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

Abstract

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). Extrapola...