Fast beam apparatus

Fig. 10.1 Schematic drawing of a fast beam apparatus. A fast atomic beam enters from the left and is excited sequentially by two different C02 lasers in electric field regions Fj and F3, respectively. F2 avoids a zero field region between them. Ions produced by highly excited atoms being ionized in the biased microwave cavity are energy selected and detected by a Johnston particle multiplier (not shown). The output signal is detected in phase with the mechanically chopped Fj laser beam (from ref. 3).

In the following sections of this article, we describe the principles of ionization cross-section measurements, including a brief description of the fast-beam apparatus and the high-resolution double-focusing mass spectrometer employed in the present studies. A comprehensive review of semiempirical calculations of total ionization cross sections is given. Comparisons between these calculated cross sections and the experimental results are presented. The decomposition of the various molecules in a low-temperature plasma is discussed on the basis of the measured ionization-cross-section data, and comparisons are made with the results of in situ plasma diagnostics studies using mass spectrometric techniques. 

A detailed description of the fast-beam apparatus and of the experimental procedure employed in the determination of absolute partial ionization cross sections has been given in previous publications (Wetzel et al, 1987 Freund et al., 1990 Tamovsky and Becker, 1992 Tamovsky and Becker, 1993). For the measurements of the cross sections of silane radicals, a dc discharge biased at typically 2 to 3 kV through SiD4 served as the primary ion source. Deuterated rather than protonated target species were used in these studies to facilitate a... 

In both the fast-beam apparatus and the double-focusing mass spectrometer, absolute cross sections can be determined with uncertainties of 15% for the parent ionization cross sections and 18% for the dissociative ionization cross sections. These error margins include statistical and all known systematic uncertainties and are typical for ionization-cross-section measurements carried out with this apparatus (Tamovsky and Becker, 1992 Tamovsky and Becker, 1993). 

How fast do gas molecules move Molecular speeds can be measured using a molecular beam apparatus, shown schematically in Figure 5-la. Gas molecules escape from an oven through a small hole into a chamber in which the molecular density is very low. A set of slits blocks the passage of all molecules except those moving in the... 

Fig. 18.3 Fast He beam apparatus used to measure the radio frequency intervals between the n=10 g, h, i and k intervals (from ref. 14).

Fig. 2 A schematic diagram of the fast-atomic beam apparatus used in this measurement...

Fig. 19. Velocity dependence of collisional ionization cross sections of nonalkali metal atoms in collisions with O2 and N2, measured using a fast alkali metal atom beam apparatus of Bukhteev and Bydin.

A crossed molecular beam apparatus was described for the study of chemiluminescent reactions such as Fj -I- I2 and Brj -I- Cl2. The effects of collision energy and beam pressure on the chemiluminescence were evaluated. A fast-flow system has been utilized for investigation of bimolecular reactions with rates up to 5 X 10 mol cm s " For an illustration of the method, the kinetics of the chemiluminescent reaction of H3B.N(CHj)3 with 0( P) atoms were determined. Other reactions that have been investigated using chemiluminescence include -f CO and the quenching of 0( Z)) by NjO and Chemilumines-... 

This article describes recent advances in the experimental determination of electron impact ionization cross sections for silane (SiH4) its radicals, SiH. (x = 1 to 3) and the Si-organic molecules tetramethylsilane (TMS), Si(CH3)4 tetraethoxysilane (TEOS), Si(0-CH2-CH3)4 and hexamethyldisiloxane (HMDSO), (CH3)3-Si-0-Si-(CH3)3, which is one of the simplest siloxane compounds. These are model substances, and the results obtained for these species may be used in efforts to predict the ionization properties of other, more complex Si-organic molecules. The ionization cross sections of the stable compounds were measured using a high-resolution double-focusing mass spectrometer. The cross-section data for the radicals were obtained in a fast-neutral-beam apparatus. 

Fig. 1. Schematic diagram of the fast-neutial-beam apparatus.

Figure 7.3 Apparatus for flash studies of fast reactions in molecular beams (1). Schematic drawing of a molecular-beam flash apparatus. The pump and probe pulses (see text. Section 4.2.4.3) are produced by a tunable dye laser, a beam-splitter, and a delay line, not shown in the figure (see Figure 7.4). The two pulses are recombined by the beam-splitter BS and sent coaxially into the molecular-beam apparatus. A supersonic jet of (e.g.) argon gas is generated by expanding the gas through a nozzle into a vacuum chamber (not shown), for time-of-flight measurements. This apparams was used in smdies of the dissociation of iodine molecules and subsequent recombination (see below. Section 7.3.4.1 and Ref. [17]). Monitoring was by laser-induced fluorescence (LIF).

The apparatus used in Fast Beam Laser Spectroscopy is shown in Fig.l. 

Fig.18.20. (a) Schematic diagram of the apparatus used for laser excitation of fast beams of Ba. The observation window is moved along the beam axis relative to the fixed laser excitation region, (b) Level scheme of showing selective ex-... 

Fig. 6. Schematic diagram of the Nottingham apparatus for IR kinetic measurements on solutions. Solid lines represent the light path, broken lines the electrical connections. L = Line tunable CO laser, S = sample cell, F = flash lamp, P = photodiode, D = fast MCT IR detector, T = transient digitizer, O = oscilloscope, and M = microcomputer. Nonfocussing optics were used throughout, and the IR laser beam was heavily attenuated by a variable path cell V, filled with liquid methanol, placed immediately in front of the detector. [Reproduced with permission from Moore et al. (61).]...

To minimize interference between the different plasma collection schemes used it was decided to use a spatially separated system. The antiproton collection, cooling, and compression is housed in the same superconducting magnet system as the recombination trap, but the positron accumulator is a stand-alone system connected to the main apparatus by a beam line incorporating differential pumping and a fast valve. Figure 1 shows a schematic lay-out of the apparatus and the following sub-sections describe the individual parts of the apparatus in more detail. 

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