ClusterTrap experiment

Metal Clusters

The properties of metal clusters, i.e. of aggregates of a few to some hundreds or thousands of metal atoms, are in general a function of the number of atoms, the “cluster size”. They vary between those of single atoms (e.g. ionization potential, vibrational modes, dissociation energies) and those of bulk matter (e.g. work function, heat capacity, sublimation heat). While for very large clusters (few thousands of atoms), the properties vary rather smooth as a function of the cluster size, for small clusters (few tens of atoms) addition or removal of one atom can have drastic effects, causing strong variations in the size-dependence.

Due to their high surface-to-volume ratio clusters are very interesting with respect to surface studies. There are also close connections to the theoretical concepts of nuclear physics, where similar models are applied to the description of finite fermion systems.

Alike the cluster size, also the charge state of a cluster affects its properties. Thus, removal or addition of one or several electrons might stabilize, or destabilize a cluster. In the later case, production of very high cationic charge states results in the imminent destruction of the cluster, caused by electrostatic repulsion between the positive charges, and being referred to as “Coulomb explosion”. Respectively high charge states of a cluster are produced by appropriate laser irradiation with high intensities at short time-scales. (See also the webpages of the SFB-652.)

Current research at the ClusterTrap experiment focusses on the opposite direction, i.e. the attachment of surplus electrons to a cluster. In contrast to the cations, the up-charching of anionic clusters is not limited by an explosive disintegration of the cluster, but by electron emission.

The reachable anionic charge states depend on the elemental composition of the cluster, on its size and on its energy content (i.e. the cluster “temperature”). In particular, a minimum cluster size (“appearance size”) is required to bound a given number of excess electrons.

A peculiarity of poly-anionic clusters is the existence of meta-stable systems. A (poly)-anionic cluster might have a negative electron affinity, EA<0, if its just small enough. This means, the binding energy of a further excess electron would be negative. Yet, the cluster would be able to bound this next excess electron to some extent. This becomes possible due to the Coulomb potential V_C (“Coulomb barrier”) of the anionic cluster: At large distances an approaching electron is repelled by the anionic cluster. However, it becomes attracted to the cluster, as soon as it gets close enough for forces due to polarization effects in the cluster act on the excess electron. Nevertheless, such a poly-anion is metastable, i.e. on a finite time-scale an electron will tunnel through the Coulomb potential and thus leave the cluster anion, reducing its charge state, again.

To some extent, the emission of electrons from clusters can be compared to field emission of electrons from bulk matter. In both cases strong electric fields at the surface are (in its very true meaning) in charge of the emission process.

For experimental research, metal clusters (neutrals and ions) need to be produced in special sources. Two basic principles apply to cluster formation:

- from the big to the small: desorption and fragmentation of bulk matter, induced by particle bombardment or laser irradiation;

- from the small to the big: nucleation and aggregation of atoms in a collision-gas environment and supported by adiabatic expansion.

For the ClusterTrap experiment, metal clusters are produced in a laser vaporization source, where material from metal wire is vaporized by a laser pulse in a background of helium gas. The vapor-gas mixture is subsequently expanded into vacuum. (See also experimental setup.)

Further reading: see publication list.