperthermal Ar beam. In this experiment, the Ar beam pulse was not chopped, so the entire beam pulse produced from the laser detonation source was allowed to strike the surface. The erosion yield was determined by placing a screen over the samples during exposure and measuring the etch depth... 

A supersonic beam is schematically described as a gas stream expanding very rapidly from a high pressure region (source), through a nozzle, to a low pressure region. The characteristics of the beam are mainly determined by the size and shape of the nozzle and by the pressure difference between the two regions [26]. Compared to effusive beams used in molecular beam... 

Figure B2.3.3. Crossed-molecular beam apparatus employed for the study of the F + D2 -> DF + D reaction. Indicated in the figure are (1) the effusive F atom source (2) slotted-disk velocity selector (3) liquid-nitrogen-cooled trap (4) D2 beam source (7) skimmer (8) chopper (9) cross-correlation chopper for product velocity analysis and (11) rotatable, ultrahigh-vacuum, triply differentially pumped, mass spectrometer detector chamber. Reprinted with permission from Lee [29]. Copyright 1987 American Association for the Advancement of Scienee. 

Guevremont, J.M., Sheldon, S., and Zaera, E, Design and characterization of collimated effusive gas beam sources Effect of source dimensions and backing pressure on total flow and beam profile. Review cf Scientific Instruments, 2000. 71(10) 3869. 

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