An aplanatic-lens velocity map imaging spectrometer with improved kinetic energy resolution for photoions

•Aplanatic Lens Velocity Map Imaging (AL-VMI) suppresses the spherical aberration of the electrode lens largely by applying a stepped voltage.•The energy resolution is improved by 9% compared to Eppink–Parker setup.•Experiments of dissociation of H+ show coincident results with the simulation.

We propose a new method to improve the energy resolution of photoions with an aplanatic velocity map imaging spectrometer. Building on the traditional VMI spectrometer with three aperture electrodes, we apply a stepped voltage on the extractor electrode of the spectrometer to reduce the spherical aberration effect of the electrostatic lens. This stepped voltage acts as a diverging electrostatic lens in ion optics. A combination of the converging electrostatic lens and the subsequent diverging electrostatic lens constitutes an aplanatic electrostatic lens. With precisely controlling the moment of the voltage, we suppress the spherical aberration effect induced by the converging electrostatic lens in the traditional VMI spectrometer. Both simulated and experimental results demonstrate that the energy resolution is significantly improved using this aplanatic-lens VMI spectrometer.

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