Suppression of hot electrons in threshold photoelectron photoion coincidence spectroscopy using velocity focusing optics

Velocity focusing of electrons is combined with photoelectron photoion coincidence (PEPICO) spectroscopy to achieve a true threshold PEPICO signal without contributions from energetic electrons. Ions are generated by a continuous vacuum ultraviolet light source. Electrons, extracted by a field of 20 V/cm, pass through a 13 cm drift region and are dispersed in space on a multichannel plate detector by velocity focusing optics. The ions are extracted in the opposite direction by the same electric field, further accelerated by a second field, and collected after passing through a 30 cm drift region. Ions are measured in coincidence with electrons collected from the central 3.2 mm electrode as well as a ring electrode (inner and outer diameters of 5.6 and 8.1 mm). The central ring electrode contains mostly true threshold electrons along with a background of "hot" electrons, whereas the outer ring electrode collects only hot electrons. By subtracting the latter from the former, true threshold photoelectron photoion coincidence spectra are obtained. The major advantages of this approach are the high electron energy resolution with the use of high direct current extraction fields, and the complete suppression of energetic electrons. (C) 2003 American Institute of Physics.