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 can reach the temperatures required to attain significant dissociation and resist corrosion by the reactive species thus produced. An alternative method of dissociation involves the use of a high-pressure discharge source. The production of a supersonic oxygen-atom beam seeded in helium buffer gas was first achieved by Miller and Patch using radio-frequency excitation. More recently, a microwave discharge source has been used to produce supersonic beams of oxygen, hydrogen, and chlorine atoms seeded in helium and neon buffer gases. The source (Figure 3) operates in a vacuum to create a discharge in the quartz tube, which is foreshortened compared with that reported in ref. 13. Gas issuing from the nozzle at the end of the quartz tube is sampled by the skimmer to yield a supersonic beam with [??]70% dissociation of oxygen, hydrogen, or chlorine molecules. Microwave excitation is generally preferable to r.f. excitation owing to its stronger coupling to the discharge plasma. However, the radio-frequency method has been extended by Lee and co-workers to produce supersonic beams of oxygen atoms seeded in argon and helium. In this source the plasma extends through the nozzle and the beam contains ions and metastable electronically excited O(1D) in addition to ground-state O(3P) oxygen atoms when using helium buffer gas. The possible production of metastable electronicallyexcited species must always be considered when using a discharge source rather than thermal dissociation. Even under the more controlled conditions of microwave excitation, where the plasma does not extend through the nozzle, the metastable electronically-excited O2(lΔg) molecule is expected to be present in the beam. Fortunately, O2(1Δg) molecules are generally much less reactive than ground-state O(3P) atoms, but the chemistry of each reaction must be checked to ensure that O2(1Δg) makes no significant contribution to the observed reactive scattering. Direct-current discharges have been used as sources of supersonic hydrogen and nitrogen atom beams with very high, ca. 6000 K, effective source temperatures. However, this very hot plasma also extends through the nozzle of these sources and the production of metastable electronically-excited species presents a very acute problem. This may inhibit the use of these sources in the study of reactive scattering by ground-state atoms particularly for the nitrogen case. On the other hand, discharge sources may be used to study reactions of metastable electronically-excited atoms that are endoergic for ground-state atoms, provided that only a single electronically-excited state contributes to the reactive scattering. It has proved possible to use the r.f. source in this way to measure reactive scattering by metastable O(1D) atoms. A supersonic beam of mercury atoms in the metastable Hg(3P0) electronically-excited state seeded in nitrogen buffer gas has been produced by illuminating a quartz nozzle tube with resonance radiation from low-pressure mercury lamps.

The conventional method of time-of-flight velocity analysis, whereby a single pulse of molecules is transmitted by a chopper disc and the time-of-flight distribution measured in computer-generated time channels before a further pulse of molecules is transmitted, suffers from a very poor duty cycle, ca. 5 %. This may be overcome if the single slot of the conventional disc is replaced by a pseudorandom sequence of slots and teeth, which constitute a Hadamard sequence of 2n – 1 elements, where n is an integer and the duty cycle is raised to ca. 50%. The arrival times of molecules at the detector resulting from the pseudorandom chopper disc are measured in computer-generated time channels and the results cross-correlated with the appropriate complementary sequence. Hirschy and Aldridge first applied this method in molecular beam experiments to measure the velocity distribution of an argon nozzle beam. A recent implementation of the cross-correlation method for measuring velocity distributions of reactive scattering is illustrated by the chopper disc shown in Figure 4 and the computer interface shown in Figure 5. In this implementation of the cross-correlation method, the rotation of the chopper disc and the advance of the channel address register in the interface are permanently synchronized. The atom or free radical beam is also modulated by a tuning-fork chopper to ensure the unambiguous measurement of scattering from the beam-intersection zone. After cross-correlation with the complementary sequence, the time-of-flight data is deconvolved and transformed to velocity space using the same algorithms as in the conventional method. Consequently, the time-of-flight system may use the cross-correlation or conventional methods with equal facility. While the cross-correlation method is preferred for measuring the low intensities of reactive scattering, the conventional method is more convenient for measuring velocity distributions of energetic seeded beams where intensities must be reduced by restricting the detector aperture and the poor efficiency of the conventional method is not detrimental.

