9.1.1.3.2 Field ion microscopy

This chapter talks about field ion microscopy. A microscope based on the process of field ionization is described. The field ion microscope achieves its high magnification from a nearly radial projection of ions formed at the emitter apex by the field ionization process. The tip is placed in an evacuated chamber opposite to a fluorescent screen or some form of an imaging detector. A low pressure of an inert gas (called the "imaging" gas) is admitted to the chamber and a positive voltage of 3-30 kV is applied to the tip. The resulting electric field, which is of the order of 10-60 V/nm, is sufficiently high to cause field ionization of the imaging gas atoms. Field ions, which are formed continuously at the tip surface, travel to the imaging detector where they form stable image spots. An example of a field ion microscope image taken from a sample of rhodium (Rh) is shown. As a point-projection microscope, the magnification of the field ion microscope is given roughly by the ratio of the tip-to- detector distance to the tip radius, just like the field emission microscope. However, unlike the field emission microscope, the resolution of the field ion microscope is not limited by the finite de Broglie wavelength of the emitted particles, but primarily by the lateral velocity component of the emerging ions. Maintaining the tip at 77 K or below reduces this velocity component sufficiently to permit atomic resolution.