Journal of Chemical Physics, Vol.105, No.13, 5358-5368, 1996
Symmetrized Shape Oscillation in the Structure of Li-6(7) Clusters Observed by Molecular-Beam Electron-Spin-Resonance
The electron-spin-resonance spectra were measured for free Li-6(7) clusters by using techniques of molecular beam magnetic resonance. The spectra show a large hyperfine constant a(1) from two equivalent nuclei and a much smaller hyperfine constant a(2) from an additional five equivalent nuclei. The cluster geometry consistent with the spectra is a regular pentagonal bipyramid. It has been found that the septemer has two isomers which give the same value for a(1) to a high accuracy and slightly different values a(2a) and a(2b) for a(2). There is a relationship between a(1) and (a) over bar(2)=(a(2a)+a(2b))/2 of the form a(1)/5 (a) over bar(2)=1.03 similar or equal to 1. The a(1) and a(2) indicate that the septemer is fluctuating in structure between two geometries, the regular pentagonal bipyramid and a deformed one of it. The former corresponds to an oblate ellipsoid of revolution in the ellipsoidal shell model, while the latter means a superposition of five equivalent deformations from the oblate ellipsoid to prolate ellipsoids. The deformation must be symmetrized by the superposition, because the ground electronic state of Li-7 has no orbital degeneracy in spite of the fivefold axial symmetry of the structure. The symmetrized shape oscillation keeps the vibrational motions of the septemer constant, because the five equivalent deformations mutually cancel all restoring forces. The two ellipsoidal configurations, oblate and prolate, are brought into resonance by a Jahn-Teller effect which takes place on the prolate side. The presence of the two like isomers provides evidence of the Jahn-Teller effect, because there are two mutually independent modes of the Jahn-Teller distortion which divide the septemers into two symmetry types. The simple relation a(1) similar or equal to 5 (a) over bar(2) further confirms that the electronic states in the two ellipsoidal configurations are in resonance.