No abstract

Stereodynamics experiments of Ne(P) reacting with Ar, Kr, Xe, and N leading to Penning and associative ionization have been performed in a crossed molecular beam apparatus. A curved magnetic hexapole was used to state-select and polarize Ne(P) atoms which were then oriented in a rotatable magnetic field and crossed with a beam of Ar, Kr, Xe, or N. The ratio of associative to Penning ionization was recorded as a function of the magnetic field direction for collision energies between 320 cm and 500 cm. Reactivities are obtained for individual states that differ only in Ω, the projection of the neon total angular momentum vector on the inter-particle axis. The results are rationalized on the basis of a model involving a long-range and a short-range reaction mechanism. Substantially lower probability for associative ionization was observed for N, suggesting that predissociation plays a critical role in the overall reaction pathway.

The stereodynamics of the Ne( 3 P2)+Ar Penning and Associative ionization reactions have been studied using a crossed molecular beam apparatus. The experiment uses a curved magnetic hexapole to polarise the Ne( 3 P2) which is then oriented with a shaped magnetic field in the region where it intersects with a beam of Ar( 1 S). The ratios of Penning to associative ionization were recorded over a range of collision energies from 320 cm −1 to 500 cm −1 and the data was used to obtain Ω state dependent reactivities for the two reaction channels. These reactivities were found to compare favourably to those predicted in the theoretical work of Brumer et al.

Detailed understanding of the mechanism of the combustion relevant multichannel reactions of O(3 P) with unsaturated hydrocarbons (UHs) requires the identification of all primary reaction products, the determination of their branching ratios and assessment of intersystem crossing (ISC) between triplet and singlet potential energy surfaces (PESs). This can be best achieved combining crossed-molecular-beam (CMB) experiments with universal, soft ionization, mass-spectrometric detection and time-of-flight analysis to high-level ab initio electronic structure calculations of triplet/singlet PESs and RRKM/Master Equation computations of branching ratios (BRs) including ISC. This approach has been recently demonstrated to be successful for O(3 P) reactions with the simplest UHs (alkynes, alkenes, dienes) containing two or three carbon atoms. Here, we extend the combined CMB/theoretical approach to the next member in the diene series containing four C atoms, namely 1,2-butadiene (methylallene) to explore how product distributions, branching ratios and ISC vary with increasing molecular complexity going from O(3 P)+propadiene to O(3 P)+1,2-butadiene. In particular, we focus on the most important, dominant molecular channels, those forming propene+CO (with branching ratio ∼0.5) and ethylidene+ketene (with branching ratio ∼0.15), that lead to chain termination, to be contrasted to radical forming channels (branching ratio ∼0.35) which lead to chain propagation in combustion systems.

The chemi-ionization of Ar, Kr, N2, H2, and D2 by Ne­(3P2) and of Ar, Kr, and N2 by He­(3S1) was studied by electron velocity map imaging (e-VMI) in a crossed molecular beam experiment. A curved magnetic hexapole was used to state-select the metastable species. Collision energies of 60 meV were obtained by individually controlling the beam velocities of both reactants. The chemi-ionization of atoms and molecules can proceed along different channels, among them Penning ionization and associative ionization. The evolution of the reaction is influenced by the internal redistribution of energy, which happens at the first reaction step that involves the emission of an electron. We designed and built an e-VMI spectrometer in order to investigate the electron kinetic energy distribution, which is related to the internal state distribution of the ionic reaction products. The analysis of the electron kinetic energy distributions allows an estimation of the ratio between the two-reaction channel Penning and associative ionization. In the molecular cases the vibrational or electronic excitation enhanced the conversion of internal energy into the translational energy of the forming ions, thus influencing the reaction outcome.