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We report high-resolution far-infrared transmission measurements on C 60 -tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated (20-650 cm Ϫ1 ), we assign intramolecular modes of the C 60 monoanion and identify low-frequency combination modes. The well-known F 1u (1) and F 1u (2) modes are split into doublets at room temperature, indicating a D 5d or D 3d distorted ball. This result is consistent with a dynamic Jahn-Teller effect in the strong-coupling limit or with a static distortion stabilized by low-symmetry perturbations. The appearance of silent odd modes is in keeping with symmetry reduction of the ball, while activation of even modes is attributed to interband electron-phonon coupling and orientational disorder in the fulleride salt. Temperature dependences reveal a weak transition in the region 125-150 K in both C 60 Ϫ and counterion modes, indicating a bulk, rather than solely molecular, effect. Anomalous softening ͑with decreasing temperature͒ in several modes may correlate with the radial character of those vibrations.

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