Effect Of Thiol-Ligands On The Optical Response Of Supported Silver Clusters

The extinction spectra of size-selected, supported Ag-20 and Agss clusters have been measured with surface cavity ring down (s-CRD) spectroscopy under ultrahigh vacuum (UHV) conditions. A single plasmonic resonance around 3.2 eV is observed. The reaction with benzenethiol shifts the localized surface plasmon resonance (LSPR) by approximate to 0,3 eV to lower energies, which is attributed to an increased dielectric function of the surrounding medium as well as to a reduction of the free-electron density inside the silver clusters. The time dependence of the LSPR redshift under exposure to benzenethiol has a double exponential behavior. A rapid redshift is caused by chemisorption of benzenethiol from the gas phase via the formation of a sulfur silver bond, whereas a slow redshift is caused by additional physisorption of benzenethiol. Comparative studies with benzene, which do not show any chemisorption but show physisorption character on silver, reveal that approximate to 0.2 eV of the overall redshift can be attributed to an increased dielectric Constant of the surrounding medium, whereas a reduction of the free-electron density accounts for approximate to 0.1 eV of the observed redshift.