Studies on the excited electronic states of fullerenes
This work focused on investigating the excited electronic states of fullerenes. Due to their highly symmetric nature and elemental purity, fullerenes are ideal model systems for studying and understanding the excitation and ionisation mechanisms of large complex systems. An intriguing characteristic that they possess is the atom-like nature of the diffuse excited electronic states known as Super-Atom Molecular Orbitals (SAMOs). SAMOs are described as low-lying Rydberg-like states where the charge distribution is centred on the centre of the hollow carbon cage. These states have been observed in gas-phase photoelectron spectroscopic studies of fullerenes, such as C60 and C70, and endohedral fullerenes, such as Sc3N@C80. The photoelectron spectra after excitation using fs or ps laser pulses, are characterized by a thermal electron background and a peak structure superimposed on it, having kinetic energies lower than the photon energy. Based on the photoelectron angular distributions and TD-DFT calculations, the peak structure can be assigned to a one-photon ionisation from the SAMO states. The electronic properties of the endohedral metallofullerene Li@C60, C60 cage with a Li atom inside, were investigated experimentally for isolated molecules in the gas-phase using laser ionisation, for surface bound molecules using an STM/STS apparatus and theoretically through a combination of DFT and TD-DFT calculations. Comparisons with the empty C60 provide information about the influence that the encapsulated atom has on the electronic system. The off-centre position of the Li inside the cage distorts the symmetry of the SAMOs which affects their binding energies and structure when probed in the gas-phase. Similar behaviours were observed from the calculations and from molecules adsorbed on a metal surface. Due to the asymmetric position of the Li atom inside the cage, Li@C60 has been speculated to be suitable to act as a molecular switch. Switching behaviour was observed for surface bound molecules were 14 reversible and stable molecular states that can be statistically accessed were identified. This represents the largest number of multiple states that have been identified in a single-molecular switch. The influences that the pulse duration, laser fluence and bandwidth of excitation have on the ionisation dynamics of C60 and the energy resolution of the SAMOs was studied. The excited electronic energy was found to couple to the vibrational degrees of freedom with a coupling constant of 240 fs, which agrees with previously published results. A 650-750 fs timescale was observed to correlate with the point that thermionic electron emission becomes the dominant decay process over the hot electron emission. In addition, the chirp of the laser pulse affected the central position of the SAMOs and resulted in the photo-electron spectra to be characterised by inhomogeneous broadening. With narrowband excitation, the energy redistribution within the C60 molecules is less efficient and evidence of high-lying Rydberg states was present in the photo-electron spectra. Lastly, a peak superimposed on the delayed ionisation tail of C60, found roughly at 6.9 µs after the prompt peak, was identified for the first time. A hypothesis for the origin of this feature is proposed which is based on energy being released from a decaying long-lived superexcited state or a group of high-lying Rydberg states.