Effect of static gas background signal on momentum imaging in electron-molecule collision experiment

The Velocity Slice Imaging technique has revolutionized charged particle-molecule interaction studies in general and electron-molecule interaction studies in particular. Multiple electrostatic lens assemblies are often used in spectrometers for resolving low kinetic energy fragments. In a crossed-beam experiment with an effusive molecular beam as a target, the background gas forms a strong extended source of ions where the charged particle beam is used as a projectile without focusing. This extended source creates artefacts on the momentum images as we try to magnify them beyond a certain size. Here, we present a systematic study of this effect on momentum imaging in the low-energy-electron molecule collisions as an example and the solutions to address this issue by background subtraction with suitable magnification. Additionally, we demonstrated that a supersonic molecular beam target helps minimize these artefacts in the image magnification by reducing the background signal. These systematic findings may bring valuable insight into the investigation of low kinetic energy release processes involving electron impact, ion impact, and merge beam experiments with large interaction volumes where high magnification is essential.