Quantum photoyield and secondary-electron distributions are presented for an unreconstructed diamond (111) surface (type-IIb, gem-quality blue-white semiconductor). This chemically inert surface exhibits a negative electron affinity, resulting in a stable quantum yield that increases linearly from photothreshold (5.5 eV) to ∼20% at 9 eV, with a very large yield of ∼40%-70% for 13≲h ν≲35 eV. For all photon energies, secondary-electron energy distributions show a dominant ∼0.5-eV-wide emission peak at the conduction-band minimum (∆<min></min>1=5.50±0.05 eV above the valence-band maximum Γ25'). In contrast with recent self-consistent calculations <article>[J. Ihm, S. G. Louie, and M. L. Cohen, Phys. Rev. B 17, 769 (1978)]</article> no occupied intrinsic surface states with ionization energies in the fundamental gap (the Fermi level was 1 eV above Γ25') were observed. Likewise, the measured photothreshold (Evac-Γ25') is significantly smaller than calculated (7.0±0.7 eV).