Probing rapidly-ionizing super-atom molecular orbitals in C₆₀: A computational and femtosecond photoelectron spectroscopy study

Super-atom molecular orbitals (SAMOs) are diffuse hydrogen-like orbitals defined by the shallow potential at the centre of hollow molecules such as fullerenes. The SAMO excited states differ from the Rydberg states by the significant electronic density present inside the carbon cage. We provide a detailed computational study of SAMO and Rydberg states and an experimental characterization of SAMO excited electronic states for gas-phase C₆₀ molecules by photoelectron spectroscopy. A large band of 500 excited states was computed using time-dependent density functional theory. We show that due to their diffuse character, the photoionization widths of the SAMO and Rydberg states are orders of magnitude larger than those of the isoenergetic non-SAMO excited states. Moreover, in the range of kinetic energies experimentally measured, only the SAMO states photoionize significantly on the timescale of the femtosecond laser experiments. Single photon ionization of the SAMO states dominates the photoelectron spectrum for relatively low laser intensities. The computed photoelectron spectra and photoelectron angular distributions are in good agreement with the experimental results. Super-atom molecular orbitals (SAMOs) in C₆₀: SAMOs are diffuse hydrogen-like orbitals. SAMO excited states differ from the Rydberg states by the significant electronic density present inside the carbon cage. They photoionize on a femtosecond time scale, several orders of magnitude faster than non-SAMO states. The computed photoelectron spectra and angular distributions are in good agreement with the experimental results obtained by gas-phase photoelectron spectroscopy.