Published: October 10, 2011 r2011 American Chemical Society 22409 dx.doi.org/10.1021/jp2066748 | J. Phys. Chem. C 2011, 115, 22409–22414 ARTICLE pubs.acs.org/JPCC Directed Long-Range Migratory Reaction of Benzene on Si(100) Krishnan R. Harikumar, John C. Polanyi,* and Amir Zabet-Khosousi Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S3H6 ’ INTRODUCTION Excitation of molecules at a surface can induce molecular motion in three possible directions: (1) toward the surface, as in “localized atomic reaction” (LAR); 1,2 (2) away from the surface, as in “desorption induced by electronic transitions” (DIET); 3,4 or (3) parallel to the surface, as in surface migration. 5À8 The first two categories have, so far, dominated the study of surface- reaction dynamics. Surface migration could, however, prove to be of considerable interest as a means of inducing reaction at a distance from the point of excitation. This requires a mechanism that can couple to a lateral coordinate, thereby inducing motion across the surface. 9 An example of such mode-coupling has been recently demonstrated in the case of laser-induced surface diffusion of chemisorbed CO on Pt(533), 8 where rocking motion (hindered rotation) preceded lateral translation, and diffusion occurred via short-range molecular hopping of less than 10 Å, since the hindered rotation was never fully released. Recently, we showed that molecular cartwheeling induced by surface reaction can lead to long-range in-plane recoil of alkene molecules across the rough surface of Si(100). 10 The recoil distance averaged 29 Å and occurred preferentially across the dimer rows of Si(100). This was in clear constrast to surface diffusion, which is short-range (averaging 5 Å) and random in direction. 11 From experiment and theory, we showed that cartwheeling rotation in a mobile physisorbed state, followed by chemisorptive reattachment, constituted the most likely explanation of the observed long-range migration. Evidence for cartwheeling was provided by the observation of molecular tumbling in the case of propene molecules, and by the observa- tion of persistent and directed motion over surface obstacles, such as defects and raised steps. Here, we extend our earlier findings to a new adsorbate, benzene, and present a further striking example of the phenomenon of directed long-range migration. We show that, upon excitation by the tip of a scanning tunneling microscope (STM), chemisorbed benzene molecules migrate, on average, 66% further than did the alkene ones, and this time preferentially along the direction of Si dimer rows rather than perpendicular to the dimer rows. In addition, we show that the range of recoil strongly depends on the polarity of the excitation voltage, thereby offering, for the first time, a means to control surface migration. We discuss the origin of the torque and the consequent cart- wheeling motion in the recoiling benzene molecule in the context of a model analogous to the MenzelÀGomerÀRedhead descrip- tion of DIET. 12 Ab initio calculations of the ground and ionic states of benzene on Si(100) show that the molecule experiences a substantial tilt and large asymmetric forces that could cause rolling, following excitation to the ionic state and subsequent reversion to the ground state. The calculations, performed for both cationic and anionic states, are in qualitative accord with the experiments. ’ METHODS The experiments were carried out in ultrahigh vacuum (base pressure ∼ 3 Â 10 À11 Torr) employing an RHK-400 STM at room temperature. Images were recorded in a constant-current mode. STM tips and silicon samples were prepared as described previously. 13 Spectrograde benzene, 99.95% purity, was obtained from ACP Chemicals Inc., and treated by several freezeÀpumpÀ thaw cycles before dosing into the vacuum chamber. Benzene was background-dosed for 75 s through a leak valve at a base pressure of 1 Â 10 À9 Torr (uncorrected ion-gauge value, Received: July 13, 2011 Revised: September 28, 2011 ABSTRACT: Electron impact on chemisorbed benzene at Si(100) is shown by scanning tunneling microscopy to cause long-range molecular recoil in the plane of the surface, followed by chemisorptive reattachment at a substantial distance (on average, 48 Å) from the originating event. Several indicators—directionality of migration, persistance of motion over obstacles, and insignificant probability of desorption—all point to a rolling mechanism in which the recoiling molecule cartwheels across the surface in a physisorbed state. In distinction to our previous report of long-range migration of alkenes on the same surface [Nat. Chem. 2011, 3, 400À408], in the present case, the benzene molecules migrate substantially further and favor motion along the dimer rows rather than across them. Ab initio theory provides a qualitive model that satisfactorily explains these experimental observations.