Seeing molecular orbitals

We present results on a well known system, C-60 on Si(111)(7 X 7), which sh ow that the scanning tunneling microscope (STM) is probing a single molecul ar orbital (MO). This MO, after comparison of experimental intramolecular s tructure with ab initio calculations, arises from the highest occupied MO ( HOMO). The adsorbed molecules keep their relevant electronic identity but s uffer a decrease in their icosahedral symmetry. The nature of the chemical bonding is proposed as being responsible for selecting one single MO in the imaging mechanisms. Our results represent an example of a strongly interac ting system that can be interpreted with a great level of detail, overcomin g substrate complexities. (C) 2000 Elsevier Science B.V. All rights reserve d.