Pseudo Jahn-Teller effect observed in Eu@C₆₀

Structures and electronic states in the ground state of endohedral fullerence are investigated by XAFS analysis and XANES spectrum. The distances between the Eu ion and the neighboring C atoms show clearly that the Eu ion is inside the C₆₀ cage. The endohedral element, Eu, in C₆₀ is located away from the central position of C₆₀ cage by 1.4. We show the detailed location of the Eu ion in the C₆₀ cage by XAFS and the first principles calculation. The perturbation theory indicates that the distortion of C₆₀ cage and the shift of the Eu ion are attributed to electron-nuclear interaction, pseudo-Jahn-Teller type interaction. XANES spectrum of Eu L_III-edge shows that the valence of the Eu ion is close to +2. The electron configuration is thus f⁷, showing the total symmetry (A_1g) of I_h group. The details of the structures and the electronic states will be discussed with both the experimental and theoretical results.