Densities of states of low-index surfaces of tantalum by photofield emission

The total energy distributions (TED) of the true photofield emission current from clean (100), (110) and (111) facets of Ta field emitters have been measured using a new method to remove the thermocurrent associated with laser heating. Each TED exhibits one or more prominent peaks that are interpreted by comparing them with ab initio calculations of the TED of the emission current based on density functional theory. The generally good agreement with experiment indicates that the same method can be used for accurate calculations of the densities of states of low-index surfaces of Ta. Each of the experimental TEDs shows, in addition to the prominent peaks, a set of weaker peaks that are not predicted by the calculation and whose spacing depends on the sharpness of the field emitter. These weaker peaks are interpreted as arising from size-effect resonances in the microcrystal at the apex of the field emitter.