| 摘要 |
A field ionization source includes a <110> oriented iridium emitter, the tip of which is initially built up in the <110> direction. A negative voltage is applied to the emitter after the emitter has been heated to approximately 2000 DEG C; thereafter, the emitter is cooled to approximately 1200 DEG C. Crystalline buildup of the pointed iridium tip occurs in the <110> direction. After buildup has occurred, the emitter is cooled sufficiently to "freeze" the tip in the built up configuration. The negative voltage is then removed. A gas containing molecules to be ionized is differentially pumped at relatively high pressure through a tube into a region immediately around the emitter tip enclosed by a cathode cap having an aperture through which the ion beam is accelerated. The iridium emitter is mounted in thermal contact with a liquid nitrogen reservoir, which maintains the emitter at near-cryogenic temperatures. The gaseous source of molecules is also maintained in thermal contact with the liquid nitrogen reservoir which cools the gas to near-cryogenic temperatures. A positive voltage of sufficient magnitude to cause ionization of molecules from the gas is applied to the emitter with respect to the cathode cap. The ions are accelerated through the aperture of the cathode cap by the electric field between the emitter and the cathode cap, thereby forming the ion beam.
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