Optical Spectroscopy of Metal Clusters: Cu sub 4 sup +.

The absorption spectrum of Cu sub 4 sup + has been recorded in the visible region of the spectrum (710-370nm) by monitoring the photodissociation of Cu sub 4 sup + to give Cu sub 3 sup +. Ten electronic transitions have been identified, and most of them show vibrational fine structure. The transitions appear to fall into two groups. A group centered around 650nm has been tentatively assigned to mainly d->s type transitions. This group of transitions can be correlated with an interband transition of bulk copper, suggesting that the incipient band structure of the bulk is apparent even in a cluster containing as few as four atoms. The vibronic structure of the band systems at 600nm, 690nm, and 565nm is discussed in detail and some plausible assignments are proposed.