First-principles insights into interaction of CO, NO, and HCN with Ag8.

We use static as well as time-dependent first-principles computations to study interaction of the CO, NO, and HCN molecules with the Ag8 nanocluster. The many-body based GW correction is applied for accurate description of the highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbital levels. It is argued that the adsorption of these molecules changes the stable structure of Ag8 from Td to the more chemically active D(2d) symmetry. We discuss that the CO, NO, and HCN molecules prefer to adsorb on the atom of the cluster with significant contribution to both HOMO and LUMO, for the accomplishment of the required charge transfers in the systems. The charge back donation is found to leave an excess energy of about 110 meV on the NO molecular bond, evidencing potential application of silver clusters for NO reduction. It is argued that CO and specially NO exhibit strong physical interaction with the silver cluster and hence significantly modify the electronic and optical properties of the system, while HCN makes very week physical bonds with the cluster. The optical absorption spectra of the Ag8 cluster before and after molecule adsorption are computed and a nontrivial red shift is observed in the NO and HCN adsorbed clusters.