How to measure the kinetic energy distribution of the precursor ion main beam in mass-analyzed ion kinetic energy spectrometry experiments

Scans of the electrostatic analyzer (ESA) across the precursor ion beam in reverse-geometry (BE) mass spectrometers that are operated under double-focusing conditions do not measure the “energy resolution of the main beam”: They only measure double-focusing resolution. The only way that ESA scans can measure the kinetic energy distribution of the main beam is to operate the instrument so that angular (directional) focusing is not achieved. Thus, the mass spectrometer is no longer double-focusing. Under double-focusing conditions, however, scans of the accelerating voltage while the magnetic field and ESA are held constant can be used to measure either the kinetic energy distribution of the main beam that enters the magnet or the energy-resolving power of the instrument. Scans at a constant ratio of B²/E can be used similarly. The energy-resolving power of any ESA is defined by its dispersion and the widths of the energy-resolving object and image slits that immediately precede and follow the ESA, respectively. The use of BE, EB, and triple-sector instruments to measure energy-resolving power and the kinetic energy distribution of the precursor ion main beam is compared and discussed.