Spectroscopy of C₆₀⁺ and C₁₂₀⁺ in the mid-infrared

Infrared spectra of C-60(+) and C-120(+), obtained via helium messenger spectroscopy, are reported. For C-60(+), new absorption features have been found just above the discrete vibrational spectrum of the ion. The absorption profile, which is broad and contains little structure, is assigned to one or more electronic absorption transitions and is in good agreement with predictions from time-dependent density functional theory. It seems likely that the transitions observed correspond to excitation from the 2A1u electronic ground state to one or both of the low-lying E-2(1u) and E-2(2u) electronic states previously identified as dark states of C-60(+). These states presumably become optically bright through vibronic coupling and specifically the Jahn-Teller effect. In the case of C-120(+), the simplest positively charged oligomer of C60, we present the first vibrational spectrum of this ion. Through a comparison with theory, vibrational features are best explained by a peanut-shaped structure for C-120(+), maintained by covalent bonding between the two C60 units. We have also discovered electronic transitions for C-120(+), which, similar to C-60(+), lie just above the vibrational spectrum.