Stable molecular orientations of a C60 dimer in a photoinduced dimer row

We present a theoretical study on stable orientations of photoinduced C-60 dimers in a C-60 monolayer film, on the basis of first-principles calculations within the framework of the density functional theory. Using Tersoff and Hamann's tunnel current theory, current images of scanning tunneling microscopy (STM) for a C-60 dimer in several different orientations are reproduced from the spatial distribution of the wave functions of an isolated C-60 dimer. Comparing the resultant current images with experimentally obtained STM images, in consequence, two stable orientations Of C-60 dimers are determined: one is that a C-C double bond shared by two adjacent hexagons faces to vacuum, and the other is that a C-C single bond shared by a hexagon and an adjacent pentagon faces to vacuum. These orientations are supported by empirical total-energy calculations. (C) 2007 Elsevier Ltd. All rights reserved.