The desire to exert active optical control over matter is a unifying theme across multiple scientific disciplines, as exemplified by all-optical magnetic switching 1,2 , light-induced metastable or exotic phases of solids 3-9 and the coherent control of chemical reactions 10,11 . Typically, these approaches dynamically steer a system towards states or reaction products far from equilibrium. In solids, metal-insulator transitions are an important target for optical manipulation, offering dramatic and ultrafast changes of the electronic 4,5 and lattice [12][13][14][15][16][17][18] properties. In this context, essential questions concern the role of coherence in the efficiencies and thresholds of such transitions. Here, we demonstrate coherent vibrational control over a metal-insulator structural phase transition in a quasi-one-dimensional solid-state surface system. An optical double-pulse excitation scheme [19][20][21][22] is used to drive the system from the insulating to a metastable metallic state, and the corresponding structural changes are monitored by ultrafast low-energy electron diffraction [23][24][25] . We observe strong oscillations in the switching efficiency as a function of the double-pulse delay, revealing the importance of vibrational coherence in two key structural modes governing the transition on a femtosecond timescale. This mode-selective coherent control of solids and surfaces could open new routes to switching chemical and physical functionalities, facilitated by metastable and non-equilibrium states.Femtochemistry entails the search for understanding and control of ultrafast reaction pathways 10,22 . To this end, coherences in the electronic and vibrational states of reactants are employed to guide the system across a complex, generally multidimensional energy landscape 10,26 . Established for small molecules, a possible transfer of this concept to extended systems and solids is complicated, e.g. due to a high electronic and vibrational density of states, and couplings to an external heat bath 27 . Low-dimensional and strongly correlated systems represent a promising intermediate between molecules and solids, with phase transitions assuming the role of a "reaction". A number of these transitions can be driven optically -either by means of transient heating 24,28 , electronic excitation [15][16][17][18]29,30 or direct resonant coupling to certain vibrational degrees of freedom [4][5][6][7]31 . The prototypical case of a phase transition governed

The covalent coupling of porphine molecules on Au( 111) is studied by scanning probe microscopy experiments and density functional theory. At sufficient temperatures, dehydrogenative C−C coupling of the unfunctionalized molecules occurs directly on the surface. Characteristic dimer structures between individual porphine molecules are observed and assigned to various binding motifs that are distinguished by specific intermolecular connections. Different preparations show that the relative abundance of these motifs depends on the temperature of the gold sample during deposition and is explained by calculated free energies and kinetic aspects that are relevant during the linking process. Observations on the gold terraces are completed by studying polymerization at step edges, giving insight into their role during the covalent linking process.

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Hot electron transmission in metals using epitaxial NiSi2/n-Si(111) interfaces Parui, S.; Wit, B.; Bignardi, L.; Rudolf, P.; Kooi, B.; van Wees, B. J.; Banerjee, T.

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