VUV photoelectron ionization source for molecular beam mass spectrometer and its application for SF₆ discharge...

This study developed a photoelectron ionization source (PEI) utilizing a compact vacuum ultraviolet (VUV) lamp for molecular beam mass spectrometer. Unlike conventional hot filament EI sources, the PEI source operates effectively at a pressure exceeding 0.1 Pa. This capability enhances sensitivity by allowing higher sample molecular density within the ionization region. The source employs a vertically oriented where the accelerated electron beam intersects the molecular beam at right angles. This orthogonal configuration enables independent control of photoelectron energy distribution, yielding a corresponding full width at half maximum (FWHM) of 2.5 eV. Optimization involved increasing the magnetic field strength to 900 Gs and adjusting the grid voltage. These modifications resulted in significantly enhanced electron yield and utilization efficiency, boosting the sensitivity for SO₂ and CO₂ by factors of 14 and 23, respectively. Furthermore, cadmium (Cd) metal was employed as the photocathode material, leading to improved ionization source stability. In the SF₆ discharge in-situ monitoring experiments, a comparison with conventional EI mass spectrometry revealed distinct changes in intermediate products (SF₄, SOF₄, NF₃) and highly reactive species (HF, NO, and NO₂). Consequently, this method promises to provide a viable solution for SF₆ equipment discharge fault diagnosis, investigation of SF₆ discharge mechanisms, and monitoring of SF₆ decomposition products.