Probing C60 Fullerenes from within Using Free Electron Lasers

Fullerenes, such as C60, are ideal systems to investigate energy redistribution following substantial excitation. Ultra-short and ultra-intense free electron lasers (FELs) have allowed molecular research in a new photon energy regime. FELs have allowed the study of the response of fullerenes to X-rays, which includes femtosecond multi-photon processes, as well as time-resolved ionization and fragmentation dynamics. This perspective: (1) provides a general introduction relevant to C60 research using photon sources, (2) reports on two specific X-ray FEL-based photoionization investigations of C60, at two different FEL fluences, one static and one time-resolved, and (3) offers a brief analysis and recommendations for future research.

Direct Visualization of Nearly Free Electron States Formed by Superatom Molecular Orbitals in a Li@C60 Monolayer

Using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we directly determine the spatial and energetic distributions of superatom molecular orbitals (SAMOs) of an Li@C₆₀ monolayer adsorbed on a Cu(111) surface. Utilizing a weakly bonded [Li⁺@C₆₀] NTf₂^- (NTf₂^-: bis(trifluoromethanesulfonyl)imide) salt makes it possible to produce a Li@C₆₀ monolayer with high concentration of Li@C₆₀ molecules. Because of the very uniform adsorption geometry of Li@C₆₀ on Cu(111), the p_z-SAMO, populated above the upper hemisphere of the molecule, exhibits an isotropic and delocalized nature, with an energy that is significantly lower compared to that of C₆₀. The isotropic overlapping of p_z-SAMOs in the condensed monolayer of Li@C₆₀ results in a laterally homogeneous STM image contributing to the formation of a free-electron-like states. These findings make an important step toward further basic research and applicative utilization of Li@C₆₀ SAMOs.

CO2 Activation Within a Superalkali-Doped Fullerene

With the aim of finding a suitable synthesizable superalkali species, using the B3LYP/6-31G* density functional level of theory we provide results for the interaction between the buckminsterfullerene C60 and the superalkali Li3F2. We show that this endofullerene is stable and provides a closed environment in which the superalkali can exist and interact with CO2. It is worthwhile to mention that the optimized Li3F2 structure inside C60 is not the most stable C2v isomer found for the "free" superalkali but the D3h geometry. The binding energy at 0 K between C60 and Li3F2 (D3h) is computed to be 119 kJ mol-1. Once CO2 is introduced in the endofullerene, it is activated, and theO C O ^ angle is bent to 132°. This activation does not follow the previously studied CO2 reduction by an electron transfer process from the superalkali, but it is rather an actual reaction where a F (from Li3F2) atom is bonded to the CO2. From a thermodynamic analysis, both CO2 and the encapsulated [Li3F2⋅CO2] are destabilized in C60 with solvation energies at 0 K of 147 and < -965 kJ mol-1, respectively.

Angle-resolved photoelectron spectroscopy and scanning tunnelling spectroscopy studies of the endohedral fullerene Li@C60

Gas phase photoelectron spectroscopy (Rydberg Fingerprint Spectroscopy), TDDFT calculations and low temperature STM studies are combined to provide detailed information on the properties of the diffuse, low-lying Rydberg-like SAMO states of isolated Li@C60 endohedral fullerenes. The presence of the encapsulated Li is shown by the calculations to produce a significant distortion of the lowest-lying S- and P-SAMOs that is dependent on the position of the Li inside the fullerene cage. Under the high temperature conditions of the gas phase experiments, the Li is mobile and able to access different positions within the cage. This is accounted for in the comparison with theory that shows a very good agreement of the photoelectron angular distributions, allowing the symmetry of the observed SAMO states to be identified. When adsorbed on a metal substrate at low temperature, a strong interaction between the low-lying SAMOs and the metal substrate moves these states to energies much closer to the Fermi energy compared to the situation for empty C60 while the Li remains frozen in an off-centre position.

Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling

Highly excited electronic states are challenging to explore experimentally and theoretically-due to the large density of states and the fact that small structural changes lead to large changes in electronic character with associated strong nonadiabatic dynamics. They can play a key role in astrophysical and ionospheric chemistry, as well as the detonation chemistry of high-energy density materials. Here, we implement ultrafast vacuum-UV (VUV)-driven electron-ion coincidence imaging spectroscopy to directly probe the reaction pathways of highly excited states of energetic molecules-in this case, methyl azide. Our data, combined with advanced theoretical simulations, show that photoexcitation of methyl azide by a 10-fs UV pulse at 8 eV drives fast structural changes and strong nonadiabatic coupling that leads to relaxation to other excited states on a surprisingly fast timescale of 25 fs. This ultrafast relaxation differs from dynamics occurring on lower excited states, where the timescale required for the wavepacket to reach a region of strong nonadiabatic coupling is typically much longer. Moreover, our theoretical calculations show that ultrafast relaxation of the wavepacket to a lower excited state occurs along one of the conical intersection seams before reaching the minimum energy conical intersection. These findings are important for understanding the unique strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules. Although such observations have been predicted for many years, this study represents one of the few where such strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules have been conclusively observed directly, making it possible to identify the ultrafast reaction pathways.

