Using time-resolved photoelectron spectroscopy, the decay channels of AuO − 2 and Au 2 O − 2 following photoexcitation with 3.1-eV photons have been studied. For AuO − 2 , a state with a rather long lifetime of 30 ps has been identified. Its decay path could not be determined but photodesorption can be excluded. For Au 2 O − 2 , the spectra indicate O 2 desorption after 3.1-eV photoexcitation on a time scale of 1 ps. While comparing these results on Au n O − 2 with analogous data on Ag n O − 2 clusters, a discernible pattern emerges: for dissociatively bound O 2 (AuO − 2 , Ag 3 O − 2), there are longliving excited states which do not decay by oxygen desorption, while for molecular chemisorption (Au 2 O − 2 , Ag 2 O − 2 , Ag 4 O − 2 , Ag 8 O − 2), the 3.1-eV photoexcitation triggers fast O 2 desorption with a high quantum yield.
The decay path of an Ag8(O2) − cluster photoexcited by a 3.1 eV photon is elucidated using time-resolved photoelectron spectroscopy. Photoabsorption results in the formation of an excited state giving rise to a peak in the photoelectron spectra with well-resolved vibrational finestructure. With a lifetime of about 100 fs this bound state decays into an anti-bonding state which dissociates into O2 and Ag − 8 on a timescale of 10 ps. In the photoelectron spectra, this corresponds to a broad maximum shifting gradually towards higher binding energy while the O2 and Ag − 8 separate. Finally, the spectrum of bare Ag − 8 appears. This process is unique to small clusters, because on metal surfaces excited state lifetimes are too short to allow for direct dissociation.
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