Figure 7.

Bright excitations (1) and (2) for undoped TiO₂ surface. Pairs of occupied and unoccupied KSOs contributing to low-energy optical transitions, for atomic model of anatase TiO₂ with (100) face exposed and covered by a monolayer of water. Red, green, blue, stand for Ti, O, H atoms. Grey clouds symbolize isosurfaces of KSO. Panels a,b,d,e show partial charge density of a given KSO. Panels c, f show partial chagre density integrated over x,y direction, as function of z, < 100 > direction is orthogonal to the plane of the figure. Lowest brightest transition HO−7 to LU at transition energy 2.40 eV with oscillator strength f=3.40 is represented by panels a–c and corrsponds to index (1) in Table 2 and scheme in Fig. 1. Panel a and pabel c, green dashes stand for HO−7. Panel b and panel c, solid red line stand for LU. Second brightest transition HO−8 to LU+3 at transition energy 2.71 eV with oscillator strength f=3.01 is represented by panels d–f and compared to index (2) in Table 2. Panel d and panel f, green dashes stand for HO−8. Panel e and panel f, solid red line stand for LU+2. Interestingly, all occupied orbitals are composed as superposition of p-orbitals of oxygen and clearly exhibit p-character. All unoccupied orbitals are composed as superposition of d-orbitals of titanium. The vision is difficult since the atoms of O and Ti are stacking in the < 001 > view directions.