Crossed beam polyatomic reaction dynamics: recent advances and new insights

Abstract

Over the past ten years or so, great advances in our understanding of the dynamics of elementary (bimolecular) polyatomic reactions in the gas-phase have occurred. This has been made possible by critical improvements (a) in crossed molecular beam (CMB) instruments with rotating mass spectrometric detection and time-of-flight analysis, especially following the implementation of soft ionization (by tunable low energy electrons or vacuum-ultraviolet synchrotron radiation) for product detection with increased sensitivity and universal detection power, and (b) in REMPI-slice velocity map ion imaging (VMI) detection techniques in pulsed CMB experiments for obtaining product pair-correlated information through high-resolution measurements directly in the center of mass system. The improved universal CMB method is permitting us to identify all primary reaction products, characterize their formation dynamics, and determine the branching ratios (BRs) for multichannel non-adiabatic reactions, such as those of ground state oxygen atoms, O(P), with unsaturated hydrocarbons (alkynes, alkenes, dienes). The improved slice VMI CMB technique is permitting us to explore at an unprecedented level of detail, through pair-correlated measurements, the reaction dynamics of a prototype polyatomic molecule such as CH (and isotopologues) in its ground state with a variety of important X radicals such as F, Cl, O, and OH. In this review, we highlight this recent progress in the field of CMB reaction dynamics, with an emphasis on the experimental side, but with the related theoretical work, at the level of state-of-the-art calculations of both the underlying potential energy surfaces and the reaction dynamics, noted throughout. In particular, the focus is (a) on the effect of molecular complexity and structure on product distributions, branching ratios and role of intersystem crossing for the multichannel, addition-elimination reactions of unsaturated hydrocarbons with O atoms, and (b) on the very detailed dynamics of the abstraction reactions of ground-state methane (and isotopologues) with atoms (F, Cl, O) and diatoms (OH), with inclusion of also rotational mode specificity in the vibrationally excited methane reactions.

Direct observation of product-pair correlations in rotationally inelastic collisions of ND3 with D2

“…The idea is that if one can accurately measure the internal state and kinetic energy of one molecular collision partner, one can infer the simultaneous excitation of the unobserved collision partner from a change in kinetic energy of the detected molecule. This method has been pioneered and successfully used in molecular reactive collision experiments, [20][21][22][23][24][25][26][27][28][29][30][31] in which vibrational product-pairs in the reaction products were probed. For rotationally inelastic scattering, however, the direct observation of rotational product-pairs is challenging, as the typical rotational energy spacing in molecules is rather small, and high image resolutions are required to resolve individual rotational channels in the scattering images.…”

Experimental Studies of Gas-Phase Reactivity in Relation to Complex Organic Molecules in Star-Forming Regions

“…s c r i p t e.g. Pan et al242 . Several classes of reactions have been tackled including direct abstraction…”

Benchmarking the polyatomic reaction dynamics of X+methane

“…As time is approaching, I am still wondering. Compiling several of my recent reviews published elsewhere [1][2][3][4][5][6] or reiterating similar topics simply does not make any sense to me-besides the ethics issue, unless I have lots of new insights to offer. A few more recent and comprehensive studies on different topics, such as that on the reactive rainbow phenomenon [7][8][9] or that on the effects of reactant's vibrational phase on reactivity, could be a good choice.…”