Crossed molecular beam reactive scattering: from simple triatomic to multichannel polyatomic reactions

Abstract

In our laboratory a recent series of experiments by means of the crossed molecular beam (CMB) scattering technique with mass-spectrometric detection and time-of-flight analysis has been instrumental in fostering progress in the understanding of the dynamics of both simple triatomic insertion reactions and complex polyatomic addition–elimination reactions exhibiting competing channels. In the first part of this review we survey the advances made in the comprehension of the dynamics of the insertion reactions involving excited carbon, nitrogen and oxygen atoms – C(1D), N(2D), O(1D) – with H2(D2), as made possible by synergistic comparisons of experimental reactive differential cross-sections with the results of exact quantum, quasiclassical trajectory and statistical calculations on reliable ab initio potential energy surfaces. Related experimental and theoretical work from other laboratories is noted throughout. In the second part, we review the progress made in the understanding of the dynamics of polyatomic multichannel reactions, such as those of ground state oxygen and carbon atoms, O(3P) and C(3P), with the simplest alkyne, acetylene, and alkene, ethylene, as made possible by the gained capability of identifying virtually all primary reaction channels, characterising their dynamics, and determining their branching ratios. Such a capability is based on an improved crossed molecular beam instrument which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power, and variable beam crossing angle for a larger collision energy range and increased angular and velocity resolution. The scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevant ab initio potential energy surfaces. These detailed studies on polyatomic multichannel reactions provide an important bridge between crossed beam dynamics and thermal kinetics research. Contents PAGE 1. Introduction 110 2. Experimental: improved crossed molecular beams apparatus 113 2.1. Soft electron-ionisation detection 114 2.2. Product angular and velocity distributions 116 2.3. Crossed molecular beam experiments with variable beam crossing angle 118 2.4. Supersonic sources of radical beams 120 3. Triatomic insertion reactions 120 3.1. Reaction C(1D) + H2 122 3.2. Reaction N(2D) + H2 126 3.3. Reaction O(1D) + H2 129 4. Polyatomic multichannel reactions 133 4.1. Reaction O(3P) + C2H2 134 4.1.1. Product angular and TOF distributions 135 4.1.2. Determination of branching ratios 138 4.1.3. Determination of product ionisation energies 138 4.2. Reaction C(3P) + C2H2 138 4.2.1. H and H2 elimination channels, and branching ratios 140 4.2.2. Ionisation energy of c/l-C3H radicals 145 4.3. Reaction O(3P) + C2H4 145 4.3.1. Observation of all product channels and branching ratios 146 4.4. Reaction C(3P) + C2H4 151 4.4.1. H-elimination channels 151 4.4.2. C–C bond fission channels 154 5. Summary and outlook 154 Acknowledgements 157 References 158