Influence of the electric field on secondary electron emission yield

We have applied a technique based on levitation of a single charged grain in the quadrupole. We have used 3-6 micrometer spherical grains front amorphous carbon. These grains were charged by an electron beam with the energy tunable up to 10 keV and the grain charge was continuously monitored. If the grain is charged by an constant energy, its surface potential is set to the value when incident electrons are slow (town to the energy where the secondary emission yield is equal to unity. Our investigations reveal that this energy changes proportionally to the grain surface field. Moreover, we have observed a shift of charging characteristics after a long-time electron bombardment.