  Nitrogen incorporated ultrananocrystalline diamond ((N)UNCD) could be an
enabling material platform for photocathode applications due to its high
emissivity. While the quantum efficiency (QE) of UNCD was reported by many
groups, no experimental measurements of the intrinsic emittance/mean transverse
energy (MTE) have been reported. Here, MTE measurement results for an (N)UNCD
photocathode in the photon energy range of 4.41 to 5.26 eV are described. The
MTE demonstrates no noticeable dependence on the photon energy, with an average
value of 266 meV. This spectral behavior is shown to not to be dependent upon
physical or chemical surface roughness and inconsistent with low electron
effective mass emission from graphitic grain boundaries, but may be associated
with emission from spatially-confined states in the graphite regions between
the diamond grains. The combined effect of fast-growing QE and constant MTE
with respect to the excess laser energy may pave the way to bright UNCD
photocathodes.
