Evaluation of photodissociation spectroscopy as a structure elucidation tool for isolated clusters: a case study of Ag4(+) and Au4(+).

Resolving the structure of clusters in the gas phase often requires the comparison of experimental data to quantum chemical calculations. Herein, we present the variation of a straightforward approach, in which photodissociation spectra of isolated clusters are compared to optical response calculations in order to elucidate cluster structures. Our absorption spectra were measured using a newly built longitudinal beam depletion spectroscopy apparatus and recorded in the photon energy range ħω = 1.9-3.5 eV. Cluster geometries were obtained using the unbiased Birmingham Cluster Genetic Algorithm coupled with density functional theory, while the optical response was calculated in the framework of time-dependent density functional theory. Experiments and excited state calculations are in excellent agreement using long-range corrected exchange correlation functionals for both ground and excited state calculations. Our methodology indicates a contribution of Y shaped Au4(+) whereas for Ag4(+) only the ground state isomer has to be considered to explain the experimental absorption spectrum. Our extended methodology shows two nearly degenerate isomers of Au4(+) probably being present in the molecular beam and therefore shows promise for the further structure determination of pure and binary transition-metal clusters.