INFRARED ROTATION AND VIBRATION-ROTATION BANDS OF ENDOHEDRAL FULLERENE COMPLEXES - K-C60(AT)

We compute the far-infrared absorption spectrum of the endohedral meta llofullerene K+ @ C60. This arises from the vibrational and rotational motion of the potassium ion about its equilibrium position at the cen ter of the buckyball cage. The spectrum is quite unusual. Although the permanent dipole moment of the complex is zero, absorption is very in tense. The spectrum contains a pure rotation band, which peaks near 55 cm-1 at 25-degrees-C. There is also a fundamental vibration-rotation band, consisting of a strong P-branch at 70 cm-1, overlapping the rota tional band, and a much weaker R-branch at 180 cm-1. Line-spacing is s trongly dependent on the rotational quantum number. Hot bands contribu te significantly to the intensity. We include a discussion of dipolar shielding effects.