IONIZATION OF LIQUID METALS, GALLIUM.

Field ionization of a liquid metal, such as Ga or In, has attracted considerable interest since it could become a source of strong focused ion emission. We have studied this phenomenon using the improved magnetic-sector atom-probe field ion microscope. The energy distributions of field ionization of Ga and critical energy deficits are measured as a function of surface temperature and electric field. The field ionization of Ga can be divided into a low and a high emission region. In the low emission region Ga** plus ions are emitted at a field of approximately 0. 5 V/A when the emitter is heated to approximately 600 degree C. The intensity of this Ga** plus emission is sufficient to yield a desorption image of the emitter surface. This implies that the ion flux is similar 10**3 ions/atomic site/second. When the emitter is kept at low tempertures (78 K) the emission is predominately Ga** plus ** plus . In both cases the ions display a sharp energy distribution ( approximately equals 1. 5 eV FWHM) and a small energy deficit ( approximately equals 2. 1 eV). In the high emission regime Ga** plus ** plus ions are produced with a broad energy distribution ( approximately equals 12 eV FWHM). The total current ranges from 10** minus **8 A or less in the low emission regime up to or greater than 10** minus **5 A in the high emission regime.