Spin-orbit effects in optical spectra of gold-silver trimers

Cationic gold-silver trimers are ideal model systems for the evaluation of relativistic electronic structure theories. The closed-shell triangles allow one to test density functional and wavefunction-based methods in their prediction of optical properties, as dependent on composition and symmetry. Here we present the gas-phase optical spectra of Ag_NAu_3-N⁺ (N = 0-3) clusters recorded by longitudinal photodissociation spectroscopy in the photon energy range 1.9-4.4 eV. The experimental data are compared to excited electronic state calculations in the framework of all-electron range-separated time-dependent density functional and equation-of-motion coupled cluster theory using two-component as well as the spin-free scalar relativistic theories. In particular, it is shown that for mixed trimers scalar-relativistic corrections are insufficient and a two-component approach becomes obligatory for a correct description of optical response properties including both spin-orbit coupling and charge-transfer effects.