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

We present a theoretical study on stable orientations of photoinduced C60 dimers in a C60 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 C60 dimer in several different orientations are reproduced from the spatial distribution of the wave functions of an isolated C60 dimer. Comparing the resultant current images with experimentally obtained STM images, in consequence, two stable orientations of C60 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.