Application of time-sliced ion velocity imaging to crossed molecular beam experiments

Abstract

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.