A TIME-OF-FLIGHT SPECTROMETER FOR LOW-ENERGY NEUTRAL AND IONIZED ALKALIS

A time-of-flight spectrometer has been developed for measuring velocity- and angle-resolved distributions of neutral and ionized alkali atoms in the range of several electron volts to several thousand electron volts. The spectrometer is mounted on a rotating platform inside a previously existing ultra-high-vacuum scattering chamber equipped with a low-energy ion beamline. A high work function platinum foil, placed after a pair of angle-defining apertures, is used to ionize the alkali atoms, regardless of their incident charge states. After striking the platinum foil, the alkali ions are accelerated and detected with a channel electron multiplier. Velocity resolution of the scattered flux is achieved using standard time-of-flight techniques. Neutral and ionized alkali atoms are detected with the same high efficiency. The ion flux can be rejected by biasing a pair of rejection elements, and therefore absolute ratios of the neutral to total flux can be determined. Such a spectrometer can be used to probe the role of charge transfer in a number of dynamical processes at surfaces.