Infrared absorption enhancement of (formula presented) on silver islands: Contribution of charge transfer and collective electron resonance

Normal-incidence infrared absorption spectra of (formula presented) films deposited on silver islands have been measured. The spectra exhibit bands at 1429 and (formula presented) due, respectively, to the infrared active (formula presented) and (formula presented) modes of (formula presented) in multilayers. These bands increase in intensity rather rapidly until 20 nm thickness: the intensity increase involves an enhancement of absorption due to the excitation of the transverse collective resonance of valence electrons in the silver islands. Additionally two bands appear at 1442 and (formula presented) both of which abruptly increase in intensity until 2 nm and saturate 10 nm thickness. These bands decrease in intensity and shift to higher wave numbers when oxygen is preadsorbed on the silver islands. The (formula presented) band is caused by the activation of the infrared inactive (formula presented) mode via electron transfer from the silver to adsorbed (formula presented) whereas the (formula presented) band is assigned to the (formula presented) mode redshifted by the charge transfer. The results reveal that both the charge transfer and the collective electron resonance contribute to the IR absorption enhancement of (formula presented) on silver islands.