Photoelectron spectroscopy of jet-cooled aluminium cluster anions

Aluminium cluster anions (Al _n ^? ) are produced by laser vaporization without additional ionization and cooled by supersonic expansion. Photoelectrons from mass-identified anion bunches (n=2...25) are detached by laser light (hv=3.68 eV) and undergo energy analysis in a magnetic bottle-type time-of-flight spectrometer. The measurements provide information about the electronic excitation energies from ionic ground states to neutral states of the clusters. In contrast to bulk aluminium these cluster photoelectron spectra partially have well-resolved bands which originate from low-lying excited bands. For small clusters, especially the aluminium dimer and trimer, quantum-chemical calculations will be compared to the measurements. The electron affinity size dependence of larger clusters shows conclusive evidence for “shell” effects.