Chemical bonding in initial building blocks of semiconductors: Geometrical structures and optical absorption spectra of isolated $CdSe2+$ and Cd₂$Se2+$ species

We present the first experimental optical absorption spectra of isolated $CdSe2+$ and Cd₂$Se2+$ species in the photon energy range ℏω = 1.9–4.9 eV. We probe the optical response by measuring photodissociation cross sections and combine our results with time-dependent density functional theory and equation-of-motion coupled cluster calculations. Structural candidates for the time-dependent excited state calculations are generated by a density functional theory based genetic algorithm as a global geometry optimization tool. This approach allows us to determine the cluster geometries present in our molecular beams by a comparison of experimental spectra with theoretical predictions for putative global minimum candidates. For $CdSe2+$⁠, an excellent agreement between the global minimum and the experimental results is presented. We identify the global minimum geometry of Cd₂$Se2+$ as a trapezium, which is built up of a neutral Se₂ and a cationic $Cd2+$ unit, in contrast to what was previously proposed. We find an excellent overall agreement between experimental spectra and excited state calculations. We further study the influence of total and partial charges on the optical and geometric properties of Cd₂Se₂ and compare our findings to CdSe quantum dots and to bulk CdSe.