A metastable atomic beam source was developed utilizing pulsed helium and argon nozzle beams. A coaxial electron gun, which can be used in a pulsed as well as in a continuous mode, excites the noble gases to their metastable states by electron bombardment in the direction of propagation of the groundhyphen;state atoms. In contrast to former work, the electron beam is controlled and accelerated by a combination of three hemispherically shaped tungsten grids. With continuous electron gun operation the source produces a metastable flux of about 1015atoms/srthinsp;s for helium and argon under optimum expansion conditions. Applying short electrical pulses to a control grid yields supersonic metastable atom pulses, with velocity distributions having a relative width as low as 3.8percnt; for helium and 6.5percnt; for argon (thinsp;FWHMthinsp;) with a 100percnt; modulation of the metastable beams. These features make the source applicable for velocity analyzed scattering experiments using the timehyphen;ofhyphen;flight technique.
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