Metastable cluster of He*4

The existence of a metastable cluster He*4 with total spin S = 2 is predicted. The cluster consists of two covalently bound excited spin-polarized triplet He*2 molecules and is rectangular in shape. The electron wave functions, the dependence of the energy of the He*4 system on the distance between the He*2 triplet molecules, the atomic spacing, the frequency spectrum of natural oscillations of the cluster and other characteristics are calculated from the first principles. It is shown that the metastable state is formed if one of the excited He*2 molecules is in the 3Σu+ state, while the other is in the 3Πg state. The radiation lifetime of the metastable cluster He*4 is calculated; it is found to range from 100 to 200 s, which is much longer than the lifetime τ ∼ 20 s of the triplet molecule He*2 (3Σu+). The height U ∼ 0.5 eV of the potential barrier preventing the departure from the local energy minimum is determined. The energy Eacc ∼ 9 eV/atom accumulated in the cluster is calculated; this energy considerably exceeds the energy of the known chemical energy carriers. It is shown that the accumulated energy is released actually completely during decomposition of the cluster He*4 into individual helium atoms. This means that helium clusters are a promising material with a high accumulated energy density (HEDM)