Reactions Studied by Molecular Beam Techniques

BY R. GRICE

1 Introduction

The objective of reactive scattering experiments in molecular beams is to gain a full understanding of the dynamics of chemical reactions. Early studies involving alkali-metal atom reactions required only relatively straightforward techniques by means of which many important results have been obtained. Effective models of the reaction dynamics have also been developed which rely on the simplicity of the chemical interactions in alkali-metal systems. These are frequently dominated by ionic potential-energy surfaces, which intersect the covalent surfaces and give rise to electron-jump transitions. However, the most important reactions of gas-phase chemical kinetics are those of atoms and free radicals, which determine the chemistry of combustion, pyrolysis, and upper atmosphere processes. The experimental techniques required for the study of such non-alkali-metal reactions are considerably more complex than those required for alkali-metal reactions. Moreover, the chemical interactions involved are frequently much more complicated than those which govern alkali-metal reactions and hence more comprehensive experimental information is required for the development of adequate models of the reaction dynamics. Experimental techniques for the study of non-alkali-metal reactive scattering have been advanced and refined extensively over the past decade, so that it is now possible to make detailed measurements of differential reaction cross-sections for an increasing range of atom and free-radical reactions. The rapid progress of laser technology is also having a major impact on reactive scattering experiments. Reactant molecules may be promoted to excited vibrational, rotational, or electronic states and their orientations thereby selected. Similarly, the vibrational and rotational state distributions of product molecules may be determined by laser fluorescence spectroscopy. In this Report we review progress in reactive scattering experiments and examine the extent to which the measurements so far accumulated demonstrate the dependence of reaction dynamics on the electronic structure of the reaction potential-energy surface.

Progress in this field was reviewed during 1976 in Faraday Discuss. Chem. Soc., 1976, 62, on 'Potential Energy Surfaces' and the meeting on 'Energy Transfer Processes', a Report of which appeared in Ber. Bunsenges. Phys. Chem., 1976, 81. Accordingly, this Report concentrates on work that has appeared since these meetings, from the beginning of 1977 to late 1979. Chemiluminescence in the gas phase, including molecular beam experiments, has recently been reviewed in this series. Thus, discussion here of chemiluminescence measurements in molecular beams will be confined to showing their relationship to other molecular beam studies; the reader is referred to the previous review for detailed discussion of spectral assignments, lifetimes, and bond energies.

2 Experimental Techniques

Early crossed-beam studies of non-alkali-metal reactive scattering employed mass-spectrometric detection with an effusive atom or free-radical source and conventional time-of-flight velocity analysis. However, an effusive source permits only crude control of the reactant translational energy, unless a mechanical velocity selector is employed at a considerable cost in beam intensity. Moreover, the inefficiency of the conventional time-of-flight method of velocity analysis and the low intensity of the effusive beam source limits the determination of differential reaction cross-sections in this form of experiment to favourable reactions with fairly large total-reaction cross-sections Q [??] 1 Å2. In order to overcome these limitations and to extend measurements of differential reaction cross-sections to a wider range of reactions, supersonic beams of atoms and free radicals seeded in inert buffer gas are now being used in place of effusive sources and cross-correlation time-of-flight analysis in place of the conventional method.

Supersonic nozzle beam sources have been used for a considerable time to produce intense beams of stable molecules with narrow velocity distributions. In a seeded supersonic expansion of a dilute mixture of a heavy gas in a light buffer gas, the heavy molecules are accelerated to the same velocity as that of the light buffer gas. Consequently, the translational energy of the heavy molecule in such a seeded beam is increased in the ratio of the molecular weight of the heavy molecule to the mean molecular weight of the gas mixture. Thus the translational energy may be controlled by varying the molecular weight of the buffer gas. Reactive scattering apparatus employing seeded supersonic nozzle beams requires powerful source differential pumping (Figure 1) by an apparatus that has pumping speeds of 4600, 1500, and 5600 1 s-1 on the source, buffer, and scattering chambers. Supersonic beams of halogen atoms and hydrogen atoms may be produced by thermal dissociation of the diatomic molecules in a high-temperature oven. Beams of fluorine atoms seeded in helium and argon buffer gases have been produced from a nickel oven at ca. 1100 K. Similarly, beams of chlorine and bromine atoms seeded in helium and argon buffer gases have been produced by use of a graphite oven (Figure 2) at a higher temperature, ca. 2000 K. Both the nickel and the graphite ovens are heated by a direct curent [??] 450 A flowing through the oven body to obtain [??] 80% dissociation of the halogen molecules at a pressure of ca. 10 mbar, with the inert buffer gas making up a total pressure [??] 1000 mbar. These oven materials are not corroded significantly by the halogens under these conditions. A supersonic hydrogen-atom beam seeded in undissociated hydrogen molecules has been produced from a tungsten oven at ca. 2800 K. Owing to the high pressure of ca. 1500 mbar of hydrogen in the source and the high bond-strength of the hydrogen molecule only a small degree of dissociation, ca. 5%, can be achieved at temperatures below the point at which tungsten softens unduly. Rather higher degrees of dissociation were obtained by using helium and neon buffer gases to produce a hypothermal supersonic hydrogen-atom beam. Beams of alkali-metal atoms seeded in helium, argon, and hydrogen buffer gases may more readily be produced by maintaining an appropriate alkali-metal vapour pressure in a stainless-steel oven. However, the thermal dissociation method is limited by the necessity of finding oven materials that...