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

We compute the far-infrared absorption spectrum of the endohedral metallofullerene K+ @ C60. This arises from the vibrational and rotational motion of the potassium ion about its equilibrium position at the center of the buckyball cage. The spectrum is quite unusual. Although the permanent dipole moment of the complex is zero, absorption is very intense. 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 rotational band, and a much weaker R-branch at 180 cm-1. Line-spacing is strongly dependent on the rotational quantum number. Hot bands contribute significantly to the intensity. We include a discussion of dipolar shielding effects.