The vibrational line shape of diatomic adsorbates on metal clusters

A decrease of at least an order of magnitude in the vibrational relaxation time T1 has been measured for CO bonded to Rh and Co clusters when the size of the cluster increases from 5 to 35 Å. We propose that this effect is mainly due to the coupling of the molecular vibration ω0 with the electron-hole excitations in the cluster. This is described via a model Hamiltonian. The finite size of the clusters give rise to a discrete electronic spectrum, and hence to a discrete pair excitation spectrum. This effect is measured in terms of D, the mean spacing between nearest-neighbor levels in the conduction band of the cluster. We find that: (1) the proposed mechanism starts to contribute to T1 only when D<ℏω0; (2) T1 is at least several hundred ps for clusters less than 15 Å in size; (3) there is a sharp decrease of T1 to about 10 ps as the cluster size increases from 15 to 40 Å; (4) T1 decreases smoothly towards the bulk value for larger clusters.