Imaging the momentum of charged particles without background noise

Velocity map imaging (VMI) spectroscopy visualizes the momentum of charged particles. When used with high-energy ultraviolet (UV) light sources, however, scattered photons knock into the device’s electrodes, leading to excessive emission of noisy photoelectrons that render the desired signal nearly invisible.

To combat this, Ladda et al. optimized the VMI spectrometer, eliminating 99.9% of the background noise without compromising imaging resolution.

A VMI spectrometer is composed of multiple electrodes, a drift region, and a detector. A laser passing between the first two electrodes ionizes particles, which are accelerated through holes in subsequent electrodes. Then, the particles are focused onto the detector by the electrostatic lens, which is formed by the entire electrode configuration.

However, when scattered UV light hits the rim of the electrode plate, it can create noisy photoelectrons. By making the electrodes thin plates, photons are less likely to hit the cross-section of the rim, reducing electron production.

The researchers found that incrementally decreasing the electrode openings through which particles pass to reach the detector physically blocked most electrons from reaching the detector. This geometric optimization reduced background noise by 95%.

Optical baffles near the incoming and back-focused laser beams blocked the path of scattered light, and clearer, smoother windows allowed the light to pass into the device without scattering.

“Because we want to excite chiral molecules with a single photon, we had to go to the deep UV range. But when we started experiments, we only saw background noise. So, we improved the spectrometer to investigate further,” said author Nicolas Ladda. “We hope people can adapt it, or improve it, to support all kinds of measurements using deep ultraviolet/vacuum ultraviolet laser light.”

Source: “Velocity map imaging spectrometer optimized for reduction of background electrons from scattered UV light,” by Nicolas Ladda, Fabian Westmeir, Sagnik Das, Wilfried Dreher, Simon T. Ranecky, Tonio Rosen, Krishna Kant Singh, Till Jacob Stehling, Sudheendran Vasudevan, Hendrike Braun, Thomas Braumert, Jochen Mikosch, and Arne Senftlenben, Review of Scientific Instruments (2025). The article can be accessed at https://doi.org/10.1063/5.0279984 .