One area of our research focuses on the ionisation dynamics of complex molecules, subject to ultrashort, intense laser pulses. In our studies, we are looking at fullerenes and and using them as a model system to describe complex molecules with a large, but finite, number of degrees of freedom. We are mainly using angular resolved photoelectron spectroscopy and mass spectrometry to probe gas-phase fullerenes and polycyclic aromatic hydrocarbons (PAHs).
The gas phase properties and formation mechanisms of fullerenes and PAHs are of interest due to their presence in the interstellar medium. A new research direction aims to study high resolution spectroscopy of vibrationally cold fullerenes and functionalised fullerenes to aid their identification in space.
Superatom Molecular Orbitals (SAMOs)
Fullerenes are nanomaterials that have properties intermediate between those of large molecules and those of bulk materials. They are becoming increasingly important as electron-acceptor constituents of organic solar cells and doped fullerene crystals show the highest critical temperatures of any “organic” superconductors. In spite of their considerable interest as organic electronic materials, surprisingly little is known about the fundamental properties of the excited electronic states of the molecules and how these develop into band structure as aggregates or crystals are formed. Evidence has been found, using scanning tunnel microscopy, for the presence of diffuse hydrogenic orbitals associated with fullerenes deposited on a metal substrate [Feng et al. Science 320, 359 (2008)]. These so-called “superatom” states (SAMO) are distinct from the molecular s- and p-orbitals that form through hybridization of the s and p orbitals on the carbon atoms. Instead of being bound to individual carbon atoms the SAMOs assume the radial and angular distributions of spherical harmonic functions that are defined by the central potential of the hollow C60 core and thus look like large, relatively simple atomic orbitals. When the fullerene molecules self-assemble into chains, the diffuse orbitals are seen to readily combine into delocalized bands and are predicted to play an important role in defining the electronic properties of fullerene-based materials. We have found evidence for the presence of these SAMOs in gas phase photoelectron spectroscopy of fullerenes and endohedral fullerenes using fs laser pulses as well as for polycyclic aromatic hydrocarbons. Gas phase studies have the potential to provide more detailed information about these unusual molecular states and provide a stringent test of theoretical predictions. We collaborate with the theoretical group of Prof. Francoise Remacle and Dr Benoit Mignolet (University of Liège) tand the STM group of Dr Renald Schaub (University of St Andrews) to explore the properties of these interesting electronic states.
Thermal Electron Emission
Studies have shown that the excitation and ionisation mechanisms of fullerenes are dependent on the pulse duration of the laser used to excite the molecule. For relatively long pulses (>1 ps), intramolecular energy coupling, between electronic and vibrational degrees of freedom, takes place and statistical ionisation and fragmentation of the molecule can be observed. By decreasing the pulse duration, it is possible to ‘outrun’ this thermalisation process so that we can observe statistical ionisation without vibrational degrees of freedom, within the molecule, being excited. We have demonstrated that the thermally emitted electrons can get a "kick" from the laser field [Johansson et al. J. Phys. Chem. 136, 164301 (2012)]. This opens up the possibility to gain more insight into the dynamics of thermal electron emission from complex organic molecules and nanoparticles.
Gas-phase fullerenes are generated in a resistively heated oven. The neutral molecules are ionised with fs/ps laser pulses of tuneable wavelength and the resulting ions can be detected in a time-of-flight mass spectrometer. The photoelectrons are extracted onto a position sensitive detector using a technique called velocity map imaging, which simultaneously records the kinetic and angular photoelectron distributions. A diagram of the experimental setup is shown below.
As gas phase spectroscopy studies move towards larger thermally labile molecules it becomes increasingly challenging to obtain gas phase targets of sufficient density to carry out detailed studies. We have developed a technique called BB-LIFT (blister-based laser-induced forward transfer) using fs laser that allows large molecules to be desorbed from surfaces without directly exposing them to the laser radiation or to high temperatures.
The technique has also been shown to provide a means of cleanly transferring fragile nanomaterials, such as 2D transition metal dichalcogenide (TMDC) monolayer crystals between substrates ,thus providing a means of fabricating devices without exposing the nanomaterials to chemical treatment that can leave residues and degrade the intrinsic electronic and electrical properties of the materials.