The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

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Here, we demonstrate that ultrafast two dimensional infrared (2D-IR) spectroscopy provides evidence for the transition state involved in a simple thermal chemical reaction, the fluxionality of Fe(CO) 5 .Fluxionality refers to the rearrangement of a molecule between chemically indistinguishable structures.These reactions produce no net change in molecular structure, yet they are important for understanding the basic chemical behavior and reactivity of molecules in solution.Department of Chemistry, University of California, Berkeley, California 94720, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 * These authors contributed equally and are listed alphabetically † corresponding author email address: cbharris@berkeley.edu 2 Fe(CO) 5 , an organometallic complex with five CO ligands arranged in a trigonal bipyramidal geometry, is a textbook example for fluxionality. In the late 1950s, nuclear magnetic resonance (NMR) spectroscopy revealed that this molecule rapidly exchanges its CO ligands between axial and equatorial sites. The 13 C NMR spectrum of 13 C labeled Fe(CO) 5 exhibits only a single peak at all accessibly measured solution temperatures, indicating that 13 C nuclei shift between axial and equatorial positions faster than NMR spectroscopy is able to distinguish these two chemical environments (2)(3)(4). Careful analysis of IR, Raman, and NMR spectra of Fe(CO) 5 and various derivatives suggests that the exchange process possesses a low barrier and occurs on a time-scale of picoseconds (4)(5)(6). Nevertheless, the dynamics in solution have not been quantified.From a general perspective, fluxional processes are simple chemical reactions in which a molecule briefly rearranges to a new symmetry and geometry as it crosses a transition state and then returns to its original geometry as it completes the reaction. We show for Fe(CO) 5 that during this process energy is exchanged between the vibrational modes of the reactive ligands. Quantification of this energy exchange provides direct information on the time-scale, transition state, and consequently, mechanism of the reaction.2D-IR spectroscopy has recently received much attention for its ability to monitor thermal reactions and chemical exchange on the femtosecond and picosecond time-scales. Conventionally, ultrafast timing of chemical reactions is achieved by photoinitiating the reactions with a short, intense laser pulse which electronically excites the molecules and typically leaves them with significant excess energy (1). In comparison, 2D-IR spectroscopy only requires vibrational excitation with an ultrafast IR laser pulse and allows the investigation of an entirely different class of thermally activated reactions in liquids at or close to equilibrium. Although similar to one-dimensional IR-pump, IR-probe experiments (1D-IR), 2D-IR spectroscopy separates the contributions to the 1D-IR spectrum into two frequency dimensions, which provides information on the correlations, anharmonicities, and exchange dynamics...