The relative stability of small cationic platinum clusters is investigated by photofragmentation experiments. Mass spectra show a smooth intensity distribution except for a local intensity minimum at Pt5+, revealing enhanced stability of the platinum tetramer Pt4+. The possibility that radiative cooling competes with statistical fragmentation after photoexcitation is examined and it is shown that clusters in the N = 3-8 size range do not radiate on the time scale of the experiment. In the absence of radiative cooling, the mass spectra of photofragmented clusters can be well explained by dissociation energies computed using density functional theory. The large calculated HOMO-LUMO gap for Pt4+ (∼1.2 eV) is attributed to its highly symmetric structure and provides an explanation for the surprisingly low reactivity of this cluster in different gas-phase reactions.
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