A high-resolution synchrotron photoionization spectrometer for the study of low-energy electron-molecule scattering

An experimental system for the study of electron-molecule scattering at high resolution is described. Synchrotron radiation from the synchrotron radiation source (SRS) at the SERC Daresbury laboratory is used to photoionize Ar at threshold (78.65 nm). The resulting photoelectrons are formed into a beam of energy spread 3.5 meV (FWHM) and crossed with a Campargue-type supersonic molecular beam. The primary electron beam energy can be varied over the range 0.04-20.0 eV. An electrostatic hemispherical monochromator with a narrow acceptance cone (+or-6 degrees ) together with a position-sensitive detector provides energy analysis of the scattered electrons. In addition a second detector comprising a cascaded pair of microchannel plates and a high-voltage anode can record without energy analysis those electrons scattered over a much wider angular range (90+or-30 degrees ). The performance of the apparatus is illustrated with data on O2, N2, SF6, NO and CO2.