Several hydrogenated diamondoids containing defect centers used in the calculations: (a) Disklike , (b) cuboidlike , (c) tetrahedron-like , and (d) sphere-like . In each model the nitrogen atom is located at the center, and the carbon cluster surface is passivated with hydrogen atoms.
(a) Calculated electron configurations of the center. The lowest excitation occurs as the -spin electron in HOMO-1 transfers to HOMO. (b) Molecular orbital maps of HOMO and HOMO-1 calculated for by CAS(6,8). In the structures superimposed on the orbital maps, the nitrogen and three carbon atoms surrounding the vacancy are labeled by N, , , and , respectively (surface hydrogen not shown). The electron densities are mainly distributed in the vacancy.
Vertical excitation energies of the transition predicted by different computational methods and cluster models. The experimental absorption maximum is (Refs. 1 and 30), shown as the rightmost data point on the plot.
Comparison of HOMO density distributions of (a) and (b) sphere-like (surface hydrogen atoms not shown). There are severe density leakage and surface interference in small clusters, while the density is well enclosed in the defect center in large models.
Basis set dependence of calculated vertical transition energies. Solid symbols: All atoms adopting the basis set (no diffuse function); open symbols: The additional diffuse function applied on the central atoms around the defect center; symbols with cross: All atoms adopting the diffuse function.
OPA cross sections of the transition predicted by different computational methods and cluster models. A blueshift of 0.15 eV in the excitation energy from the vertical excitation was assumed in the calculation. Shown on the rightmost of the plot is the experimental value of (Ref. 30).
(a) TPA cross sections and (b) TPA/OPA cross section ratios of the transition predicted by different computational methods and cluster models. In addition to the assumption of a blueshift of 0.15 eV in the excitation energy from the vertical excitation, linear polarization and single-beam excitation were also assumed in the calculation. The experimental TPA cross section is (Ref. 30), shown as the rightmost data point in the upper panel.
Computational results of the transition of the defect center in diamond, where is the vertical excitation energy, is the magnitude of the transition dipole moment, is the configuration interaction expansion coefficient for the first single excitation (i.e., the -spin electron transition), and is the one-photon absorption cross section of the transition.
Two-photon absorption cross sections calculated for the transition of the defect center in diamond.
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