An optical absorption spectrum of a silver tetramer ion, Ag+4, was obtained in the photon-energy range of 2.0-5.2 eV by measuring its photofragmentation 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 width of less than 0.1 eV, indicating a molecular nature of the present cluster 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 excitation, whereas only Ag+2 was produced at 4.3 eV excitation. The fragmentation was found to proceed in a nonstatistical manner. These processes were further characterized by translational kinetic energies of the photofragments 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.
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