Calculated linear and nonlinear optical absorption spectra of phosphine-ligated gold clusters.

Although prediction of optical excitations of ligated gold clusters by time-dependent density functional theory (TDDFT) is relatively well-established, limitations still exist, for example in the choice of the exchange-correlation functional. In aiming to improve on the accuracy of the calculated linear absorption, we report a theoretical study on phosphine-ligated gold clusters, specifically Au 9 (PR 3 ) 8 3+ and Au 8 (PR 3 ) 7 2+ characterized by highly resolved UV/Vis spectra, using mass-selective electronic absorption photofragmentation spectroscopy (A. Cirri, H. M. Hernández and C. J. Johnson, J. Phys. Chem. A , 2020, 124 , 1467-1479, and references therein). The optical absorption spectra of the Au 9 (PR 3 ) 8 3+ and Au 8 (PR 3 ) 7 2+ clusters were calculated using TDDFT and the many-body GW (G-Green's function, and W-screened Coulomb interaction)-BSE (Bethe Salpeter Equation) method, and compared to the experimental measurements. The evGW-BSE results demonstrated fair agreement with the experimental data, comparable to the TDDFT results, but with less dependence on the reference exchange-correlation functional. Experimentally observed ligand-effects in these materials were reproduced in our calculations as well. Finally, to assess the utility of the materials for nonlinear optical absorption, a theoretical evaluation of two-photon absorption cross-sections is included.