A large aperture magnification lens for velocity map imaging

We have designed and implemented a large aperture electrostatic Einzel lens that magnifies the images of low energy ions or electrons in a standard velocity map imaging apparatus by up to a factor of 5 while allowing the normal use of the apparatus (without blocking any part of the detector). The field strength in the interaction region remains reasonably constant with or without magnification, and the lens can be used in the normal "crush" mode or with any of the different variants of the "slicing" mode. We have characterized the performance of the lens by imaging ion recoil due to two-photon resonant three-photon ionization (2+1) REMPI] of O((3)P(2)) atoms and by imaging slow NO molecules from the near-threshold photodissociation of the NO-Ar van der Waals complex.