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 their inception, velocity map imaging (VMI) techniques have received continued interest in their expansion from 2D to 3D momentum measurements through either reconstructive or direct methods. Recently, much work has been devoted to the latter of these by relating electron time-of-flight (TOF) to the third momentum component. The challenge is having a timing resolution sufficient to resolve the structure in the narrow (<10 ns) electron TOF spread. Here, we build upon the work in VMI lens design and 3D VMI measurement by using a plano–convex thick-lens (PCTL) 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 geometry, the resulting plano–convex electrostatic field extends the detectable electron cutoff energy range while retaining the high resolution. This design also extends the electron TOF range, allowing for a better momentum resolution 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 ∼7 eV with a TOF spread of ∼30 ns, both of which are improved compared to a previous work by factors of ∼1.4 and ∼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 in a gas mixture. These techniques have the potential to lend themselves to more advanced measurements involving systems where the electron momentum distributions possess non-trivial symmetries.

Implementation of 3D-focusing in an ion momentum spectrometer

“…So far, the best ion kinetic energy release (KER) resolution in the COLTRIMS is about 0.3 eV at 11.2 eV [22]. Numerous efforts have been undertaken to focus all three momentum components (P x , P y , P z ) simultaneously to improve the energy resolution [23][24][25][26][27][28].…”

Velocity map imaging with no spherical aberrations

“…8 A comprehensive review of the different implementations of VMI is beyond the scope of this paper; we will only note that configurations with a large number of electrodes spanning the entire volume from the interaction region to the detector (known as 'thick lens' designs) are increasingly being used. [10][11][12][13][14][15][16] However, as the complexity increases, finding the optimal design and voltages becomes more challenging requiring extensive iterative simulations. 13 According to a recent work, 15 the current state of the art enables focusing a beam with a width of a few mm in the on-source region to a C0.1 mm spot on the detector, resulting in a 'focusing factor' (also known as a 'blurring factor') of C0.05.…”