Theoretical investigation of the C₆₀ infrared spectrum

A semiempirical model of the infrared (IR) spectrum of the C₆₀ molecule is proposed. The weak IR-active modes seen experimentally in a C₆₀ crystalline sample are argued to be combination modes caused by anharmonicity. The origin of these two-mode excitations can be either mechanical (anharmonic interatomic forces) or electrical (nonlinear dipole-moment expansion in normal mode coordinates). It is shown that the electrical anharmonicity model exhibits basic features of the experimental spectrum while nonlinear dynamics would lead to a qualitatively different overall picture. © 1996 The American Physical Society.