Near-infrared photoabsorption by C₆₀ dianions in a storage ring

We present a detailed study of the electronic structure and the stability of C60 dianions in the gas phase. Monoanions were extracted from a plasma source and converted to dianions by electron transfer in a Na vapor cell. The dianions were then stored in an electrostatic ring, and their near-infrared absorption spectrum was measured by observation of laser induced electron detachment. From the time dependence of the detachment after photon absorption, we conclude that the reaction has contributions from both direct electron tunneling to the continuum and vibrationally assisted tunneling after internal conversion. This implies that the height of the Coulomb barrier confining the attached electrons is at least ∼1.5 eV. For C₆₀^2- ions in solution electron spin resonance measurements have indicated a singlet ground state, and from the similarity of the absorption spectra we conclude that also the ground state of isolated C₆₀^2- ions is singlet. The observed spectrum corresponds to an electronic transition from a t1u lowest unoccupied molecular orbital (LUMO) of C₆₀ to the t_ag LUMO+1 level. The electronic levels of the dianion are split due to Jahn-Teller coupling to quadrupole deformations of the molecule, and a main absorption band at 10723 cm^-1 corresponds to a transition between the Jahn-Teller ground states. Also transitions from pseudorotational states with 200 cm^-1 and (probably) 420 cm-1 excitation are observed. We argue that a very broad absorption band from about 11 500 cm^-1 to 13 500 cm^-1 consists of transitions to so-called cone states, which are Jahn-Teller states on a higher potential-energy surface, stabilized by a pseudorotational angular momentum barrier. A previously observed, high-lying absorption band for C 60 - may also be a transition to a cone state.