The application of lasers to reactive scattering experiments continues to be determined by the availability of certain types of high-powered lasers. The hydrogen halide chemical lasers are particularly effective at producing vibrational excitation of reactant hydrogen halide beams, as the laser transition coincides with the absorbing transition of the beam molecules. This method has been to excite hydrogen chloride and fluoride molecules from the vibrational ground state to specific rotational levels J = 1 — 4 of the first vibrationally-excited state. Electronic excitation of reactant molecules usually depends upon chance coincidences between an available intense laser line and an absorption transition of a reactant molecule. The excitation of the v' = 43, J' = 13 vibration–rotational level of the I2[B3 Π(0+u)] state by an argon ion laser has proved a useful example. Indeed, excitation of a diatomic molecule by plane or circularly polarized light yields excited molecules with selected orientations and this excitation has recently been used to study the orientation dependence of the reaction of I2[B3 Π(O+u)] with indium and thallium atoms. Similarly, rotation-vibrational excitation of hydrogen fluoride has been used to study the orientation dependence of its reaction with strontium atoms. Atoms and molecules with low-lying excited electronic states may be excited selectively by a tunable dye laser. This method has been used to excite beams of sodium atoms to the 2P electronically-excited state, strontium and barium atoms to the 3P electronically-excited state, and sodium dimers to the 1Πu electronically-excited state. In each of these cases of excitation to electronically-excited states the laser must illuminate the beam-intersection zone, since the excited atoms and molecules travel only a short distance, [??]0.1 mm, during their radiative lifetimes. However, excitation of a diatomic molecule prior to the beam-intersection zone allows fluorescent decay to vibration-rotation states of the ground electronic state different from the vibration-rotation state from which it was excited. Indeed excitation of a beam of sodium dimers to the Na2(1Πu) electronically- excited state by a tunable dye laser followed by fluorescent decay has been used to modulate specific-rotational-state populations of the ground vibrational and electronic state.

The laser-induced fluorescence method of determining product internal-state distributions has been improved by the introduction of optical-fibre techniques in the construction of the rotatable detector (Figure 6). This permits measurement of angular distributions of product scattering while maintaining the beam sources stationary as required for the use of seeded nozzle beam sources. When the laser line used in laser-induced fluorescence detection is sufficiently narrow the Doppler shift due to the component of molecular velocity along the direction of the laser beam may be observedZ6 and information on the product velocity distribution thereby obtained. Indeed it has been suggested that full contour maps of reactive scattering of specific product internal states as a function of laboratory scattering angle and velocity may be determined by Fouriq transformation of Doppler profiles measured by laser-induced fluorescence. In this method the laser beam passes through the beam intersection region and Doppler profiles are measured for different angles of incidence of the laser beam with respect to the molecular beams. Thus components of the laboratory product velocity distribution along the laser beam are measured by the Doppler profile at each angle of incidence and the full set of Doppler profiles may be inverted to obtain a contour map of the product angle-velocity distribution by standard Fourier transform techniques. Such a contour map has recently been obtained for the hydroxy-radical product scattering from the reaction of an effusive hydrogen-atom beam with a cross-beam of NO2 molecules. The detection of hydroxy-radicals and also of bromine atoms by the laser-induced fluorescence method illustrates the extension of tunable dye laser technology into the near-uv., which promises to bring many more species within the scope of this detection method.

A high-power argon ion laser has been used to produce nonresonant two-photon ionization of alkali-metal dimers, and tunable dye lasers to produce resonant multiphoton ionization of alkali-metal dimer, iodine, aniline, and benzene molecules. Resonant multiphoton ionization is a promising method of detecting scattered reaction products since it is insensitive to scattered laser light and the detection of ions is much more efficient than the detection of photons in the laser-induced fluorescence method. Indeed resonant multiphoton ionization has been used to detect BaCl product molecules from the reaction of barium atoms with hydrogen chloride molecules. However, the observed ionization was not readily related to the internal-state distribution of the reaction products. This appears to present a still more severe problem in the multiphoton ionization of polyatomic molecules where fragmentation may be more extensive than in electron bombardment ionization.