Cluster Physics

Clusters are aggregates of 2-106 atoms. The physics and chemistry of clusters addresses an impressive set of problems, ranging from fundamental to applied ones. Clusters have applications in various aspects in many fields of science such as astrophysics, chemistry and material science. For example, clusters play an important role in the formation mechanism of cosmic dust. Clusters have properties intermediate between the atomic scale and the bulk scale. In a manner of speaking, clusters bridge the gap between the microscopic and the macroscopic worlds. Clusters provide a useful laboratory for investigating the structure of matter and how its properties (electric, magnetic and optic) evolve with size. These properties can be divided into 2 regimes : (i) a scalable regime where properties vary smoothly until they reach those of bulk limit and (ii) a non-scalable regime where variation is highly non-monotonic. In the later regime, many phenomena arise, e.g. non-metallic systems become metallic and so on.

A very interesting branch of cluster physics is their stability, namely their formation and destruction. The stability is usually studied through interactions (collisions) of the clusters with other atoms, electrons or photons.

Size effects in small carbon clusters

The cross section for electron impact detachment from size selected carbon anions was measured in our lab using the electrostatic ion beam trap. Figure 1 shows the measured cross section for electron detachment from carbon cluster anions after interacting with electrons at 20 eV. Data is presented together with model calculations (dotted blue line). There are two interesting effects:  First,  the cross section increases in average with cluster size. Second,  The cross section changes non-monotonically with cluster size. The experimentally known binding energy to detach an electron from carbon clusters are presented in figure 2. The odd-even effect in the cross-section as a function cluster size follows the odd-even oscillation in the binding energies. Thus, stronger binding energy correspond to smaller cross section. The odd-even oscillations  in the binding energies was understood from the Jellium model. Thus, the most stable clusters (having larger binding energies) have full electronic shells, and by adding or subtracting a single atom the binding energy may change by few electron volts.












More on this subject:

[1] "Size effects in the interaction between ionic clusters and low-energy electrons", M. Eritt et al., Phys. Scr. 73 (2006) C32-C35.

[2] "UPS of 2-30 atom carbon clusters: chains and rings", S. Yang et al., Chem. Phys. Lett.,144, 431 (1988).

Cluster Cooling

We are also interested in the thermodynamics of clusters, namely how their cooling and heating processes. The cooling process for clusters can be divided in three classes: Evaporation, Radiative cooling, thermionic emission. What is the number of atoms in a cluster where thermodynamic concepts behave same as in bulk? How does the temperature defined for a system of few atoms?

Using the electrostatic ion beam trap, we are studying the cooling processes of hot clusters. In these experiments, the clusters are produced in a hot ion source, accelerated and trapped. The cooling process is probed via the interaction of laser light with the clusters.

The radiative cooling of Al4- has been measured in our lab and was found very similar to black body. The aluminum clusters were produced very hot and trapped in the electrostatic ion beam trap . The figure below shows the temperature of the Al4- clusters as a function of storage time. The solid line is generalized Stefan-Boltzmann law with I(T)~T3.5, where T is the cluster initial temperature.




More Literature:

[1] Clusters and Small particles, Boris M. Smirnov, Springer (2000).
[2] Radiative Cooling of Al4-, Y. Toker et al., Phys. Rev. A 76, 053201 (2007).