Big Experiment for Research on Temperature controlled cluster Anions and cations (Berta)
Researching PhD student: Lukas Weise
The total setup
Sodium and aluminum clusters were produced in an evaporation respectively magnetron sputter source. For sodium atoms were evaporated from an oven, condensed to clusters in a liquid-nitrogen cooled He gas flow, and ionized by an electric discharge. The clusters were trapped in a 12-pole RF ion trap connected to a cold-head for two purposes: The trap accumulates ions from the continuously running source and bunches them to much higher intensities, and the trap serves to cool clusters via thermalization with buffer gas atoms (~10-3 mbar He, trap temperature 6K). Single mass clusters were selected with a multi-wire mass gate at the first time-focusing point of a time-of-flight spectrometer. By crossing the cluster beam with a laser beam, photoelectrons were detached from the cluster ions and guided to a two-dimensional electron detector (single MCP and phosphor screen) while expanding according to their initial velocities. The resulting electron distribution were recorded with a fast CCD camera from the outside of the vacuum chamber.
Experimental results for Sodium clusters
- For all investigated cluster sizes the β parameters show a smooth evolution with the excitation energy.
- Especially for bigger sizes β reaches strongly negative values. This can only occur by the interference of two outgoing partial waves which leads to the assumption of a high coherence during the excitation process, even for very big many-particle systems as Na147-.
Comparison of the approximate angular momentum eigenstates for different cluster sizes
- Characteristic differences exist for the evolution of β parameters of jellium states with different l quantum numbers.
- Energetically split jellium states from the same cluster size show a very similar evolution of β if plotted versus the kinetic energy of the detached electron.
- Another similarity is observed between β parameters for jellium states with the same l quantum number but from different cluster sizes.
→ The quantum number l still satisfyingly characterizes the bound states even for nonspherical symmetries.
Data of Ag55 and Cu55 clusters and comparing with Na55
- Splitting of 1g and 1f level because of icosahedral structure.
- In contrast to sodium and copper the both outer peaks of silver don't overlay.
- DFT calculations show very similar electron densities for silver and sodium.
- Peaks of the 1g level: compared to sodium the minima of the distributions of copper and silver are shifted to higher kinetic energies.
- Peak of the 2p level: the distributions of copper and silver are very similar.
- Minima of the Betaparameter distributions are similar when plotted against the kinetic energy multiplied by the clustersize.