Design and performance of an (e, e+ion) spectrometer for studies of fragmentation of dipole and non-dipole states of gaseous molecules

An (e, e+ion) spectrometer has been constructed that has the capability to measure time-of-flight (TOF) mass spectra of gases in coincidence with energy selected inelastically scattered electrons, as a function of impact energy, electron energy loss and electron scattering angle. Relative to earlier dipole-regime implementations of the (e, e+ion) technique, this spectrometer can be used to study molecular fragmentation under both dipole and non-dipole electronic excitation and ionization conditions. The spectrometer uses a position-sensitive electron detector and a TOF tube positioned at 90° with respect to the electron impact and scattering plane. The TOF design makes it possible either to extract all ions from the interaction region, or to discriminate preferentially for ions that have kinetic energy along the axis of the TOF tube, thus allowing one to collect spectra with maximum efficiency, or to study the dynamics of the production of the molecular and fragment ions. The design and construction of the spectrometer is described, along with preliminary results for ionization in the S 2p excitation and ionization region of SF6.