A novel experimental approach to the investigation of surface adsorbate reaction dynamics is presented. The direct time-resolved monitoring of the surface reaction transition state and product formation dynamics were accomplished via pump-probe mass spectrometry. As an example, methyl iodide molecules adsorbed at submonolayer coverage on an ultrathin magnesia film on Mo(100) were photoexcited to the A-band by ultrafast laser pulse irradiation. Employing time-delayed multiphoton ionization the dynamics of the dissociative methyl iodide transition state and of the emerging methyl photoproduct could be detected with femtosecond resolution. The reaction times deduced from the temporal evolution of the methyl ion mass signal indicate a strong interaction of the methyl fragment with the substrate surface prior to desorption.

Metal-oxide-semiconductor junctions are central to most electronic and optoelectronic devices. Here, the element-specificity of broadband extreme ultraviolet (XUV) ultrafast pulses is used to measure the charge transport and recombination kinetics in each layer of a Ni-TiO 2 -Si junction. After photoexcitation of silicon, holes are inferred to transport from Si to Ni ballistically in ~100 fs, resulting in spectral shifts in the Ni M 2,3 XUV edge that are characteristic of holes and the absence of holes initially in TiO 2 . Meanwhile, the electrons are observed to remain on Si. After picoseconds, the transient hole population on Ni is observed to back-diffuse through the TiO 2 , shifting the Ti spectrum to higher oxidation state, followed by electron-hole recombination at the Si-TiO 2 interface and in the Si bulk. Electrical properties, such as the hole diffusion constant in TiO 2 and the initial hole mobility in Si, are fit from these transient spectra and match well with values reported previously.

The ultrafast dynamics of the bimetallic cluster Ag2Au is investigated by pump-probe negative ion-to-neutral-to-positive ion (NeNePo) spectroscopy. Preparation of the neutral cluster in a highly nonequilibrium state by electron detachment from the mass-selected anion, and subsequent probing of the neutral nuclear dynamics through two-photon ionization to the cationic state, leads to strongly probe-energy-dependent transient cation-abundance signals. The origin of this pronounced time and wavelength dependence of the ionization probability on the femtosecond scale is revealed by ab initio theoretical simulations of the transient spectra. Based on the analysis of underlying dynamics, two fundamental processes involving geometry relaxation from linear to triangular structure followed by ultrafast intramolecular vibrational energy redistribution (IVR) have been identified and for the first time experimentally observed in the frame of NeNePo spectroscopy under conditions close to zero electron kinetic energy.

Femtosecond-laser photoemission spectra were obtained from a Mo(100) single crystal surface covered with stoichiometric magnesia thin films of well-defined thickness. Depending on the excitation wavelength and the MgO layer thickness, either one or two photon photoemission was detected at 333 and at 402 nm. This photoemission was assigned to originate from the molybdenum d band states even for 10 monolayers MgO coverage. At 263 nm excitation, however, the photoemission power dependence indicated that the signal contained both contributions from the Mo metal d electrons and from the MgO O2p valence band. While one nominal monolayer of MgO only slightly influenced the photoemission from the molybdenum surface, a considerable reduction of the surface electronic work function was observed for ultrathin magnesia overlayers of 2 to 3 monolayers in thickness. No significant change in the work function was measured for more than 3 up to 10 monolayers.