4.7 Article

Improved sliced velocity map imaging apparatus optimized for H photofragments

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JOURNAL OF CHEMICAL PHYSICS
卷 138, 期 14, 页码 -

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AMER INST PHYSICS
DOI: 10.1063/1.4798929

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  1. National Science Foundation [CHE-0951976]
  2. Division of Chemical Sciences, Geosciences, and Biosciences of the Department of Energy, Basic Energy Sciences [DE-FG02-05ER15629]

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Time-sliced velocity map imaging (SVMI), a high-resolution method for measuring kinetic energy distributions of products in scattering and photodissociation reactions, is challenging to implement for atomic hydrogen products. We describe an ion optics design aimed at achieving SVMI of H fragments in a broad range of kinetic energies (KE), from a fraction of an electronvolt to a few electronvolts. In order to enable consistently thin slicing for any imaged KE range, an additional electrostatic lens is introduced in the drift region for radial magnification control without affecting temporal stretching of the ion cloud. Time slices of similar to 5 ns out of a cloud stretched to >= 50 ns are used. An accelerator region with variable dimensions (using multiple electrodes) is employed for better optimization of radial and temporal space focusing characteristics at each magnification level. The implemented system was successfully tested by recording images of H fragments from the photodissociation of HBr, H2S, and the CH2OH radical, with kinetic energies ranging from <0.4 eV to >3 eV. It demonstrated KE resolution less than or similar to 1%-2%, similar to that obtained in traditional velocity map imaging followed by reconstruction, and to KE resolution achieved previously in SVMI of heavier products. We expect it to perform just as well up to at least 6 eV of kinetic energy. The tests showed that numerical simulations of the electric fields and ion trajectories in the system, used for optimization of the design and operating parameters, provide an accurate and reliable description of all aspects of system performance. This offers the advantage of selecting the best operating conditions in each measurement without the need for additional calibration experiments. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4798929]

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