On the linewidth in photoelectron spectra of size-selected clusters

A systematic analysis of the average linewidth of features in the photoelectron spectra of size-selected elemental clusters consisting of up to 10 atoms is presented. With increasing atomic weight, the average linewidth decreases. Several possible reasons for this trend are discussed. Obvious effects such as experimental resolution, vibrational temperature, and lifetime broadening can be excluded. The only remaining explanation is a mass-dependence of the Franck-Condon envelope. Each photoelectron peak corresponds to an electronic transition, which exhibits a Frank-Condon envelope. Its full width of half maximum depends on the spatial expansion of the nuclear wave functions in the initial state. With increasing atomic mass, the nuclear wave functions narrow down.