Crossed beam

For conventional probes, acoustic verification aims at characterizing the beam pattern, beam crossing, beam angle, sensitivity, etc., which are key characteristics in the acoustic interaction between acoustic beam and defect. For array transducers, obviously, it is also a meaning to check the acoustic capabilities of the probe. That is to valid a domain (angle beam, focus, etc.) in which the probe can operate satisfactorily. 

There are many experimental methods by which photodissociation of ions have been studied. The earliest were crossed-beams experiments on Hjbeginning in the late 1960s [24, 25 and 26] and experiments on a... 

Two teclmiques exist for measuring the angular distribution of products. In the crossed-beam setup, the... 

The first half of this section discusses the use of the crossed beams method for the study of reactive scattering, while the second half describes the application of laser-based spectroscopic metliods, including laser-mduced fluorescence and several other laser-based optical detection teclmiques. Furtlier discussion of both non-optical and optical methods for the study of chemical reaction dynamics can be found in articles by Lee [8] and Dagdigian [9]. 

In a crossed-beam experiment the angular and velocity distributions are measured in the laboratory coordinate system, while scattering events are most conveniently described in a reference frame moving with the velocity of the centre-of-mass of the system. It is thus necessary to transfonn the measured velocity flux contour maps into the center-of-mass coordmate (CM) system [13]. Figure B2.3.2 illustrates the reagent and product velocities in the laboratory and CM coordinate systems. The CM coordinate system is travelling at the velocity c of the centre of mass... 

Figure B2.3.2. Velocity vector diagram for a crossed-beam experiment, with a beam intersection angle of 90°. The laboratory velocities of the two reagent beams are and while the corresponding velocities in the centre-of-mass coordinate system are and U2, respectively. The laboratory and CM velocities for one of the products (assumed here to be in the plane of the reagent velocities) are denoted if and u, respectively.

Thennal dissociation is not suitable for the generation of beams of oxygen atoms, and RF [18] and microwave [19] discharges have been employed in this case. The first excited electronic state, 0( D), has a different spin multiplicity than the ground 0( P) state and is electronically metastable. The collision dynamics of this very reactive state have also been studied in crossed-beam reactions with a RF discharge source which has been... 

Laser photolysis of a precursor may also be used to generate a reagent. In a crossed-beam study of the D + FI2 reaction [24], a hypertliennal beam of deuterium atoms (0.5 to 1 eV translational energy) was prepared by 248 mn photolysis of DI. This preparation method has been widely used for the preparation of molecular free radicals, both in beams and in experiments in a cell, with laser detection of the products. Laser photolysis as a method to prepare reagents in experiments in which the products are optically detected is fiirtlier discussed below. 

Keil and co-workers (Dhamiasena et al [16]) have combined the crossed-beam teclmique with a state-selective detection teclmique to measure the angular distribution of HF products, in specific vibration-rotation states, from the F + Fl2 reaction. Individual states are detected by vibrational excitation with an infrared laser and detection of the deposited energy with a bolometer [30]. 

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

With spectroscopic detection of the products, the angular distribution of the products is usually not measured. In principle, spectroscopic detection of the products can be incorporated into a crossed-beam scattering experiment of the type described in section B2.3.2. There have been relatively few examples of such studies because of the great demands on detection sensitivity. The recent work of Keil and co-workers (Dhannasena et al [16]) on the F + H2 reaction, mentioned in section B2.3.3, is an excellent example of the implementation... 

Kaiser R I and Suits A G 1995 A high-intensity, pulsed supersonic carbon course with C( P ) kinetic energies of 0.08-0.7 eV for crossed beam experiments Rev. Sc/. Instrum. 66 5405-11... 

Liu K, Macdonald R G and Wagner A F 1990 Crossed-beam investigations of state-resolved collision dynamics of simple radicals int. Rev. Phys. Chem. 9 187-225... 

Hemmi N and Suits A G 1998 The dynamics of hydrogen abstraction reactions crossed-beam reaction Cl +... 

Another test used to determine the shear modulus and shear strength of a composite material is the sandwich cross-beam test due to Shockey and described by Waddoups [2-17]. The composite lamina... 

Quer-achse, /. transverse axis, -arm, m. cross arm, cross bar. -balken, m. cross beam, cross bar. -beweguug, /. transverse motion. 

Turner et al. (23) have measured the cross-section for this reaction with a cross-beam apparatus using ion energies down to 4 e.v. These results are given in Table I. The agreement with the present results is gratifying in view of the uncertainties discussed above. 

A mass spectrometer provides an example of a molecular beam, in this case a beam of molecular ions. Molecular beams are used in many studies of fundamental chemical interactions. In a high vacuum, a molecular beam allows chemists to study the reactions that take place through specifically designed types of collisions. For example, a crossed-beam experiment involves the intersection of two molecular beams of two different substances. The types of substances, molecular speeds, and orientations of the beams can be changed systematically to give detailed information about how chemical reactions occur at the molecular level. Chemists also have learned how to create molecular beams in which the molecules have very little energy of motion. These isolated, low-energy molecules are ideal for studies of fundamental molecular properties. 

Fig. 2. Schematic of the rotatable sources, crossed-beam machine. 

Compared to the H-atom Rydberg tagging technique,65 the resolution of the present method is somewhat worse, by about a factor of two. This loss in resolution, however, is realized in general only for photodissociation studies. In a typical crossed beam experiment, the product translational energy resolution is usually limited by the energy spread of the initial collision energy rather than the detection scheme. On the other hand, the present... 

The setup used for crossed beam experiments is basically the same apparatus used in the H2O photodissociation studies but slightly modified. In the crossed beam study of the 0(1D) + H2 — OH + H reaction and the H + HD(D2) — H2(HD) + D reaction, two parallel molecular beams (H2 and O2) were generated with similar pulsed valves. The 0(1D) atom beam was produced by the 157 photodissociation of the O2 molecule through the Schumann-Runge band. The 0(1D) beam was then crossed at 90° with the... 

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