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Using spot profile analysis low energy electron diffraction, we studied the growth mode and strain state of ultra-thin epitaxial Bi2Se3(111) films grown by molecular beam epitaxy on Si(111). The first layer grows as complete quintuple layer and covers the Si substrate before the next layer nucleates. Its lateral lattice parameter is increased by 1% compared with the value of a‖ = 4.136 Å for a 6-nm-thick film. With increasing film thickness, a continuous change of the lattice parameter is observed to an asymptotic value, which is explained by a van der Waals-like bonding between the Bi2Se3 film and the Si substrate.

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A disordered spacer layer (red) allows to form an ordered CO2 layer (green), which is not influenced by the substrate.

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.