Theoretical modelling of Jahn-Teller distorted C60 anions on a surface

Abstract

Through the use of scanning tunnelling microscopy (STM), it is possible to directly observe the molecular orbitals associated with a particular molecule. For the charged ions of the C60 fullerene, the interpretation of these experimental images has an additional complication due to the inherent presence of the Jahn-Teller (JT) effect. In this work, the influence of the JT effect on STM images is examined. We also include interactions between the molecule and both the surface substrate on which it rests, and, when present within a monolayer, the nearest neighbours. Simple symmetry arguments are used to explain the effects of these external interactions on the energy of the different JT wells relating to molecular distortions of D3d, D5d, and D2h symmetry. We first investigate the C-60 monoanion, and then, through the construction of multi-electron states, move on to consider anions with higher charges. It is found that for high symmetry orientations of the molecule on the surface, the wells that are degenerate in the absence of external interactions split into equal energy subgroups, with the grouping dependent on the orientation of the molecule. Hückel molecular orbital theory is then used to investigate the effect this has on STM images. We show that when dynamic JT effects are considered, the images are always formed from some linear combination of the squares of the individual single electron molecular orbitals that make up the lowest unoccupied molecular orbital of the neutral molecule.