Relaxationsdynamik und Femtochemie in massenselektierten Metall-Clustern

Abstract

In this work, the relaxation dynamic of photo-excited pure Cu-clusteranions as well as the photodesorption of oxygen from Ag8O2- are studied. For that purpose, copper was vaporized in a PACIS cluster source and the mass-selected clusters were investigated by time-resolved pump-probe-photoelectron spectroscopy using ultra-short femto-laserpulses. The focus of this research was directed to the possible application of cluster building-blocks in future technologies for catalysis or energy storage and conversion.1. The investigation of dynamical properties of pure Cu-clusteranions offers the basic research of the dominant relaxation processes in these clusters. The measured dynamics of Cun- were compared to the known behaviour of other sp-metal clusters, like Ag-, Au- and Al-clusteranions, to obtain a systematic understanding. The main interest was on extraordinary long-lasting relaxation dynamics. Pump-probe-signals could be measured from Cun- with n = 5 9, 11 13, 18 and 19. The most relaxation times hit the average of 100 fs. An exception is the electronically magic cluster Cu7-, which shows a remarkable long-living excited state of some picoseconds. Contrary to expectations, the next bigger, electronically magic Cluster, Cu19-, has a relatively fast relaxation dynamic, which does not differ perceptibly from those of the other neighbouring, non-magic clusters. Qualitatively, this special behaviour was explained on the basis of internal conversion via conical intersection and was illustrated by using a Clemenger-Nilsson-diagram. As sp-metal clusters have a low density of states (DOS) in the neighbourhood of the highest occupied molecular orbital (HOMO), fast relaxations via Auger-like processes are unlikely. But the dominating sp-character in the clusters bonds opens an additional relaxation channel: The flexible geometrical structure enables a fast, radiationless relaxation via deformation of the geometry in terms of internal conversion. This ability was already proposed to be responsible for the unexpected short relaxation times in the case of previously studied photo-excited doubly magic Al13-. Qualitatively and quantitatively, small Ag-clusteranions show almost identical dynamics in comparison of measured Cun-. This can be attributed to similarities in their electronic and geometric structures. On the contrary, Au-clusteranions have a clearly more rigid geometric structure due to relativistic effects and stronger influence of d-orbital in the bonds. Therefore, in Au-clusters the fast relaxation channel via geometrical deformation is not likely, resulting in conspicuously longer-lasting relaxation dynamics.2. Time-resolved photoelectron spectra of Ag8O2- show pump-probe signals shifting to higher binding energies with increasing time delay between pump- and probe-pulse. The signals finally localize at energies which can be attributed to the ones of the pure Ag8-. The temporal evolution of the measuring signals was explained on the basis of a direct photodesorption process: First, the pump-pulse photoexcites the system in an excited binding state, which develops quickly in an excited anti-binding state. The direct desorption of O2 from Ag8O2- occurs out of this anti-binding state. Meanwhile, the potential energy of the state is reduced, which means the binding energy grows. Correspondingly, the measured spectra show a shifting of the time delay depending signal to higher binding energies as the time delay increases. This process results in the breaking of the O2-Ag8-bond, leaving the pure Ag8- as fragment product. This is consistent with the measured data showing a stop of the shifting of the pump-probe signals at energies which agree well with the known ones of Ag8-. The discussed photoelectron spectra are typical for the direct photo-induced desorption. In contrast, the indirect process takes place via an intermediate step of thermalization of the excited state and is well known for metal surfaces.