Total energy distribution (TED) curves of ions emitted from a gallium liquid metal ion source exhibit full width at half maximum (FWHM) value that increases from 5 to 30 eV with an increasing total current of 1–30 &mgr;A. An energy deficit of 4 eV at threshold supports a field evaporation mechanism of ion formation. The lowest TED FWHM value of 5 eV corresponds to a source angular brightness of 20 &mgr;A/sr.