Collisions in crossed beams

A wide variety of works deal with rotational angular momenta polarization of the products in reactive collisions between molecular beams. Usually an atom-transfer reaction of type 

The conversion of the orbital angular momentum of the reagents to rotational angular momentum of the products in reaction (6.13) appears not only for heavy atom transfer reactions in heavy + heavy-light A+BC reactive scattering, but is also expected to be the case for other mass combinations and also for lower reactional orbital momentum values. As was shown in [337], the kinematic polarization of the reaction products can be 

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Fig. 12.14a,b> Measurement of differential cross sections for inelastic atom-molecule collisions in crossed beams, (a) Schematic level scheme, (b) experimental arrangement [12.31]... 

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... 

The concept of reaction mechanism is very broad and its exact meaning depends to considerable extent on the point of view from which a given problem is to be analysed. Thus, for example, reaction mechanisms can be understood differently by a chemical physicist analysing a given reaction at the level of elementary collisions in crossed molecular beams, and by an organic chemist analysing the reaction course by the formalism of phenomenological kinetics. This implies that if one wants to speak about the mechanism of the reaction it is always necessary to specify also the point of view, from which the reaction is analysed. Thus, for example, in the case of usual reactions performed on the preparative scale, the term reaction mechanism is used to denote the detailed specification of whether the reaction proceeds in one elementary step or whether some, more or less stable, intermediates intervene. 

These requirements can be met in a so-called crossed molecular-beam experiment, which is sketched in Fig. 2.1.1. Here we can generate beams of molecules with well-defined velocities and it is possible to determine the speed of the product molecules, e.g., vc = vc, by the so-called time-of-Sight technique. The elimination of multiple scattering in the reaction zone and collisions in the beams are obtained by doing the experiments in high vacuum, that is, at very low pressures. 

Collision Experiments with Laser-Excited Atoms in Crossed Beams, I. V. Hertel and W. Stoll... 

The experimental conditions for the spectroscopy of reactive collisions are quite similar to those for the study of inelastic collisions. They range from a determination of the velocity-averaged reaction rates under selective excitation of reactants in cell experiments to a detailed state-to-state spectroscopy of reactive collisions in crossed molecular beams (Sect. 8.5). Some examples shall illustrate the state of the art ... 

An example of measurements of inelastic collisions between excited atoms A and molecules M in crossed beams is the experiment by Hertel et al. [1083], where laser-excited Na (3P) atoms collide with molecules such as N2 or CO and the energy transfer... 

J. Grosser, O. Hoffmann, S. Klose, F. Rebentrost, Optical excitation of collision pairs in crossed beams determination of the NaKr fi Z -potential. Europhys. Lett. 39,147 (1997)... 

In a full-collision experiment, e.g. in crossed-beam, beam-gas or gas cell arrangements, the reference axis is the relative velocity vector. Conceptually, the vector correlation is identical to that of photodissociation, only now the relative-velocity vector rather than the electric-field vector defines the symmetry. Thus, the reagents electronic orbital alignment can influence the product yield of a chemical reaction. Imagine, for... 

Meanwhile, energy transfer is being investigated in CH(X n)-D2 and He collisions by state-to-state integral cross section measurements in crossed beam experiments with LIF detection. Finally, theoretical studies on the interaction of open shell atoms with open shell molecules, 0( P2 i q)... 

Bonaimo R, Bouhner J, Weiner J. (1983) Determination of the absolute rate constant for associative ionization in crossed-beam collisions between Na 3 P3/2 atoms. Phys. Rev. A 28 604 08. 

There is, apparently, no need for large-scale molecular-dynamic calculations in every concrete case. Representative reactions should, however, be analyzed from this angle so as to understand possible deviations of the true trajectories of motion over the PES from the MERP and to reveal optimal energy zones of a reaction defined by the character of the vibration excitation. Furthermore, the importance of such an analysis consists in the fact that here, unlike the case with the MERP, there exists in principle a possibility of experimental verification of correctness of the trajectories calculated. In this connection, one may refer in the first place to such experiments as the collisions in crossing molecular beams and the selective laser excitation of vibration-rotational states [3, 103]. 

Scattering of electrons by laser excited sodium atoms in crossed beams [12.35] can result in elastic (3p->3p), inelastic (3p- 3d,4s), or superelastic (3p->3s) collisions. Since the orientation of the excited Na atoms depends on the polarization of the pump laser, orientational effects on the cross section can be studied. 

I.V. Hertel, W. Stoll "Collision Experiments with Laser Excited Atoms in Crossed Beams", Advances in Atomic and Molecular Physics, Vol. 13 (Academic Press, New York 1977) pp. 113-228... 

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. 

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. 

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