An optical absorption spectrum of a silver tetramer ion, Ag-4(+) was obtain
ed in the photon-energy range of 2.0-5.2 eV by measuring its photofragmenta
tion action spectrum. Three major absorption peaks were observed at photon
energies of about 3.1, 4.0, and 4.3 eV. Each transition has a narrow line w
idth of less than 0.1 eV, indicating a molecular nature of the present clus
ter ion at low temperature. Fragmentation channels were strongly dependent
on excited states; Ag-3(+) was the dominant product ion at 3.1 and 4.0 eV e
xcitation, whereas only Ag-2(+) was produced at 4.3 eV excitation. The frag
mentation was found to proceed in a nonstatistical manner. These processes
were further characterized by translational kinetic energies of the photofr
agments evaluated by the measurements of velocity distributions of neutral
fragments. It was shown that photofragments are emitted with relatively low
translational energies and that, consequently, they are highly excited in
internal modes.