Electron Emission from a Lattice Step on Clean Tungsten

Electron emission has been observed from the edges of the outermost atom layer of a (110) crystal face of clean tungsten. A bright ring appears in the (110) area of the emission pattern on a pulsed $T-F$ emission electron microscope when current densities exceeding ${10}^7$ amp/${\mathrm{cm}}^2$ are drawn. When the emitter is maintained at an elevated temperature, $T\geqq 1700°$K, the ring decreases in radius and eventually disappears. This is followed by the appearance of a new ring at the periphery of the dark (110) region, making possible the observation of a sequence of such collapsing rings. An experiment is reported which associates the ring collapse with the loss of an atom layer from the cathode tip, causing a change in its length, which is equal on the average to the interplanar spacing of the (110) crystal direction. This association proves that electron emission is observed from a lattice step and provides a convenient means for the study of the corresponding processes by which cathode material is transported.