Manifestation of dynamic Jahn–Teller distortions and surface interactions in scanning tunnelling microscopy images of the fullerene anion C⁻₆₀

Abstract

Using scanning tunnelling microscopy (STM), it is possible to observe detailed structure of the molecular orbitals (MOs) of fullerene anions C − 60 . However, understanding the experimental observations is not straightforward because of the inherent presence of Jahn-Teller (JT) interactions, which (in general) split the MOs in one of a number of equivalent ways. Tunnelling between equivalent distortions means that any observed STM image will be a superposition of images arising from the individual configurations. Interactions with the surface substrate must also be taken into account. We will show how simple ideas involving a symmetry analysis and Hückel molecular orbital theory can be used to understand observed STM images without need for the more usual but more complicated density functional calculations. In particular, we will show that when the fullerene ion is adsorbed with a pentagon, hexagon or double-bond facing the surface, STM images involving the lowest unoccupied molecular orbital (LUMO) can be reproduced by adding together just two images of squares of components of the LUMO, in ratios that depend on the strength of the JT effect and the surface interaction. It should always be possible to find qualitative matches to observed images involving any of these orientations by simply looking at images of the components, without doing any detailed calculations. A comparison with published images indicates that the JT effect in the C − 60 ion favours D 3d distortions.

High-resolution photoelectron imaging of cold ${\rm C}_{60}^ -$C60− anions and accurate determination of the electron affinity of C60

“…29 Much attention has been paid to studying the JT vibronic interactions in C − 60 because of the significant role it plays in the electron-phonon coupling mechanism in the superconductivity of alkali doped fullerides. 10,[26][27][28][30][31][32][33][34] We have designed a PE imaging system, which replaced the magnetic-bottle electron analyzer on our first generation ESI-PES apparatus at room temperature, 35 and studied the angular distributions of photoelectrons from multiply charged anions. [36][37][38][39] We have recently developed a highresolution PE imaging system, which can achieve electron kinetic energy resolutions down to 1.4 cm −1 for low energy electrons.…”

Interplay between Coulomb and Jahn-Teller effects in icosahedral systems with triplet electronic states coupled toh-type vibrations

“…Also, scanning tunneling microscopy images of C 60 molecules on an alkylthiol self-assembled monolayer 34 have been interpreted in terms of JT parameters that would cause a D 3d distortion (with an additional perturbation to further lower the symmetry due to interactions with the surface substrate). 35 Early semiempirical molecular orbital calculations also predicted D 3d symmetry, 36 as did density functional theory (DFT) calculations. 37 All of these results suggest that the quadratic coupling parameters in this problem must lie within the range of the green triangle in Fig.…”

Symmetry restoration in mean-field approaches

“…Finally worth mentioning is the use of projection-operator techniques to describe the dynamic Jahn-Teller effect in natural molecules in the case of tunneling between equivalently distorted energy-minimum configurations of the adiabatic potential energy surface (Hallam et al 1992 andDunn et al 2012). Naturally, due to the very large masses of the ionic cores, the explicit symmetry-broken wave-packet state localized within a single minimum can be observed in cases when tunneling is suppressed (Bersuker 2016).…”