  The electron emission properties of a single-crystal nitrogen-doped
diamond(001) photocathode inserted in a 10kV DC photoelectron gun are
determined using a tunable (235-410nm) ultraviolet laser radiation source for
photoemission from both the back nitrogen-doped substrate face and the front
homo-epitaxially grown and undoped diamond crystal face. The measured spectral
trends of the mean transverse energy (MTE) and quantum efficiency (QE) of the
emitted electrons are both anomalous and non-monotonic, but are shown to be
consistent with (i) the known physics of electron photoexcitation from the
nitrogen substitution states into the conduction bands of diamond, (ii) the
energy position and dispersion characteristics of the conduction bands of
diamond in the (001) emission direction, (iii) the effective electron affinity
of the crystal faces, (iv) the strong electron-(optical)phonon coupling in
diamond, and (v) the associated hot electron transport dynamics under energy
equipartition with the optical phonons. Notably, the observed hot electron
emission is shown to be restricted parallel to the photocathode surface by the
low transverse effective masses of the emitting band states - a transverse
momentum filtering effect.
