Calculation of One- and Two-Photon Absorption Spectra of Thiolated Gold Nanoclusters using Time-Dependent Density Functional Theory.

The one- (OPA) and two-photon (TPA) absorption spectra have been calculated for a gold dimer, for a monothiolated gold dimer anion, for a thiolated gold cluster [Au25(SH)18](-1), whose structure has been determined, and for a proposed cluster [Au12(SR)9](+1) using time-dependent density functional theory (TDDFT). Geometry optimization with different exchange-correlation (X-C) functionals yielded small differences which had significant consequences in the spectra calculations. The calculated excitation energies of Au25(SH)18(-1) are in excellent agreement with experiment when the local density approximation Xα-optimized geometry is used with the B3LYP X-C functional. The CAMB3LYP and mCAM functionals yielded OPA results in good agreement with experiment for the dimer systems and the larger clusters. The super-atom theory was useful in analyzing the electronic transitions in the larger clusters. TPA was dominated by resonance effects, and the calculated cross-sections displayed a strong X-C functional dependence.