Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen

TLDR

Open lens electrodes avoid transmission loss, trajectory deflections, and blurring that plague grid electrodes. The study demonstrates that two‑photon Rydberg excitation of O₂ at 225 nm yields ion and electron images with markedly improved spatial resolution. A three‑plate aperture lens assembly coupled to 2D detection implements velocity‑map imaging, mapping particles of identical velocity to a single detector point, and its performance was compared to grid electrodes. Electrostatic lenses substantially improve 2D ion/electron imaging, producing higher spatial resolution in O₂ photodissociation images, with trajectory simulations agreeing with experiment.

Abstract

The application of electrostatic lenses is demonstrated to give a substantial improvement of the two-dimensional (2D) ion/electron imaging technique. This combination of ion lens optics and 2D detection makes “velocity map imaging” possible, i.e., all particles with the same initial velocity vector are mapped onto the same point on the detector. Whereas the more common application of grid electrodes leads to transmission reduction, severe trajectory deflections and blurring due to the non-point source geometry, these problems are avoided with open lens electrodes. A three-plate assembly with aperture electrodes has been tested and its properties are compared with those of grid electrodes. The photodissociation processes occurring in molecular oxygen following the two-photon 3dπ(3Σ1g −)(v=2, N=2)←X(3Σg −) Rydberg excitation around 225 nm are presented here to show the improvement in spatial resolution in the ion and electron images. Simulated trajectory calculations show good agreement with experiment and support the appealing properties of this velocity mapping technique.