Molecular beams collision dynamics

As with most methods for studying ion-molecule kinetics and dynamics, numerous variations exist. For low-energy processes, the collision cell can be replaced with a molecular beam perpendicular to the ion beam [106]. This greatly reduces the thennal energy spread of the reactant neutral. Another approach for low energies is to use a merged beam [103]. In this system the supersonic expansion is aimed at the tluoat of the octopole, and the ions are passed tluough... 

For example, energy transfer in molecule-surface collisions is best studied in nom-eactive systems, such as the scattering and trapping of rare-gas atoms or simple molecules at metal surfaces. We follow a similar approach below, discussing the dynamics of the different elementary processes separately. The surface must also be simplified compared to technologically relevant systems. To develop a detailed understanding, we must know exactly what the surface looks like and of what it is composed. This requires the use of surface science tools (section B 1.19-26) to prepare very well-characterized, atomically clean and ordered substrates on which reactions can be studied under ultrahigh vacuum conditions. The most accurate and specific experiments also employ molecular beam teclmiques, discussed in section B2.3. 

Kinetics on the level of individual molecules is often referred to as reaction dynamics. Subtle details are taken into account, such as the effect of the orientation of molecules in a collision that may result in a reaction, and the distribution of energy over a molecule s various degrees of freedom. This is the fundamental level of study needed if we want to link reactivity to quantum mechanics, which is really what rules the game at this fundamental level. This is the domain of molecular beam experiments, laser spectroscopy, ah initio theoretical chemistry and transition state theory. It is at this level that we can learn what determines whether a chemical reaction is feasible. 

Bernstein, R.B. (ed) (a) Chemical Dynamics via Molecular Beam and Laser Techniques, Clarendon Press, Oxford, New York (1982). (b) Atom-Molecule Collision Theory A guide to experimentalist, Plenum Press, New York (1979). 

Experiments have also played a critical role in the development of potential energy surfaces and reaction dynamics. In the earliest days of quantum chemistry, experimentally determined thermal rate constants were available to test and improve dynamical theories. Much more detailed information can now be obtained by experimental measurement. Today experimentalists routinely use molecular beam and laser techniques to examine how reaction cross-sections depend upon collision energies, the states of the reactants and products, and scattering angles. 

The theory of molecular scattering has now been developed to the point that scattering calculations can be made with an accuracy sufficient for comparison with current experiments. Thus any discrepancy between theory and experiment should be traced to an inadequate knowledge of the interaction potentials, or to experimental errors, rather than to approximations in the collision dynamics. This tighter coupling of theory and experiment should permit a much more fruitful utilization of the results of molecular beam scattering. 

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. 

In this chapter we summarize the current status of the low-energy scattering of noble-gas metastable atoms in molecular beams. A brief summary of potential scattering theory that is relevant to the understanding of collision dynamics, as well as a description of the experimental method, precedes the presentation of experimental findings. The experimental results presented are mainly from the authors laboratories. 

Detailed reaction dynamics not only require that reagents be simple but also that these remain isolated from random external perturbations. Theory can accommodate that condition easily. Experiments have used one of three strategies. (/) Molecules in a gas at low pressure can be taken to be isolated for the short time between collisions. Unimolecular reactions such as photodissociation or isomerization induced by photon absorption can sometimes be studied between collisions. (2) Molecular beams can be produced so that motion is not random. Molecules have a nonzero velocity in one direction and almost zero velocity in perpendicular directions. Not only does this reduce collisions, it also allows bimolecular interactions to be studied in intersecting beams and increases the detail with which unimolecular processes that can be studied, because beams facilitate dozens of refined measurement techniques. 

Transition state theory, especially with its recent developments, has proved a very powerful tool, vastly superior to collision theory. It has only recently been challenged by modem advances in molecular beams and molecular dynamics which look at the microscopic details of a collision, and which can be regarded as a modified collision theory. These developments along with computer techniques, and modem experimental advances in spectroscopy and lasers along with fast reaction techniques, are now revolutionizing the science of reaction rates. 

In the previous chapter, we have discussed the reaction dynamics of bimolecular collisions and its relation to the most detailed experimental quantities, the cross-sections obtained in molecular-beam experiments, as well as the relation to the well-known rate constants, measured in traditional bulk experiments. Indeed, in most chemical applications one needs only the rate constant—which represents a tremendous reduction in the detailed state-to-state information. 

One of the practical applications of the optical polarization of molecular angular momenta is the investigation of the stereochemical forces in the process of molecule-atom collisions. The most complete information on the dependence of atom-molecule interaction potential on the orientation of the molecule with respect to the relative collision velocity can be obtained by the method of molecular beams, and often in conjunction with inhomogeneous magnetic and electric fields which orient the molecules. Such investigations are undoubtedly very complex, and their realization rather costly. To convince oneself of it one might just peruse the Proceedings of the First Workshop on Dynamic Stereochemistry in Jerusalem in 1986 [66] see also [67, 341],... 

Which experimental approach can best reveal the chemical dynamics of carbon-centered radicals Recall that since the macroscopic alteration of combustion flames, atmospheres of planets and their moons, as well as of the interstellar medium consists of multiple elementary reactions that are a series of bimolecular encounters, a detailed understanding of the mechanisms involved at the most fundamental microscopic level is crucial. These are experiments under single collision conditions, in which particles of one supersonic beam are made to collide only with particles of a second beam. The crossed molecular beam technique represents the most versatile approach in the elucidation of the energetics and... 

The data available from experiments such as molecular beam scattering are now becoming very detailed and include measurements of the number of product molecules in individual vibration-rotation states as a result of reactive encounters . The first reasonably unambiguous resonance in reactive collisions (in the F -f Hj reaction) has recently been observed . These phenomena can only be understood through dynamical studies of... 

---