Angular intensity of a gas-phase field ionization source

Angular intensities of 1 μA sr−1 have been measured for a gas-phase field ionization source in an optical column under practical operating conditions. The source, which was differentially pumped and cooled to 77 K, utilized a 〈110〉-oriented iridium emitter and precooled hydrogen gas at 10−2 Torr. The ion beam was collimated with an electrostatic lens and detected below an aperture subtending 0.164 msr. A transmitted current of ∼10−10 A was measured at voltages corresponding to a field of ?2.2 V/Å at the emitter.