Crossed molecular beam scattering

In crossed molecular beam scattering experiments, the direction of approach of the reactants is fixed by the configuration of the colliding beams [41]. Since by convention the incoming flux vector is directed along the space-fixed z-axis (and parallel to the field vectors E and B), we have = 0, and the general expression for the scattering amplitude (4.14) reduces to... 

We highlight some recent work from our laboratory on reactions of atoms and radicals with simple molecules by the crossed molecular beam scattering method with mass-spectrometric detection. Emphasis is on three-atom (Cl + H2) and four-atom (OH + H2 and OH + CO) systems for which the interplay between experiment and theory is the strongest and the most detailed. Reactive differential cross sections are presented and compared with the results of quasiclassical and quantum mechanical scattering calculations on ab initio potential energy surfaces in an effort to assess the status of theory versus experiment. 

Our previous speaker, P. Casavecchia, just discussed the interaction potential for the open-shell F( P) with Ar( S) in the ground state, which was determined from crossed molecular beam scattering experiments. The... 

Fig. 8.3. Schematic representation of crossed molecular beam scattering apparatus with laser ionization detection of the scattering product. Ion Imaging is used to measure the velocity of the laser-produced ions.

Flowever, in order to deliver on its promise and maximize its impact on the broader field of chemistry, the methodology of reaction dynamics must be extended toward more complex reactions involving polyatomic molecules and radicals for which even the primary products may not be known. There certainly have been examples of this notably the crossed molecular beams work by Lee [59] on the reactions of O atoms with a series of hydrocarbons. In such cases the spectroscopy of the products is often too complicated to investigate using laser-based techniques, but the recent marriage of intense syncluotron radiation light sources with state-of-the-art scattering instruments holds considerable promise for the elucidation of the bimolecular and photodissociation dynamics of these more complex species. 

Figure B2.3.7. Schematic apparatus of crossed molecular beam apparatus with synclirotron photoionization mass spectrometric detection of the products [12], To vary the scattering angle, the beam source assembly is rotated in the plane of the detector. (By pemrission from AIP.)...

The "time of flight" mass spectrometer has been used to confirm that this highly radioactive halogen behaves chemically very much like other halogens, particularly iodine. Astatine is said to be more metallic than iodine, and, like iodine, it probably accumulates in the thyroid gland. Workers at the Brookhaven National Laboratory have recently used reactive scattering in crossed molecular beams to identify and measure elementary reactions involving astatine. 

CROSSED MOLECULAR BEAM REACTIVE SCATTERING TOWARDS UNIVERSAL PRODUCT DETECTION BY SOFT ELECTRON-IMPACT IONIZATION... 

Keywords Reactive scattering reaction dynamics crossed molecular beams elementary reactions. 

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

In this chapter we have discussed the successful implementation in our laboratory, for the first time, of the soft (i.e. low energy) electron-impact ionization method for product detection in crossed molecular beams reactive scattering experiments with mass spectrometric detection. Analogous to the approach of soft photoionization by tunable VUV synchrotron radiation,... 

One of the first approaches to study the microscopic kinetics i.e. state-to-state cross sections and reaction probabilities of a chemical reaction was the crossed molecular beam experiments. The principle of the method consists of intersecting two beams of the reactant molecules in a well-defined scattering volume and catching the product molecules in a suitable detector (Fig. 9.33). 

Crossed-molecular-beam studies of differential scattering of metastable noble-gas atoms with ground-state noble-gas atoms or simple molecules is the major topic of this chapter. These studies have been carried out recently at several laboratories with the common goal of finding both real and imaginary parts of the interaction potentials to further our understanding of the dynamics of collision processes involving metastable noble-gas atoms. 

Crossed molecular beams have been used to study nearly as wide a range of alkali metal atom reactions as has been examined by diffusion flames. An excellent review has been provided by Herschbach2. The multi-step mechanism displayed for chemiluminescence studies does not apply to the scattering experiments. Only the initial bimolecular reaction is important at the low pressures used. 

N. Balucani, G. Capozza, F. Leonori, E. Segoloni, P. Casavecchia, Crossed molecular beam reactive scattering From simple triatomic to multichannel polyatomic reactions, Int. Rev. Phys. Chem. 25 (2006) 109. 

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. 

Potential energy surfaces can be built starting from experimental data (e.g., bond strengths, geometries, infrared and fluoresence spectra, molecular beam scattering cross sections, viscosity, diffusion coefficients, line broadening... 

Fig. 3. A schematic view of a crossed-molecular beam apparatus used for studying the reactions of chlorine atoms with halogen molecules. The mass spectrometer detector is rotatable about the scattering centre for measuring the angular distributions of the reaction products whose recoil velocities are determined by time-of-flight analysis. (Reproduced from ref. 558 by permission of the authors and the American Institute of Physics.)...

The reactions of alkaline earth atoms with alkali halide molecules are especially noteworthy because laser-induced fluorescence has been employed in these crossed-molecular beam experiments to measure the product internal state distributions as a function of scattering angle. For Ba + KC1 and Ca + NaCl, both the atomic and diatomic products were detected. 

The more exoergic reaction Ba + NzO has a smaller reaction cross section ( 90 A2 or 27 A2) [347, 351] and crossed-molecular beams studies [349] show that the BaO product is backward-scattered with a large amount of internal excitation ((Fr) < 0.20). Laser-fluorescence measurements [348] of the BaO(X Z+) product for the reaction in the presence of an argon buffer gas, find population of vibrational states up to v = 32. The relative populations have a characteristic temperature of 600 K for v = 0—4 and 3600 K for v = 5—32 with evidence of non-thermal population of v — 13—16. This study also observes population of A n and a 3II states of BaO with v = 0—4. A molecular beam study of Ba + N20 with laser-induced fluorescence detection indicates that the BaO( X) product is formed with a very high rotational temperature. 

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