## 4 Discussion
As a result, an [N-V]\({}^{0}\)-center is formed in the excited state \({}^{2}\)A\({}_{2}\). The transition from excited state \({}^{2}\)A\({}_{2}\) to the ground state occurs due to the emission of a photon at a wavelength of 575 nm. Thus, by irradiating a [N-V]-center with radiation at 637 nm, it is possible to obtain emission at a shorter wavelength of 575 nm. In the studied HP-HT nanodiamonds, the [N-V]\({}^{-}\)-centers were not specifically created. They are formed only upon irradiation of diamond particles containing substitutional N with high-energy electrons with subsequent annealing at high temperature [47; 48]. However, the studied HP-HT nanodiamonds contain Ni- and N- centers [11; 16; 19; 20; 45; 49; 50]. From Figure 4, it can be seen that there are peaks in the luminescence spectrum at the wavelengths of 485 and 528-529 nm. These luminescence peaks are related to the Ni- [16] and [Ni-N]-complexes [19]. It is noteworthy that there are the same peaks in the up-conversion luminescence spectrum obtained at the wavelength of 633 nm (see Figure 4d). Therefore, the investigated HP-HT nanodiamonds have a system of energy levels that allow the resonant absorption of one or several He-Ne laser photons at 1.96 eV. This enables the population of the upper levels of energy transitions of complexes [Ni] and [Ni-N]. If the population of the upper levels occurs then blue-green up-conversion luminescence is observed [51]. Synchronously, the absorption of an additional photon by the impurity centers at these upper-levels with relatively long lifetime might occur. This leads to the transition of the impurity centers to the ionized state, similar to the transition of the [N-V]-centers during laser illumination [15; 42]. Consequently, free electrons arise in the laser beam spot on the film, beginning to efficiently absorb incident red light at 633 nm. This causes the sharp temperature increase in the laser beam spot, which results in the diamond into sp\({}^{2}\)-carbon transformation, i.e. blackening. Subsequent laser illumination of the blackened area without the diamond impurity complexes does not lead to resonance photon absorption and, due to the low intensity of the laser used, the film cannot achieve a high enough temperature to emit thermally induced light, as it was described in [39]. It is known that impurities incorporated into diamond lattice during synthesis are non-uniformly distributed within individual particles and among different particles [52]. Therefore, the impurity centers are randomly distributed on the film, which explains the blackening effect that mostly takes place when the laser beam spot is scanning the surface of the film. The motion of the film greatly increases the probability of the intersection of randomly distributed impurity centers with the beam waist of the laser. The number of impurity centers affected by the laser beam in a unit of time also increases. As soon as the laser affects such center, efficient absorption of the laser radiation begins and the diamond to sp\({}^{2}\)-carbon transformation occurs. The thickness of the film also plays an important role, since a certain amount of impurity centers within the film volume illuminated by the beam along the film depth needs to be activated by the laser beam simultaneously to raise the local temperature to a critical point when the graphitization occurs. As the graphitization starts, it also spreads into the nearby regions (as wide as 10 \(\upmu\)m across the direction of the beam movement, Figure 9), even though the density of the impurity centers there is low. There is a recent paper regarding laser-induced broadband light emission and structural changes in HP-HT micro-size diamonds after continuous excitation at near-infrared wavelengths, proposing a model of an sp\({}^{2}\)-sp\({}^{3}\) hybridization switch process assisting the multiphoton ionization and free to bound recombination of electron between two carbon atoms [53].