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Cluster structures

The field of cluster science continues to rapidly expand.This has several reasons; the connection to the field of nanoscale science and the fact that clusters represent ideal model systems, with well-defined structures and selection rules.

Cluster science and nanoscale science

In the field of nanoscale science, clusters offer the exciting prospect of serving as building blocks for new materials, whose desired properties may be tailored through selection of size and composition [1,2]. 

Indeed, new materials of nanoscale dimensions can be formed using clusters as building blocks: an approach “from the bottom up” [3,4].

Therefore, efforts should be focused on systems whose properties dramatically vary with composition, one atom at a time. Most appealing among these are clusters that display interesting behavior, whose composition can be selectively chosen, and whose individual characteristics might be retained when assembled into an extended material or one comprising nanoscale composites [3].

intro

Ideal model systems give new insights

Such clusters represent ideal model systems, with well-defined structures and selection rules. Such systems allow for unique tests of various solid-state models, that are hidden from the experimentalists in the bulk samples by defects, inhomogeneities, etc. This will result in considerable insight into strongly correlated phenomena. Quantum confinement effects often govern the behavior of matter in this size regime, and studying the interplay of structure, geometry, electronic and magnetic properties provide unique information on the behavior of such quantum objects. How atoms join together to form a crystal structure is a complicated process, even the initial stages of which are not understood yet. Solving this problem for real matter is thus of utmost importance.

Combination of approaches

To make a continuous transition from an atom to a macroscopic unit, a combination of approaches is imperative. Clusters that are few atoms large, must be isolated from the environment and therefore require gas-phase techniques. Large clusters need to be supported by a substrate, but are also more stable against environmental effects. The experimental tools evolve accordingly. Moreover, also theoretical understanding needs combined efforts of analytical many-body theory methods with computational first-principles approaches.

Determination structure gas-phase clusters

The structure of the gas-phase clusters cannot be determined by any known microscopy method. Here we use infrared (IR) vibrational spectroscopy to unravel the geometric arrangement of the atoms. However, the density of the molecular beam containing the cluster is too low to use regular absorption spectroscopic techniques. After the clusters are created in a laser ablation source, they travel through the setup at approximately 1000 m/s. The produced distribution is analyzed in a time-of-flight mass spectrometer revealing the mass of the clusters and their abundance. Because it is only possible to “count” the number of clusters in the beam we have to rely upon the action of the IR light on the cluster and try to record this (“action spectroscopy”).

Method 1: Attach messenger atom

One of the methods is to attach an inert messenger atom to the cluster. Upon the excitation of a vibrational mode, the messenger atom is shed from the cluster which can be detected using the mass spectrometer by comparing the intensity of the cluster-messenger complex to the bare cluster. By changing the frequency of the IR light and monitoring the depletion of the cluster-messenger complex a vibrational spectrum can be obtained.

Method 2: IR excitation and UV ionization

A second method is to combine IR excitation with UV ionization. The number of neutral clusters in the molecular beam is obtained by ionization of the clusters using a UV light source. The energy is tuned such that it is close to the ionization threshold of the cluster of interest. When the clusters are irradiated with the IR light, prior to UV ionization, an increase in the ion yield is observed when the IR is resonant with a vibrational mode. In this case an IR photon is absorbed increasing the internal energy of the cluster overcoming the ionization energy assisted by the UV pulse.

The information on the structure of clusters which can be extracted from vibration spectra alone is very limited. The vibration spectrum is a fingerprint of the cluster geometry, a comparison with theoretical calculated spectra can identify the cluster geometry. Using ab-initiomethods like density functional theory all possible cluster geometries can be calculated. For the isomers lowest in energy, a comparison of the theoretical vibration spectrum with the experiment can be made. Thus vibration spectroscopy can provide us with the structure of the clusters involved.

2: H.H. Anderson, Ed.;Small Particles and Inorganic Cluster; Springer: New York, 1997.