The Role of Super-Atom Molecular Orbitals in Doped Fullerenes in a Femtosecond Intense Laser Field

The interaction of gas phase endohedral fullerene Ho₃N@C₈₀ with intense (0.1–5 × 10¹⁴ W/cm²), short (30 fs), 800 nm laser pulses was investigated. The power law dependence of Ho₃N@C₈₀^q+, q = 1–2, was found to be different from that of C₆₀. Time-dependent density functional theory computations revealed different light-induced ionization mechanisms. Unlike in C_60, in doped fullerenes, the breaking of the cage spherical symmetry makes super atomic molecular orbital (SAMO) states optically active. Theoretical calculations suggest that the fast ionization of the SAMO states in Ho₃N@C₈₀ is responsible for the n = 3 power law for singly charged parent molecules at intensities lower than 1.2 × 10¹⁴ W/cm².

Low-lying, Rydberg states of polycyclic aromatic hydrocarbons (PAHs) and cyclic alkanes

TD-DFT calculations of low-lying, Rydberg states of a series of polycyclic hydrocarbons and cyclic alkanes are presented.

Transition from SAMO to Rydberg State Ionization in C60 in Femtosecond Laser Fields

The transition between two distinct ionization mechanisms in femtosecond laser fields at 785 nm is observed for C₆₀ molecules. The transition occurs in the investigated intensity range from 3 to 20 TW/cm² and is visualized in electron kinetic energy spectra below the one-photon energy (1.5 eV) obtained via velocity map imaging. Assignment of several observed broad spectral peaks to ionization from superatom molecular orbitals (SAMOs) and Rydberg states is based on time-dependent density functional theory simulations. We find that ionization from SAMOs dominates the spectra for intensities below 5 TW/cm². As the intensity increases, Rydberg state ionization exceeds the prominence of SAMOs. Using short laser pulses (20 fs) allowed uncovering of distinct six-lobe photoelectron angular distributions with kinetic energies just above the threshold (below 0.2 eV), which we interpret as over-the-barrier ionization of shallow f-Rydberg states in C₆₀.

Direct observation of photocarrier electron dynamics in C60 films on graphite by time-resolved two-photon photoemission

Time-resolved two-photon photoemission (TR-2PPE) spectroscopy is employed to probe the electronic states of a C₆₀ fullerene film formed on highly oriented pyrolytic graphite (HOPG), acting as a model two-dimensional (2D) material for multi-layered graphene. Owing to the in-plane sp²-hybridized nature of the HOPG, the TR-2PPE spectra reveal the energetics and dynamics of photocarriers in the C₆₀ film: after hot excitons are nascently formed in C₆₀ via intramolecular excitation by a pump photon, they dissociate into photocarriers of free electrons and the corresponding holes, and the electrons are subsequently detected by a probe photon as photoelectrons. The decay rate of photocarriers from the C₆₀ film into the HOPG is evaluated to be 1.31 × 10¹² s⁻¹, suggesting a weak van der Waals interaction at the interface, where the photocarriers tentatively occupy the lowest unoccupied molecular orbital (LUMO) of C₆₀. The photocarrier electron dynamics following the hot exciton dissociation in the organic thin films has not been realized for any metallic substrates exhibiting strong interactions with the overlayer. Furthermore, the thickness dependence of the electron lifetime in the LUMO reveals that the electron hopping rate in C₆₀ layers is 3.3 ± 1.2 × 10¹³ s⁻¹.

Super-atom molecular orbital excited states of fullerenes

Super-atom molecular orbitals are orbitals that form diffuse hydrogenic excited electronic states of fullerenes with their electron density centred at the centre of the hollow carbon cage and a significant electron density inside the cage. This is a consequence of the high symmetry and hollow structure of the molecules and distinguishes them from typical low-lying molecular Rydberg states. This review summarizes the current experimental and theoretical studies related to these exotic excited electronic states with emphasis on femtosecond photoelectron spectroscopy experiments on gas-phase fullerenes.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Multi-reference approach to the calculation of photoelectron spectra including spin-orbit coupling

X-ray photoelectron spectra provide a wealth of information on the electronic structure. The extraction of molecular details requires adequate theoretical methods, which in case of transition metal complexes has to account for effects due to the multi-configurational and spin-mixed nature of the many-electron wave function. Here, the restricted active space self-consistent field method including spin-orbit coupling is used to cope with this challenge and to calculate valence- and core-level photoelectron spectra. The intensities are estimated within the frameworks of the Dyson orbital formalism and the sudden approximation. Thereby, we utilize an efficient computational algorithm that is based on a biorthonormal basis transformation. The approach is applied to the valence photoionization of the gas phase water molecule and to the core ionization spectrum of the [Fe(H2O)6](2+) complex. The results show good agreement with the experimental data obtained in this work, whereas the sudden approximation demonstrates distinct deviations from experiments.

Coherent electronic wave packet motion in C(60) controlled by the waveform and polarization of few-cycle laser fields

Strong laser fields can be used to trigger an ultrafast molecular response that involves electronic excitation and ionization dynamics. Here, we report on the experimental control of the spatial localization of the electronic excitation in the C_{60} fullerene exerted by an intense few-cycle (4 fs) pulse at 720 nm. The control is achieved by tailoring the carrier-envelope phase and the polarization of the laser pulse. We find that the maxima and minima of the photoemission-asymmetry parameter along the laser-polarization axis are synchronized with the localization of the coherent electronic wave packet at around the time of ionization.