Velocity map imaging of the dynamics of bimolecular chemical reactions

Abstract

The experimental technique of velocity map imaging (VMI) enables measurements to be made of the dynamics of chemical reactions that are providing unprecedented insights about reactive scattering. This perspective article illustrates how VMI, in combination with crossed-molecular beam, dual-beam or photo-initiated (PHOTOLOC) methods, can reveal correlated information on the vibrational quantum states populated in the two products of a reaction, and the angular scattering of products (the differential cross section) formed in specific rotational and vibrational levels.Reactions studied by VMI techniques are being extended to those of polyatomic molecules or radicals, and of molecular ions. Subtle quantum-mechanical effects in bimolecular reactions can provide distinct signatures in the velocity map images, and are exemplified here by non-adiabatic dynamics on coupled potential energy surfaces, and by experimental evidence for scattering resonances.2

Velocity map imaging the dynamics of the reactions of Cl atoms with neopentane and tetramethylsilane

“…The energetics and kinetics of these reactions are very similar to those for reactions (2) and (3); the reactions are all rapid and direct, have either a low or no barrier to reaction and the exothermicities of the primary hydrogen abstraction reactions are typically about -3 kcal mol -1 . They show cold rotational distributions of the HCl products, broad angular scattering and a substantial fraction of the available energy (~20-50%) is deposited into internal energy of the radical coproduct.…”

“…1,2 In addition to numerous measurements of reaction rates, 1 the dynamics of these reactions (and their deuterated analogs) have been examined using a variety of experimental techniques. In an early dynamical study, Flynn and co-workers employed infra-red absorption spectroscopy of the DCl products of reaction of Cl atoms with d 12 -cyclohexane, 3 but the subsequent combination of resonance enhanced multi-photon ionization (REMPI) or vacuum ultraviolet (VUV) ionization with velocity resolution of reaction products has proved most informative.…”

Chemical Reaction Dynamics in Liquid Solutions

“…Integration of the rate equations gives time dependent populations of the vibrational levels, and spectral band intensities then depend on the difference in population of levels connected by an IR transition. 22 This method provides an excellent description of the experimental observations, as exemplified in figure 2 by the fit to a representative data set for the CN + TMS reaction in CDCl 3 . The values of the resultant rate coefficients for the reactive processes are indicative of the nascent HCN(v 1 v 2 v 3 ) populations.…”

“…Moreover, VMI experiments can simultaneously map out the differential cross sections that quantify the angular scattering distributions for products in selected quantum states. 3,4 The outcomes of such experiments can be interpreted both qualitatively and quantitatively in terms of the kinematics of the reaction, locations and angular dependence of energy barriers on a potential energy surface (PES), the shapes of transition states (TS) and of the PES in the pre-and post-TS regions, crossings between PESs, and the nuclear dynamics on the PES. 1,2 Here, theoretical and experimental studies of fundamental chemical reaction mechanisms meet, and critical comparisons can be drawn between the outcomes of quasi-classical trajectory (QCT) or quantum mechanical (QM) scattering calculations and experimental data.…”

“…1,2 Isolated collisions can be observed in experiments that employ molecular beam methods under high vacuum conditions, and, in combination with laser-based probes, electronic, vibrational, rotational and translational energy content of the products can be resolved, as well as preferred directions of scattering and of orientation of rotational or electronic angular momenta. 2, 3 In favourable cases, such as for the reaction F + CD 4 → DF + CD 3 , methods such as velocity map imaging (VMI) reveal correlations between the vibrational energy content of both products, and indeed whether specific vibrational modes of the products are excited. Moreover, VMI experiments can simultaneously map out the differential cross sections that quantify the angular scattering distributions for products in selected quantum states.…”