A three-dimensional (3D) ion velocity imaging method was developed to measure the product velocity distributions in crossed molecular beam experiments. While maintaining conventional two-dimension velocity mapping, the third velocity component was mapped linearly to the ion time of flight. A weak extraction field was used to spread the ion turnaround time to several hundred nanoseconds, which permits good resolution for selection of the longitudinal velocity. A fast gated (⩾5 ns) intensified charge coupled device camera was used to record time-sliced ion images. Calibration of the apparatus was done by measuring O+ images from the multiphoton dissociation/ionization of O2. The resolution in velocity achieved was about 1% (Δv/v) in slicing through the center of a Newton sphere. The overall performance was examined by observing product ion images from the F+CD4→DF+CD3 reaction. To detect CD3+ with kinetic energy release of about 1 eV, 50 ns time slicing provides sufficient velocity resolution, such that resolution of the image is mainly limited by the spread in velocity of the two molecular beams. These ion optics can focus on a large volume of ion cloud, which is crucial in crossed molecular beam experiments. Direct 3D imaging also simplifies data analysis. This direct 3D ion imaging method provides a powerful tool with which to study systems with no cylindrical symmetry.

When a chemical reaction forms two molecular products, even if the state-resolved differential cross section (DCS) for each product is obtained individually, the coincident attributes of the coproducts are still lacking. We exploit a method that provides coincidence information by measuring the state-resolved, pair-correlated DCS. Exemplified by the reaction F + CD4 --> DF + CD3, a time-sliced ion velocity imaging technique was used to measure the velocity distribution of a state-selected CD3 product and to reveal the information of the coincident DF in a state-correlated manner. The correlation of different product state pairs shows a striking difference, which opens up a new way to unravel the complexity of a polyatomic reaction.

In the course of an extensive investigation aimed at understanding the detailed mechanisms of a prototypical polyatomic reaction, several remarkable observations were uncovered. To interpret these findings, we surmise the existence of a reactive resonance in this polyatomic reaction. The reaction of concern is F+CH4-->HF+CH3, and the abnormal attributes were revealed only near the reaction threshold. The discovery of reactive resonance in a polyatomic reaction is more than just an extension from a typical atom+diatom reaction. As shown here, it holds great promise to disentangle the elusive intramolecular vibrational dynamics of transient collision complex in the critical transition-state region.

Differential cross section polarization moments: Location of the D-atom transfer in the transition-state region for the reactions Cl+C 2 D 6 →DCl (v ′ =0,J ′ =1)+ C 2 D 5 and Cl+CD 4 →DCl (v ′ =0,J ′ =1)+ CD 3