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The motion of atoms on interatornic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids., In a crystal at room temperature, vibrational excitations, or phonons, only slightly perturb the crystalline order. In contrast, liquids explore a wide range of configurations set by the topology of [...]

Ultrafast two-photon photoemission has been used to study electron solvation at two-dimensional metal/polar-adsorbate interfaces. The molecular motion that causes the excess electron solvation is manifested as a dynamic shift in the electronic energy. Although the initially excited electron is delocalized in the plane of the interface, interactions with the adsorbate can lead to its localization. A method for determining the spatial extent of the localized electron in the plane of the interface has been developed. This spatial extent was measured to be on the order of a single adsorbate molecule., The interaction of excess electrons at interfaces is not only of fundamental interest, but affects processes such as charge injection and transport in molecular electronics and electrochemistry (1, 2). Electron [...]

The mechanism for hydrogen bond (H-bond) switching in solution has remained subject to debate despite extensive experimental and theoretical studies. We have applied polarization-selective multidimensional vibrational spectroscopy to investigate the H-bond exchange mechanism in aqueous NaCl[O.sub.4] solution. The results show that a water molecule shifts its donated H-bonds between water and perch[orate acceptors by means of large, prompt angular rotation. Using a jump-exchange kinetic mode[, we extracted an average jump angle of 49 [+ or -] 4, in qualitative agreement with the jump angle observed in molecular dynamics simulations of the same aqueous NaCI[O.sub.4] solution. doi: 10.1126/science.1187707