Velocity map imaging spectrometer optimized for reduction of background electrons from scattered UV light

Velocity map imaging spectroscopy is a powerful technique for detecting the momentum distribution of photoelectrons resulting from the ionization of atoms, molecules, clusters, or surfaces. However, when used with ultraviolet light sources, scattered photons can lead to the emission of photoelectrons from the spectrometer's electrodes, giving rise to severe noise superimposing the desired signal. We present a velocity map imaging spectrometer optimized to reduce such unwanted background signals. The primary modifications to the conventional design include spectrometer electrode geometries with small cross sections exposed to the scattered photons, blocked pathways for photoelectrons from the electrodes to the detector, and the incorporation of optical baffles to confine the solid angle of scattered light. Compared to a conventional design optimized solely on the spectrometer's photoelectron momentum resolution, we have achieved the elimination of 99.9% of the background noise without noticeably compromising the resolution. Note that most of the improvements were achieved without the necessity of high-grade windows, reducing the sensitivity to window degradation by UV light. We give general guidelines on efficiently coping with the long-standing experimental problem of electron background originating from scattered light by considering it already in the design stage of a new spectrometer.