A time-of-flight electron spectrometer for studies of forward electron emission in ion-atom collisions

Complete text of publication follows. We designed and constructed a new kind of time-of-flight electron spectrometer for measurements of the spectra of electrons emitted in forward direction in ion-atom collisions. The principle of operation of the spectrometer is shown in Figure 1. The electrons ejected in a small angular range (typically ±1deg) around 0 deg from a gas jet target by a well-collimated ion beam (diameter: 0.5 mm) are reflected by an electrostatic mirror in backward angles. The angle between the normal of the mirror and the direction of the ion beam is 10 deg, so the angle of the backscattering is 160 deg. The backscattered electrons are detected by an array of channel electron multipliers (CEMs) mounted at a distance of about 30 cm from the mirror. The detector array consists of four CEMs allowing the measurement of the angular distribution of the electrons. Choosing a pair of CEMs, one can also carry out electron-electron coincidence measurements. The time of flight of the electrons is measured by detecting the electrons in coincidence with the charge-state analysed outgoing projectiles. The electrostatic mirror consists of eight electrodes. The optimal electrode potentials were determined by calculating the electron trajectories with help of the SIMION program. To avoid the production of secondary electrons, we did not use any grid in the mirror. The spectrometer was tested by C+ on Ar collisions in the impact energy range between 100 and 300 keV. A typical velocity spectrum taken by 200 keV C+ ions is shown in Figure 2. The electrons were detected in coincidence with the outgoing C2+ ions. The broad peak at v = 0:81 a.u. (electron energy = 9.08 eV) is the 'Electron Loss to the Continuum' cusp. [1]. The narrower peaks located symmetrically on the low- and high-velocity wing of the cusp are autoionization lines originating from the projectile. The time resolution of the spectrometer (determined from the width of the autoionization lines) is about 6 ns. (author)