Ultra-thin insulating films (e.g. bilayer of NaCl) can be used to decouple molecules from the underlying metal surface. This allows the investigation of the properties of essentially isolated molecules with STM. This strategy has been used to directly probe molecular orbitals in real space [1], visualize bond formation between metal atoms and molecules [2], and to study electrically excited luminescence from single molecules [3].
The insulator extends the life-time of the transient charged state of the molecule following electron injection from the STM tip. In contrast to molecules adsorbed directly on a metal surface, this enhances the coupling of the injected electron to molecular vibrations, and results in the formation of vibronic levels [4].
We will present experiments on a number of different planar molecules with a delocalized π-system. These measurements show that we can map out the vibronic energy levels in real space and to probe the coupling of both localized and delocalized vibrational modes to different molecular orbitals.
References
1. J. Repp, G. Meyer, S.M. Stojković, A. Gourdon, C. Joachim, Phys. Rev. Lett. 94, 026803 (2005).
2. J. Repp, G. Meyer, S. Paavilainen, F.E. Olsson, M. Persson, Science 312, 1196 (2006).
3. E. Ćavar, M.-C. Blüm, M. Pivetta, F. Patthey, M. Chergui, W.-D. Schneider, Phys. Rev. Lett. 95, 196102 (2005).
4. X.H. Qiu, G.V. Nazin, W. Ho, Phys. Rev. Lett. 92, 206102 (2004).
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