The effect of impurity doping on the photoactivity of TiO2 rutile single crystals was subjected to a combined surface-science and bulk-analysis study. The incorporation of nitrogen ions, N-, into TiO2 single crystals was achieved by sputtering with N-2(+)/Ar+ mixtures and subsequent annealing to 900 K under ultrahigh vacuum conditions. This procedure leads to a 90 A thick structurally modified near-surface region, which, by the use of cross sectional transmission electron microscopy, can be described as rutile grains imbedded within a monocrystalline strained rutile matrix. The presence of N- ions distributed in the first 200 Angstrom below the surface was revealed by X-ray photoelectron spectroscopy, in agreement with sputter depth profiles obtained by secondary ion mass spectroscopy. The concentration of N- doping is about 10(20) cm(-3) in the first 200 Angstrom of the near-surface region. The photodesorption of O-2 was employed to study the changes in the photochemical properties of nitrogen-implanted crystals. The action curves for O-2 photodesorption exhibit an unexpected blueshift compared to undoped crystals. The effect is attributed to the deposition of electronic charge in the lower levels of the conduction band (band-filling mechanism), causing allowed indirect photoexcitation processes to shift to energies higher than the band gap.