KRYPTON an inert gas – is generally reluctant to enter into any kind of chemical relationship, either with itself or with another element. Now, however, chemists at the University of Sussex have made ‘lumps’ or ‘clusters’ of krypton atoms that contain as many as 340 atoms.
Such clusters bridge the gap between the world of atoms and small molecules, and the world of bulk matter. Chemists are interested in such clusters because they are useful in the study of catalysts.
Paul Lethbridge and Tony Stace at the University of Sussex have carried out an investigation into how krypton clusters are formed and, subsequently, fall apart (Journal of Chemical Physics, vol 91, p 7685).
To create the clusters, the chemists passed gaseous krypton into a vacuum through a hole only 200 micrometres in diameter. This cooled the atoms, so that when they collided, the weak Van der Waals force between them was sufficient to hold them together (see New Scientist, Science, 4 November 1989).
Lethbridge and Stace used this method to make a wide range of electrically neutral clusters, Krn, where n varied from 10 to 340. They then ionised the clusters with an electron beam and studied the positive ions in a mass spectrometer. This is an instrument that separates particles according to the ratio of their mass to charge.
By analysing the spectrum, Lethbridge and Stace were able to discover which masses were formed. Their analysis was complicated, however, because krypton occurs naturally in six abundant isotopes, each with a different mass. Isotopes of an element have the same numbers of protons in their nucleus but different numbers of neutrons.
The various possible combinations of the isotopes give rise to many extra peaks in the mass spectra. For example, for the cluster ion Kr100+, there are more than 100 million different permutations of the isotopes. Lethbridge and Stace needed a computer to unravel the complications.
Chemists have a theory which predicts which sizes of cluster should be favoured. The theory, which is based on three-dimensional structures known as Mackay icosahedra, predicts that favoured clusters should contain 13, 55, 147, 309, 561, and 923 atoms. These numbers are also known as ‘magic number combinations’.
In their mass spectra, Lethbridge and Stace did indeed see intense peaks which corresponded to Kr147+ and Kr309+.
The theory to explain why some cluster sizes are favoured is based on a simple picture of the way in which the atoms in a cluster pack together. According to the theory, a cluster grows around a small ‘core’, consisting of a few atoms. Further atoms are then added until the core is completely surrounded by an outer layer, which forms a closed ‘shell’.
Structures with a closed shell are thought to be particularly stable. The magic numbers simply correspond to the numbers of atoms needed to complete closed shells of increasing size. The clusters Kr147+ and Kr309+ contain three and four closed shells.
Clusters have icosohedral symmetry – that is, 20 regular faces. One of the interesting things about such structures, says Stace, is that there is no equivalent structure for the bulk material. When the inert gases become solids, they always have a structure which crystallographers refer to as face-centred cubic.
The chemists also want to find evidence for similar changes in structure in clusters of metal atoms. If metal clusters show the same kind of behaviour, they may give rise to new types of catalytic activity. This is because catalysis is a phenomenon which occurs on surfaces and such clusters will come in a large variety of structures, all with high surface areas.