The nitrogen-vacancy centre (NV) has drawn much attention for over a decade, yet detailed knowledge of the photophysics needs to be established. Under typical conditions, the NV can have two stable charge states, negative (NV-) or neutral (NV0), with photo induced interconversion of these two states. Here, we present detailed studies of the ionization dynamics of single NV centres in bulk diamond at room temperature during illumination in dependence of the excitation wavelength and power. We apply a recent method which allows us to directly measure the charge state of a single NV centre, and observe its temporal evolution. Results of this work are the steady state NV- population, which was found to be always < 75% for 450 to 610 nm excitation wavelength, the relative absorption cross-section of NV- for 540 to 610 nm, and the energy of the NV- ground state of 2.6 eV below the conduction band. These results will help to further understand the photo-physics of the NV centre.
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