The electron transport and emission properties of hydrogenated and cesiated single-crystal and chemical-vapor-deposited (CVD) diamond are investigated using secondary electron emission spectroscopy. The kinetic energy of the electrons and the height of the surface energy barrier are measured relative to the conduction band minimum, E-c, which is identified in the spectra. In spectra measured from hydrogenated and cesiated diamond surfaces, electron emission appears at energies E < E-c which gives direct evidence of a negative electron affinity. The strongest emission is observed from cesiated samples, which produced very high yields (delta(max) similar to 80-130 at E-b = 2900 eV). The energy distributions from all three samples are sharply peaked at similar to 0.50-0.65 eV above E-c and have a full width at half maximum similar to 0.55-0.75 eV, except in the case of the cesiated CVD diamond samples. The energy distributions measured from cesiated CVD diamond are peaked at lower energy and are much broader due to lower emission-onset energies. An emission model, which invokes band bending near the surface, is deduced that accounts for the observed energy spectra from the samples in terms of the surface properties of the C(100) and CVD diamond and the internal electron energy distributions.