Excited-state thermionic emission in III-antimonides: Low emittance ultrafast photocathodes

The normalized rms transverse emittance of an electron source is shown to be proportional to m*, where m* is the effective mass of the state from which the electron is emitted, by direct observation of the transverse momentum distribution for excited-state thermionic emission from two III-V semiconductor photocathodes, GaSb and InSb, together with a control experiment employing two-photon emission from gold. Simulations of the experiment using an extended analytical Gaussian model of electron pulse propagation are in close agreement with the data.