Probing Rapidly‐Ionizing Super‐Atom Molecular Orbitals in C₆₀: A Computational and Femtosecond Photoelectron Spectroscopy Study

Abstract

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 C(60) 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.

Coherent Electronic Wave Packet Motion inC60Controlled by the Waveform and Polarization of Few-Cycle Laser Fields

“…The analysis of the fragmentation patterns is usually complex-even for simple diatomic molecules-and quickly becomes prohibitively complicated for large polyatomic molecules because of the large number of fragmentation channels [3]. Angularly resolved photoionization by ultrashort laser pulses has been advocated for probing the electronic dynamics before the onset of significant nuclear motion (see, e.g., [27][28][29][30][31] [36,37], and efficient high-harmonic generation [38][39][40]. The ionization and fragmentation of C 60 have been investigated extensively in the past (see, e.g., [35,[41][42][43][44][45][46][47]).…”

“…Fullerenes are nanometer-size systems with interesting physical properties, including high polarizability [32], superatomic molecular orbitals [33,34] with macroatom behavior [35], large photoionization cross sections [36,37], and efficient high-harmonic generation [38][39][40]. The ionization and fragmentation of C 60 have been investigated extensively in the past (see, e.g., [35,[41][42][43][44][45][46][47]).…”

Super-atom molecular orbital excited states of fullerenes

“…The calculated photoionization widths can be converted to photoionization lifetimes of the order of femtoseconds for the SAMO states but picoseconds to nanoseconds for the non-SAMO states [19]. This explains very clearly why the SAMO peaks are so dominant for ionization with visible laser wavelengths.…”

“…Time-dependent (TD)-DFT calculations were described in detail by Mignolet et al [19]. In these studies, the electronic structure of the 500 lowest excited states of neutral C 60 was computed at the TD-DFT/B3LYP/6+31 G(d) [8] or the TD-DFT/CAM-B3LYP/6-31( [19].…”

Fullerene Dynamics with X-Ray Free-Electron Lasers

“…Fullerenes and their derivatives, characterized by their hollow geometric structures and nanometersized outer diameter, draw a great deal of interest due to their wide range of applications and "supramolecular" physical and chemical properties [1]. Fullerenes have displayed molecular [2] and bulk [3] behavior and have proven to be an excellent testing ground for experiments and theories [4].…”