Goals: Probe the mechanisms of polyatomic reactions in detail, with particular emphasis on the effects of different modes of reactant vibrational excitation on reactivity, and on energy disposal in the products. Understand the circumstances when mode-selective control of chemistry may be possible.
Motivation: Vibrational mode-selected differential scattering is a powerful tool for probing reaction mechanisms of small polyatomic systems. When effects of specific reactant vibrational modes are observed, they provide insight into the dynamics of the early time part of the collisions, before collisional interactions can scramble the initial excitation. Recoil angular distributions probe the collision dynamics, revealing whether reaction is direct or complex-mediated, and giving evidence of collision time scales. Recoil energy distributions probe partitioning of available energy into the internal v.s. relative degrees of freedom, and provide further insight into collision time scales. When carried out over a wide range of collision energies, and augmented by theory, quite a complete picture of the reaction dynamics can be deduced, even for complicated reactions.
Experimental Techniques: Several different methods are used to generate reactant ions with controled excitation in different vibrational modes. Multiphoton ionization through Rydberg intermediate states can generate state-selected ions with good intensity for molecules where the spectroscopy is suitable. In other cases, multiphoton Mass Analyzed Threshold Ionization (MATI) is needed. In special cases, rotational selection is also possible. Reactions are studied with high collision energy resolution using guided ion beam (GIB) methods, including measurement of reaction cross sections, product branching, and differential cross sections (i.e., recoil energy and angular distributions of the products).
Computational Techniques: In all cases, we use ab initio electronic structure calculations to map out the energetics of the reaction coordinate (complexes, transition states, ...). For select systems, we also carry out ab initio Direct Dynamics trajectory calculations, using a cluster of dedicated workstations. These calculations are far more infomative about the dynamics, allowing us to probe the origins of experimentally measured vibrational effects, and other details of the observed scattering behavior.
The Instrument (MATI configuration)
Typical Results: (Note: the slide shows may be too big if you have a slow internet connection).Ions are created by MATI or MPI at the intersection of a molecular beam with the laser(s). By using appropriate ionization schemes, we can produce ions with variable excitation in particular vibrational modes (i.e. stretch, bends, ...). For small molecules we can also select the rotational state of the reactant ion. The ions are focused by an rf-quadrupole ion guide through a gating system that mass and kinetic energy-selects the beam. The reactant beam is then guided through a scattering cell where reactions take place under single collision conditions. Products are collected by the ion guide, and their time of flight (TOF) is measured. By measuring TOFs under variable guiding field conditions, we can measure angular and energy distributions of the scattered ions.
FIGURE 1. Guided ion beam instrument for mode-selective differential scattering studies