Giovanni Capozza

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

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

Dynamics of the C(D1)+D2 reaction: A comparison of crossed molecular-beam experiments with quasiclassical trajectory and accurate statistical calculations

In this paper we report a combined experimental and theoretical study on the dynamics of the insertion reaction C((1)D)+D(2) at 15.5 kJ mol(-1) collision energy. Product angular and velocity distributions have been obtained in crossed beam experiments and quasiclassical trajectory (QCT) and rigorous statistical calculations have been performed on the recent and accurate ab initio potential energy surface of Bussery-Honvault, Honvault, and Launay at the energy of the experiment. The molecular-beam results have been simulated using the theoretical calculations. Good agreement between experiment and both QCT and statistical predictions is found.

Soft electron impact ionization in crossed molecular beam reactive scattering: The dynamics of the O(3P)+C2H2 reaction

Soft ionization by low-energy, tunable electrons is implemented for the first time in crossed molecular beam reactive scattering experiments with mass-spectrometric detection. The power of the method, which permits the suppression of the dissociative ionization of interfering species, is exemplified with the study of the O((3)P)+C(2)H(2) multichannel reaction.

A crossed-beam study of the reaction C(1D)+H2(X1Σ+, v=0)→CH(X2Π, v′)+H(2S)

Abstract Product angular and time-of-flight distributions have been measured for the first time for the prototypical insertion reaction C( 1 D)+H2 using the crossed-beam technique with mass spectrometric detection at the collision energy of 1.86 kcal mol−1. Center-of-mass angular and kinetic energy distributions have been derived for CH(X 2 Π , v′=0)+H products and compared with those of statistical calculations based on phase-space theory.

Crossed beam studies of elementary reactions of N and C atoms and CN radicals of importance in combustion.

The dynamics of some elementary reactions of N(2D), C(3P,1D) and CN(X2 sigma +) of importance in combustion have been investigated by using the crossed molecular beam scattering method with mass spectrometric detection. The novel capability of producing intense, continuous beams of the radical reagents by a radio-frequency discharge beam source was exploited. From angular and velocity distribution measurements obtained in the laboratory frame, primary reaction products have been identified and their angular and translational energy distributions in the center-of-mass system, as well as branching ratios, have been derived. The dominant N/H exchange channel has been examined in the reaction N(2D) + CH4, which is found to lead to H + CH2NH (methylenimine) and H + CH3N (methylnitrene); no H2 elimination is observed. In the reaction N(2D) + H2O the N/H exchange channel has been found to occur via two competing pathways leading to HNO + H and HON + H, while formation of NO + H2 is negligible. Formation of H + H2CCCH (propargyl) is the dominant pathway, at low collision energy (Ec), of the C(3P) + C2H4 reaction, while at high Ec formation of the less stable C3H3 isomers (cyclopropenyl and/or propyn-1-yl) also occurs; the H2 elimination channel is negligible. The H elimination channel has also been found to be the dominant pathway in the C(3P,1D) + CH3CCH reaction leading to C4H3 isomers and, again, no H2 elimination has been observed to occur. In contrast, both H and H2 elimination, leading in comparable ratio to C3H + H and C3(X1 sigma g+) + H2(X1 sigma g+), respectively, have been observed in the reaction C(3P) + C2H2(X1 sigma g+). The occurrence of the spin-forbidden molecular pathway in this reaction, never detected before, has been rationalized by invoking the occurrence of intersystem crossing between triplet and singlet manifolds of the C3H2 potential energy surfaces. The reaction CN(X2 sigma +) + C2H2 has been found to lead to internally excited HCCCN (cyanoacetylene) + H. For all the reactions the dynamics have been discussed in the light of recent theoretical calculations on the relevant potential energy surfaces. Previous, lower resolution studies on C and CN reactions carried out using pulsed beams are noted. Finally, throughout the paper the relevance of these results to combustion chemistry is considered.

