## 4 Discussion  
The structural transformation of sp\({}^{3}\)- to sp\({}^{2}\)-carbon was previously observed by [PERSON] et al. [39] while irradiating powders of detonation nanodiamonds [40; 41] with a _cw_ 325 nm helium-cadmium laser (1.3 kW/cm\({}^{2}\)). Such transformation occurred because of high-temperature heating of the irradiated region of the sample that was caused by absorption of the incident UV light by the nanodiamond particles and it was accompanied by thermoluminescence in a wide range of wavelengths (325-700 nm) [39]. However, our experimental results cannot be explained by simple heating to the high-temperatures for the following reasons. Firstly, Figure 10a shows that prolonged focused laser irradiation of a motionless film at a maximum intensity of 65 kW/cm\({}^{2}\) does not cause blackening and structural changes. Only the luminescence amplitude decreases in a wide range of wavelengths at the point of laser exposure, as the exposure time increases (Figure 10d). Secondly, laser-induced blackening only happens when the film and the focused beam move relative to each other. Thirdly, the laser modification of the film is accompanied by the green luminescence. Fourth, after laser modification, the absorption coefficient of the film at the wavelength of 633 nm is several times higher, as can be seen from Figure 7a; however, re-exposure of the same spot of the film at the highest intensity of the laser (65 kW/cm\({}^{2}\)) does not lead to any additional green emission or structural changes, i.e. green emission and laser blackening are one-time events. Also note that green emission and laser blackening occurs not only in ambient atmosphere, but also in the Ar atmosphere. The results that are presented above can be explained on the assumption that the HP-HT nanodiamond films contain impurity centers that are distributed non-uniformly among different diamond particles. The well-studied color centers in diamond are [N-V]-centers. The main \({}^{3}\)A\({}_{2}\), \({}^{2}\)E,  
Figure 10: (**a**) Raman spectra of a randomly chosen place of the stationary HP-HT nanodiamond film irradiated by 65 kW/cm\({}^{2}\) laser radiation for different intervals of time (0.5, 10, and 120 min. ); (**b**) HP-HT nanodiamond Raman peak intensity and its Raman shift of a randomly chosen place of the stationary film as a function of laser exposure time at 65 kW/cm\({}^{2}\); (c) Raman spectra of a randomly chosen place of the stationary HP-HT nanodiamond film irradiated by 65 kW/cm\({}^{2}\) laser radiation for intervals of time (curve 1) \(\Delta t\) = 0.5 min. and (curve 2) \(\Delta t\) = 120 min. measured at 65 kW/cm\({}^{2}\) excitation intensity after subtraction of baselines with the peaks fitted; (**d**) luminescence intensity at Stokes Raman frequency shifts of 1000, 1100, and 1500 cm\({}^{-1}\) of a randomly chosen place of the stationary HP-HT nanodiamond film irradiated by 65 kW/cm\({}^{2}\) laser radiation as a function of laser exposure time. and excited \({}^{3}\)E, \({}^{2}\)A\({}_{2}\) energy levels of the electronic transitions \({}^{3}\)A\({}_{2}\)\(\rightarrow\)\({}^{3}\)E and \({}^{2}\)E \(\rightarrow\)\({}^{2}\)A\({}_{2}\) of [N-V]\({}^{-}\)-and [N-V]\({}^{0}\)-centers, respectively, lie between the valence and the conduction band of diamond [42; 43; 44]. Moreover, the zero phonon line related to the \({}^{3}\)A\({}_{2}\)\(\rightarrow\)\({}^{3}\)E and \({}^{2}\)E \(\rightarrow\)\({}^{2}\)A\({}_{2}\) transitions have the wavelengths of 637 nm (1.945 eV) and 575 nm (2.156 eV), respectively [15; 45; 46]. The ground state \({}^{3}\)A\({}_{2}\) of the [N-V]\({}^{-}\)-center is above the valence band by 2.94 eV (recombination energy) and lower by 2.6 eV (ionization energy) of the conduction band [42]. Upon irradiation of [N-V]\({}^{-}\)-centers at the wavelength of 637 nm, resonance excitation of the upper-level \({}^{3}\)A\({}_{2}\)\(\rightarrow\)\({}^{3}\)E with its subsequent ionization is possible, in accordance with [PERSON] et al. [42].