The photochemical attachment of 10-amino-dec-1-ene molecules protected with a trifluoroacetic acid group (TFAAD) on hydrogen-terminated single-crystalline chemical vapor deposited (CVD) diamond is characterized by total photoyield spectroscopy (TPYS), conductivity, Hall-effect, spectrally resolved photoconductivity (SPC), optical transmission experiments, and, for the first time, by in situ internal photoemission (IPE) spectroscopy applied in the spectral regime from 4 to 6 eV on the alkene/diamond (liquid/solid) heterostructures. These experiments are performed on undoped, (100) oriented, single-crystalline CVD diamond films, which contain no grain boundaries and have negligible bulk and surface defect densities. X-ray photoelectron spectroscopy (XPS) is used to investigate the chemical bonding of alkene molecules to diamond. The spectroscopic set of data shows that the photochemical reaction window of H-terminated diamond is shifted below the optical gap of diamond because of the negative electron affinity. In situ IPE experiments reveal electron emission between 4.5 and 5.2 eV. A model is introduced and discussed in which valence-band electrons are optically excited into empty hydrogen-induced surface states of diamond from where they tunnel into empty pi states of alkene molecules. We theoretically discuss the fastest attachment time to achieve a saturated TFAAD layer of about 2 x 10(14) cm(-)(2) on diamond, which is experimentally detected to be 7 h. In the case of direct optical electron excitations from diamond, the bonding efficiency will be one TFAAD molecule attachment arising from about 1600 emitted electrons.