Dynamics of the insertion reaction C(1D) + H2: A comparison of crossed molecular beam experiments with quasiclassical trajectory and quantum mechanical scattering calculations

In this paper we report a combined experimental and theoretical study on the dynamics of the prototype insertion reaction C(1D) + H2. Product angular and velocity distributions have been obtained in crossed beam experiments at two collision energies of 7.8 and 16.0 kJ mol−1. Quasiclassical trajectory (QCT) and quantum mechanical (QM) scattering calculations have been carried out on a recent accurate ab initio potential energy surface at the energies of the experiments. The molecular beam results have been simulated using the theoretical calculations. Reasonably good agreement between experiment and theory is found.

Combined crossed-beam studies of C(3PJ)+C2H4→C3H3+H reaction dynamics between 0.49 and 30.8 kJ mol−1

The reaction C(3PJ)+C2H4(X 1A1)→C3H3+H(2S1/2) has been studied using complementary crossed molecular beam techniques. Integral cross sections have been obtained in the range of relative translational energies ET=0.49–24.9 kJ mol−1 in experiments conducted with pulsed supersonic beams coupled with laser-induced fluorescence detection of H(2S1/2) atoms. The major reaction pathway leading to HCCCH2 (propargyl)+H has been found without any barrier, with relative integral cross sections that are proportional to (ET)−0.60±0.03 below 8 kJ mol−1. Threshold for a minor pathway, leading also to H formation, occurs around 6 kJ mol−1; the relative importance of this second pathway increases with relative translational energy. Differential cross sections have been obtained at three relative translational energies: ET=9.1, 17.2, and 30.8 kJ mol−1 in experiments conducted with continuous supersonic molecular beams coupled with universal mass spectrometric detection and time-of-flight analysis. At the lowest ET of 9.1 kJ...

Dynamics of the C+C2H2 reaction from differential and integral cross-section measurements in crossed-beam experiments

The reaction between atomic carbon and acetylene has been investigated using complementary crossed molecular beam techniques. Differential cross sections have been obtained for the reactions of both ground and excited carbon atoms, C(3PJ, 1D2)+C2H2(X 1Σg+), in experiments conducted with continuous supersonic beams, mass spectrometric detection, and time-of-flight analysis at a relative translational energy of 29.3 kJ mol−1. The reaction C(3PJ)+C2H2(X 1Σg+) has been found to lead to C3H+H and C3+H2 products in comparable amounts. Both H and H2 elimination pathways are found to proceed through the formation of a C3H2 long-lived intermediate complex whose lifetime may be comparable to its rotational period. The spin-forbidden H2 elimination channel is attributed to the occurrence of intersystem-crossing between the triplet and singlet manifolds of the C3H2 potential-energy surfaces. The reaction C(1D2)+C2H2(X 1Σg+) has been found to lead to formation of C3H+H, with a C3H center-of-mass angular distribution s...

Crossed beam studies of radical-radical reactions: O(3P) + C3H5 (allyl).

The dynamics of the radical-radical reaction O((3)P) + C(3)H(5) has been investigated by means of the crossed molecular beam technique with mass spectrometric detection at a collision energy of 73.0 kJ mol(-1); the reaction mechanism of the H-displacement channel has been elucidated, while experimental evidence of the occurrence of one or more C-C bond-breaking channels at this collision energy has been obtained.

The dynamics of the prototype abstraction reaction Cl(2P3/2,1/2)+ H2: A comparison of crossed molecular beam experiments with exact quantum scattering calculations on coupled ab initio potential energy surfaces

To investigate the relative reactivity of the two spin–orbit states of atomic Cl with molecular hydrogen, we have measured laboratory-frame differential cross sections (DCSs) using an atomic Cl beam with a known concentration of the ground (2P3/2) and excited (2P1/2) spin–orbit states. The experimental results are compared with a complete determination of the appropriate centre-of-mass DCSs from quantum mechanical scattering calculations on the Capecchi–Werner coupled ab initio potential energy surfaces (PESs). The multi-electronic-state quantum scattering prediction differs somewhat from the experimental results. This disagreement is likely due to an underestimation of the degree of rotational excitation of the HCl product, due to residual imperfections in the exit channel of the ab initio PESs. In particular, an increase in the reactivity of the excited spin–orbit state would result in poorer agreement with experiment.