Time-Resolved Spectroscopy of Bare and Reacted Gold and Silver Clusters : Materials for New Photochemistry?

Abstract

Led by the vision of searching for suitable candidates for future applications in catalysis and energy conversion, systematic studies on the dynamical properties of several size-selected clusters have been performed. The following questions were addressed:- What are the basic relaxation mechanisms of clusters consisting of s/p-metals? Do bare silver clusters show similar properties to the previously studied gold or aluminum clusters?- Do metal clusters exhibit different dynamical behavior from bulk metals with respect to photon induced desorption?- Can species with long-lived excited states be identified?In order to address these issues, Agn- (n up to 21), AgnO2- with n = 2, 3, 4, 8, Ag2O3-, Au2O- and AunO2- for n = 1, 2 were synthesized using a PACIS and investigated systematically by time-resolved photoelectron spectroscopy: The clusters were excited by a pump pulse and the time-dependent development of the excited system was observed by recording photoelectron spectra using a second (probe) pulse. Different optical setups were installed within this work, because pump and probe energies had to be modified in order to access and probe excited states for various clusters.1. Bare Silver ClustersAgn- clusters were photoexcited by a 1,55 eV photon and photoelectron spectra were taken using a photon energy of 3.1 eV. For most of the species the excited state signal disappears on a time scale considerably shorter than 1 ps. Since the spacing between neighboring electronic levels is of the same order as the excitation energy and much larger than the typical energy of vibrational modes, relaxations via combinations of Auger-like processes and coupling of electronic and vibrational excitations are unlikely. The ability of these small clusters to deform after optical excitation is proposed to be responsible for the short relaxation times analogous to the previously studied case of Al13-. Qualitatively, the relationship of electronic shells and their gaps and the deformation of the clusters is described by the Nilsson diagram. With help of this diagram, a first qualitative understanding of the experimental data can be gained.2. Silver Oxide ClustersExcited states of AgnO2- clusters with even n = 2, 4 and 8 were discovered, having high photoabsorption cross sections for a pump energy of 3.1 eV. The excited states of these species are similar, although the electronic structure varies strongly with increasing number of Ag atoms in the cluster. The state is localized close to the oxygen molecule and decays for Ag2O2- into Ag2- and O2 via direct photoinduced desorption. On metal surfaces, direct desorption is unlikely, because any excited state is quenched effectively by the DOS near the Fermi energy. In clusters the DOS is low, allowing longer lifetimes and competing processes become more likely.This interpretation is supported by the finding of an excited state with a long liftetime for Ag3O2-. Here, O2 is suggested to be dissociatively chemisorbed and the only remaining "fast" relaxation channel (desorption) is blocked. The lifetime increases by more than one order of magnitude to 5.4 ps.3. Gold Oxide ClustersPhotofragmentation of Au2O- induced by excitation with a 3.1 eV photon was observed using one-color TR-PES. The data indicate the existence of an excited state of the anion with a very long lifetime of more than 100 ps. From this excited state, the parent anion decayed into Au- + AuO or AuO- + Au. Fragmentation did not start immediately after excitation but with a delay of 7 ps. A detailed analysis of the spectra at short delay times indicates that the delayed onset for fragmentation corresponds to the time for geometric rearrangement.Au1O2- was studied using the blue-UV setup. For this cluster, oxygen is bound dissociatively, inhibiting relaxation by desorption of O2. Several excited states could be identified, all having relaxation times of about 31 ps. Relaxation via vibrational autodetachment is proposed for energy dissipation of this cluster.For Au2O2-, a rather short-lived excited state (< 1 ps) could be observed using the blue-UV pump-probe scheme. Au2- could be identified in the spectra, suggesting photodesorption similar to the even numbered silver oxide clusters and Au2O-.In summary, the results obtained in this work demonstrate that for small metal clusters fast thermalization is not always the dominant relaxation channel. Furthermore, direct photodesorption and long-lived excited states have been identified, which have not been observed from metal surfaces. Thus, photoactivation or photodesorption may prevail as dominant processes for such small clusters, suggesting unique photochemical properties and making them very interesting for photochemistry and promising candidates for future applications in energy storage, energy conversion and catalysis.