Molecular beam deflection

Collision-induced dipoles manifest themselves mainly in collision-induced spectra, in the spectra and the properties of van der Waals molecules, and in certain virial dielectric properties. Dipole moments of a number of van der Waals complexes have been measured directly by molecular beam deflection and other techniques. Empirical models of induced dipole moments have been obtained from such measurements that are consistent with spectral moments, spectral line shapes, virial coefficients, etc. We will briefly review the methods and results obtained. 

Many factors and considerations are germane to future research in this area. On the technical side, achieving cluster size selection stands as one of the most important and sought-after goals. It would be most desirable to achieve this while maintaining sufficiently high densities for studies of photoinitiated reactions to be carried out with product state resolution and/or ultrafast time resolution. The two methods that, in our opinion, are most viable are molecular beam deflection, as pioneered by Buck (1994) and coworkers, and laser-based double-resonance methods. Less direct approaches are deemed inferior. 

The molecular beam deflection method is shown schematically in Figure 3-17 (Buck et al. 1985). It is based on momentum transfer between clusters entrained in a molecular beam and rare gas atoms which are the constituents of a second molecular beam at 90° to the cluster beam. Collisions between the rare gas atoms and the clusters under single-collision conditions deflect a small percentage of the clusters from their original path. The maximum deflection angle depends on the mass of the cluster. For example, binary clusters may be deflected into a broad range of angles with a well defined upper limit set by the momentum conservation... 

Figure 3-17. Schematic of the molecular beam deflection method. Under single-collision conditions, collisions between clusters and He atoms deflect the clusters to angles whose maxima are determined by momentum conservation.

Figure 3-18. Experimental demonstration of the molecular beam deflection method, from Buck et al. (1985). The Newton circles show the peak positions associated with the different clusters S) denotes the velocity of the center-of-mass for different clusters. The TOF data were recorded at a lab angle of 10° note the correspondence between peaks 2 and 3 and the corresponding intersection points on the Newton circles.

The difficulty In distinguishing permanent dipole from Induced dipole moment effects In a molecular beam deflection... 

Signs of electric dipole moments, which must be obtained by techniques other than Stark effect measurements, are listed in the column Varia, remarks . There exist some molecular beam deflection data of electric dipole moments which have reached the accuracy of dipole moments determined by microwave spectroscopy or may even be better. These values have been included in columns 3-"6. If there are experimental dipole moments from other spectroscopic regions they are listed too. The methods are given with abbreviations as explained in part 1. 

CsK a = 99(6) 10 J determined by molecular beam deflection techniques. 93Tar ... 

Another method, based on an old idea about radiation pressure, uses the local separation of different isotopes in atomic or molecular beams. If the laser beam which crosses the molecular beam at right angles is tuned to an absorption line of a defined isotope in a molecular beam containing an isotopic mixture, the recoil from the absorption of the laser photons results in a small additional transverse velocity component. This leads to a beam deflection for the absorbing molecules which enables the desired isotope to be collected in a separate collector 154g)... 

Other measurements. Induced dipole moments can be measured by most of the familiar methods that are designed to measure permanent dipole moments. We mention in particular the beam deflection method by electric fields, using van der Waals molecules, and molecular beam electric resonance spectroscopy of van der Waals molecules [373, 193, 98]. 

Molecular beam experiments study reactive and non-reactive collisions between molecules by observing the deflection of each molecule from its original path as a result of the collision. By studying the amount of scattering into various angles, much useful kinetic information can be found. 

Here a collision is seen in terms of the deflection of the molecules from their original paths as they approach each other. Molecular beam experiments are ideally suited to... 

The relation between the deflection angles in the two coordinate systems is derived below, in the special case where the target atom is at rest before the collision. This case represents of course not the typical situation in a crossed molecular-beam experiment. However, it greatly simplifies the relation and the derivation displays the essence of the problem. The general case is considered in Appendix C. 

FIGURE 28 Deflection of a molecular beam of CS2 using the dipole force of a focused laser beam. [Reproduced with permission from Stapelfeldt, H., Sakai, H., Constant, E., and Corkum, P. B. (1997). Phys. Rev. Lett. 79, 2787.]... 

P PAE PD PDS PEC PL PLE PMBE PPC PPPW PR PV PWP PWPP pi-MODFET precipitate power added efficiency photodetector photothermal deflection spectroscopy photoelectrochemical photoluminescence photoluminescence excitation spectroscopy plasma-assisted molecular beam epitaxy persistent photoconductivity pseudo-potential plane-wave photoreflectance photovoltage plane-wave pseudo-potential plane-wave pseudo-potential piezoelectric modulation doped field effect transistor... 

For the reactions K + HBr and K + DBr, the KBr recoil energy distribution has been determined in a crossed-molecular beam experiment using a mechanical velocity selector. No difference was found in the form of the translational energy distributions for the two reactions for which a value of 0.30 may be derived. Although all the angular momentum appears in the product rotation, the moments of inertia for the alkali halides are large, which implies that the mean product rotational energy is quite small ( 0.21, 0.21 and 0.09 for K, Rb, Cs + HBr, respectively [3] these values are derived from the rotational temperatures obtained by electric deflection analysis). 

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