Search

Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion (1,2). Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site (3-11) that need to be controlled to optimize complexes for photocatalytic hydrogen production (8) and selective carbon-hydrogen bond activation (9-11). An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe[(CO).sub.5] in solution, that the photo-induced removal of CO generates the 16-electron Fe[(CO).sub.4] species, a homogeneous catalyst (12,13) with an electron deficiency at the Fe centre (14,15), in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe[(CO).sub.5] (refs 4, 16-20), was made possible by the ability of femto-second X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes., In our experimental setup (Fig. 1a), the valence electronic structure of Fe[(CO).sub.5] is probed with femtosecond-resolution resonant inelastic X-ray scattering (RIXS) at the Fe [L.sub.3]-edge (Fe [L.sub.3]-RIXS, illustrated in Fig. [...]

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 [...]

The electronic properties of the ternary potassium-ammonium graphite intercalation compound K(NH3)(sub 4)C24 are studied using generalized gradient-corrected density functional theory. This results in the K 4s electron being promoted to a very diffuse state extending in the outer region of the K(NH3)4 cluster, with the electron only loosely bound to the nearly planar K(NH3)(sub 4)(super +) system.

Time-resolved two-photon photoemission is utilized to study solvation dynamics at two-dimensional interfaces. Collective reorientation of a dipolar adsorbate in the presence of an image potential states electron results in dynamic reduction of the local work function, which is manifest in the observed dynamic changes of the photoelectron kinetic energies.

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

Time-resolved diffuse X-ray scattering with 50 fs, 9.5 keV X-ray pulses from the Linear Coherent Light Source was used to study the structural dynamics in materials undergoing rapid melting and ablation after fs laser excitation.

Hydrogen bond population dynamics are extricated with exceptional detail using ultrafast (<50 fs) infrared multidimensional stimulated vibrational echo correlation spectroscopy with full phase information and frequency resolved infrared pump–probe experiments performed on the hydroxyl stretch of methanol–OD oligomers in CCl[sub 4]. Hydrogen bond breaking makes it possible to acquire data for times much greater than the hydroxyl stretch vibrational lifetime. The correlation spectra and detailed calculations demonstrate that vibrational relaxation leads to hydrogen bond breaking for oligomers that have hydroxyl stretch frequencies on the low energy (red) side of the hydroxyl stretch spectrum, the spectral region that is associated with the strongest hydrogen bonds. Frequency resolved pump–probe data support the conclusions drawn from the correlation spectra. Using a global fit to the pump–probe spectra, in conjunction with assignments made possible through the correlation spectra, it is demonstrated that the residual ground state and photoproduct of hydrogen bond breaking are prepared near their thermal equilibrium distribution. The spectrum of the hydrogen bond breaking photoproduct and the residual ground state approach the steady-state temperature difference spectrum on the tens of picoseconds time scale, indicating the system thermalizes on this time scale. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A study shows that time and spectrally resolved ultrafast infrared vibrational pump-probe experiment on the dynamics of hydrogen bond dissociation and reformation in methanol-d clusters dissolved in carbon tetrachloride. It was concluded that hydroxyl stretch excitation lead to hydrogen bond dissociation and has clarified the variety of hydrogen bond breaking and reformation processes and their time scales.

The electronic dynamics at the naphthalene/Ag(111) and the anthracene/Ag(111) interfaces are investigated with femtosecond time and angle resolved two photon photoemission. The results show that the transition from a near resonant to a non resonant interaction between the image potential and the adsorbate electron affinity levels causes significant variations in binding energies.

A study was conducted to investigate electron relaxation processes at metal surfaces in the presence of thin layers of insulating or wide energy gap materials. The experiment demonstrated that the perpendicular spatial extent of the electronic wavefunction at the interface primarily determines the inter-band relaxation rate of electrons or relaxation from the interfacial states to the metal bulk bands. Experimental results and implications are discussed.