  We present femtosecond laser-induced electron emission from
nanodiamond-coated tungsten tips. Based on the shortness of the femtosecond
laser pulses, electrons can be photo-excited for wavelengths from the infrared
(1932 nm) to the ultraviolet (235 nm) because multi-photon excitation becomes
efficient over the entire spectral range. Depending on the laser wavelength, we
find different dominant emission channels identified by the number of photons
needed to emit electrons. Based on the band alignment between tungsten and
nanodiamond, the relevant emission channels can be identified as specific
transitions in diamond and its graphitic boundaries. It is the combination of
the character of initial and final states (i.e. bulk or surface-near, direct or
indirect excitation in the diamond band structure), the number of photons
providing the excitation energy and the peak intensity of the laser pulses that
determines the dominant excitation channel for photoemission. A specific
feature of the hydrogen-terminated nanodiamond coating is its negative electron
affinity that significantly lowers the work function and enables efficient
emission from the conduction band minimum into vacuum without energy barrier.
The properties of these tips are encouraging for their use as laser-triggered
electron sources in applications such as ultrafast electron microscopy and
diffraction and novel photonics-based laser accelerators. Emission is stable
for bunch charges up to 400 electrons per laser pulse. We infer a normalized
emittance of < 0.20~nm~rad and a normalized brightness of > $1.2 \cdot 10^{12}
\text{A} \text{ m}^{-2} \text{ sr}^{-1}$.
