Probing Reaction via Controlled Single-Molecule Collisions

Collisions of atoms and molecules are fundamental to any chemical reaction as they are required for the formation of bonds. Understanding what influences the dynamics of these collisions, and what consequently determines the outcome of a reaction, lies therefore at the heart of chemistry. The molecular motions during these bimolecular encounters play a major role in determining the outcome of the collision. They depend on the collision energy, the collision geometry, and the miss-distance between the centers of mass of the colliding reagents (called the impact parameter). While the influence of the collision energy on the collision dynamics has been illuminated in gas-phase studies of single-molecule collisions, the role played by the impact parameter is not as well understood. This is because the incoming species in general will randomly miss the targets center of mass. As such, the ability to control the impact parameter is generally considered as the forbidden fruit of reaction dynamics and to pluck this forbidden fruit is a challenging task since it requires to spatially restrict the paths along which the colliding reagents could travel to the sub-nanometer scale. In carrying out these reactions at a solid surface, rather than in the gas-phase, the collisions between reagents can be confined to the surface plane and thus to two-dimensions. Furthermore, the surface itself can restrict the possible impact parameters, as was recently demonstrated in studies involving difluorocarbene (CF2) molecules on the Cu(110) surface. Importantly, the close-packed Cu-rows of this surface served to collimate the motion of the CF2 molecules, restricting them to travel along a single dimension. Unfortunately, the possible adsorption configurations of the chemisorbed targets in these studies allowed for only a single collision geometry for each impact parameter, thus precluding the ability to study how varying collision geometry, at a fixed impact parameter, changes reactivity. This project seeks to overcome this limitation, and thus furnish the missing element required for a more general selection of collision geometry, by employing methods developed to precisely control the position and alignment of the molecular targets on metal surfaces under ultrahigh vacuum conditions and at cryogenic temperatures. Using a scanning-tunneling microscope (STM), which can visualize processes occurring at the surface with atomic precision as well as induce the molecular translation, rotation, desorption, or dissociation one molecule at a time, these reactions will be set up and monitored. This approach will allow the ability to induce collisions at a selected distance from, and alignment with, the center of mass of a molecular target, and is thus of great interest in the field of molecular reaction dynamics. As such, this project will provide an until-now unprecedented understanding for how the collision geometry contributes to the outcome of reactions.