A plano-convex thick-lens velocity map imaging apparatus for direct, high resolution 3D momentum measurements of photoelectrons with ion time-of-flight coincidence

Abstract

Since its inception, velocity map imaging (VMI) has been a powerful tool for measuring the 2D momentum distribution of photoelectrons generated by strong laser fields. There has been continued interest in expanding it into 3D measurements either through reconstructive or direct methods. Recently much work has been devoted to the latter of these, particularly by relating the electron time-of-flight (TOF) to the third momentum component. The technical challenge here is having timing resolution sufficient to resolve structure in the narrow (< 10 ns) electron TOF spread. Here we build upon work in the fields of VMI lens design and 3D VMI measurement by using a plano-convex thick-lens VMI in conjunction with an event-driven camera (TPX3CAM) providing TOF information for high resolution 3D electron momentum measurements. We perform simulations to show that, with the addition of a mesh electrode to the thick-lens VMI geometry, a plano-convex electrostatic field is formed which extends the detectable electron cutoff energy range while retaining high resolution. Further, the thick-lens also extends the electron TOF range which allows for better resolution of the momentum along this axis. We experimentally demonstrate these capabilities by examining above-threshold ionization in Xenon where the apparatus is shown to collect electrons of energy up to $\sim$7 eV with a TOF spread of $\sim$30 ns, both of which are improvements on previous work by factors of $\sim$1.4 and $\sim$3.75 respectively. Finally, the PCTL-VMI is equipped with a coincident ion TOF spectrometer which is shown to effectively extract unique 3D momentum distributions for different ionic species within a gas mixture. These techniques have potential to lend themselves to more advanced measurements, particularly involving systems where the electron momentum distributions possess non-trivial symmetries and require high resolution.