Rare Gas And Molecular Clusters

4.4.1. Inert-Gas Clusters

Table 4.2 lists a number of different kinds of nanoparticles. Besides metal atoms and semiconducting atoms, nanoparticles can be assembled from rare gases such as krypton and xenon, and molecules such as wafer. Xenon clusters are formed by adiabatic expansion of a supersonic jet of the gas through a small capillary into a vacuum. The gas is then collected by a mass spectrometer, where it is ionized by an electron beam, and its mass : charge ratio measured. As in the case of metals, there are magic numbers, meaning that clusters having a certain number of atoms are more stable than others. For the case of xenon, the most stable clusters occur at particles having 13, 19, 25, 55, 71, 87, and 147 atoms. Argon clusters have similar structural magic numbers. Since the inert-gas atoms have filled electronic shells, their magic numbers are structural magic numbers as discussed in Chapter 2. The forces that bond inert-gas atoms into clusters are weaker than those that bond metals and semiconducting atoms. Even though inert-gas atoms have filled electron shells, because of the movement of the electrons about the atoms, they can have an instantaneous electric dipole moment, An electric dipole moment occurs when a positive charge and a negative charge are separated by some distance. This dipole produces an electric field 2P,/R3 at another atom a distance R away. This, in turn, induces a dipole moment, P2, on the second atom, laPi/R3, where a is called the electronic polarizability. Thus two inert-gas atoms will have an attractive potential

This is known as the van der Waab potential, and it is effective at relatively large separations of the atoms. As the two atoms get much closer together, there will be repulsion between the electronic cores of each atom. Experimentally this has been shown to have the form B/R12. Thus the overall interaction potential between two inert-gas atoms has the form

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