Though largely influencing the efficiency of a reaction, the molecular‐scale details of the local environment of the reactants are experimentally inaccessible hindering an in‐depth understanding of a catalyst's reactivity, a prerequisite to maximizing its efficiency. We introduce a method to follow individual molecules and their largely changing environment during a photochemical reaction. The method is illustrated for a rate‐limiting step in a photolytic reaction, the dissociation of CO2 on two catalytically relevant surfaces, Ag(100) and Cu(111). We reveal with a single‐molecule resolution how the reactant's surroundings evolve with progressing laser illumination and with it their propensity for dissociation. Counteracting processes lead to a volcano‐like reactivity. Our unprecedented local view during a photoinduced reaction opens the avenue for understanding the influence of the products on reaction yields on the nanoscale.

The restriction imposed by the lattice structure of different surfaces is used to investigate the influence of the distance between two monomers on their ability to bind to each other. We compare the interaction of ammonia monomers at two distinct distances imposed by the surface structure of a Cu(511) highindex surface to that of a Cu(110) low-index surface using lowtemperature scanning tunneling microscopy, inelastic tunneling spectroscopy, and density functional theory. Frustrated translational and rotational modes, the Mulliken and Bader charge analyses, and electrostatic potential mapping indicate chemisorption of ammonia monomers on both surfaces, with their dipoles oriented perpendicular to the surface plane. At a larger intermolecular distance of around 0.51 nm on step edges of Cu( 511), the monomers slightly repel each other due to electrostatic repulsion. At a shorter distance of around 0.36 nm perpendicular to the close-packed rows on Cu(110), a noticeable charge transfer between adjacent monomers indicates binding, that is, dimer formation in parallel orientation. This binding energy of the molecules compensates for the electrostatic repulsion. Our results outline how the choice of the surface structure may be utilized to alter the intermolecular interaction of solvent molecules and to enforce or suppress dimer formation.

Though largely influencing the efficiency of ar eaction, the molecular-scale details of the local environment of the reactants are experimentally inaccessible hindering an in-depth understanding of ac atalystsr eactivity,aprerequisite to maximizing its efficiency.W ei ntroduce am ethod to follow individual molecules and their largely changing environment during aphotochemical reaction. The method is illustrated for arate-limiting step in aphotolytic reaction, the dissociation of CO 2 on two catalytically relevant surfaces,A g(100) and Cu(111). We reveal with as ingle-molecule resolution howt he reactantss urroundings evolve with progressing laser illumination and with it their propensity for dissociation. Counteracting processes lead to av olcano-like reactivity.O ur unprecedented local view during aphotoinduced reaction opens the avenue for understanding the influence of the products on reaction yields on the nanoscale.