Time-of-flight spectrometer for measuring inelastic to elastic differential cross-section ratios for electron-gas scattering

We describe a crossed electron beam-atomic beam apparatus which utilizes a pulsed electron gull and field free drift tube to obtain time-of-flight (TOF) spectra of electrons scattered from atoms and molecules. This apparatus was constructed for the purpose of obtaining inelastic-to-elastic differential cross-section (DCS) ratios in the energy range extending from threshold to several eV above the threshold of the inelastic channel. The TOF approach eliminates the need for complicated calibration procedures required when using conventional electrostatic electron energy-loss spectroscopy (EELS) at these low energies. The characteristics of the apparatus will be given, along with representative TOF spectra from carbon monoxide. From those spectra we obtained DCS ratios at 90 degrees scattering angle for excitation of the a(3) Pi state of CO, in the impact energy range of 6-15 eV. These ratios were measured with uncertainties as small as +/-4%, which represents a substantial improvement over previous measurements in this energy range. This demonstrates the feasibility of using the TOF technique to measure DCS ratios which in turn can serve as secondary standards to normalize other inelastic DCSs obtained from measurements with EELS. (C) 1996 American Institute of Physics.