Quantitative Determination of the Raman Enhancement of Ag₃₀(CO)₂₅ and Ag₅₀(CO)₄₀ Matrix Isolated in Solid Carbon Monoxide

Size-selected Ag clusters in the range Ag₃–Ag₅₀ were prepared by sputtering a silver substrate, mass-selecting Ag cation clusters using a Wien filter, neutralizing and matrix-isolating them at cryogenic temperatures in solid CO. The Raman spectra of the resulting silver-cluster carbonyls were recorded using excitation wavelengths in the range 457.9 to 514.5 nm. For Ag₃₀ and Ag₅₀, the “adsorbed” carbon monoxide (which we estimated to number ∼25 and ∼40, respectively) gave rise to broad Raman bands centered at ∼2110 cm^–1. Because both the metal cluster and the CO density were measured quantitatively, a good estimate was computed for the increase in the Raman scattering cross-section per CO molecule adsorbed on the silver particle. For Ag₅₀ and 457.9 nm laser excitation, an enhancement of ∼1850 was measured, which dropped to ∼1350 at 514.5 nm excitation. For Ag₃₀ (and 457.9 nm excitation) the enhancement was ∼530. The enhancements of Ag₃, Ag₅, and Ag₉ were too low to measure accurately (i.e., <10). Extrapolating the enhancements obtained with blue and green wavelengths to the “plasmonic” band center, which for an Ag₅₀ cluster is expected to be at ∼370 nm, and assuming the excitation band to be a Lorentzian with a fwhh of 0.8 eV, the maximum Raman enhancement per CO ligand in Ag₅₀CO₄₀ was estimated to be ∼12000, in good agreement with computed results using a time-dependent density functional quantum calculation, carried out on a Ag₂₀ cluster complex by Jensen et al. (Jensen et al. Size-Dependence of the Enhanced Raman Scattering of Pyridine Adsorbed on Ag_<i>n</i> (<i>n</i> = 2–8, 20) Clusters. <i>J. Phys. Chem. C</i> <b>2007</b>, <i>111</i>, 4756–4764)