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 utiliz es 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 ine lastic channel. The TOF approach eliminates the need for complicated c alibration procedures required when using conventional electrostatic e lectron energy-loss spectroscopy (EELS) at these low energies. The cha racteristics 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 st ate of CO, in the impact energy range of 6-15 eV. These ratios were me asured with uncertainties as small as +/-4%, which represents a substa ntial improvement over previous measurements in this energy range. Thi s demonstrates the feasibility of using the TOF technique to measure D CS ratios which in turn can serve as secondary standards to normalize other inelastic DCSs obtained from measurements with EELS. (C) 1996 Am erican Institute of Physics.