At metal-oxide/protic-solvent interfaces, partially hydrated or "wet electron" states represent the lowest energy pathway
for electron transfer. Here we study the photoinduced charge transfer at the H2O/TiO2(110) interface by means of timeresolved
two-photon photoemission spectroscopy and electronic structure theory. At ~1 monolayer coverage of H2O on
partially hydroxylated TiO2 surfaces we find an unoccupied electronic state 2.4±0.1 eV above the Fermi level. Density
functional theory shows this to be a two-dimensional "wet electron" state, which is distinct from hydrated electrons
observed on water-covered metal surfaces. The decay of electrons from the wet electron state by the resonant charge
transfer to the conduction band of TiO2 occurs in ≤15 femtoseconds. Similar unoccupied electronic structure is observed
for CH3OH covered TiO2(110) surfaces; however, the electron dynamics are considerably more complex. The wet
electron state dynamics of CH3OH/TiO2 exhibit both energy and population decay. The excited state lifetime is strongly
coverage dependent increasing to >100 fs range above 1 ML CH3OH coverage. Significantly, a pronounced deuterium
isotope effect (CH3OD) indicates a strong correlation between the interfacial electron transfer and the motion of protons
in the molecular overlayer.
Photoinduced electron-hole excitation and relaxation in bulk, at interfaces, and at surfaces of solid state materials play a key role in a variety of physical and chemical phenomena that are important for surface photochemistry, and device physics. The possibility of controlling the charge carrier dynamics by the means of the optical phase may open up new possibilities in these fields. The control of electron distribution excited in Cu(111) through optical phase of the excitation light is demonstrated. Two-photon photoemission from the Cu(111) surface is excited by a pair of approximately 15 fs laser pulses with a mutual delay fixed to an accuracy of +/- 0.025 fs. A consequence of the interference between several coherent excitation pathways in the two-photon excitation process, the photoemission spectra do not only depend on the frequency, as in conventional spectroscopy, but also on the phase of the excitation light. Though coherent control is demonstrated for electrons at a metal surface, the excitation scheme is identical to optical Ramsey fringe experiments, and therefore, is a general phenomenon in multi-photon ionization in atomic, molecular, and condense phase environments.
Reflective transient grating experiments were conducted using two different experimental configurations to study carrier dynamics. Using an 800 nm pump and 400 nm probe, a signal attributed to bleaching was observed, and the carrier energy relaxation time was measured to be approximately 600 fs. Experiments were also conducted with a 400 nm pump and 800 nm pump. For this configuration, the observed TG signal decay was attributed to carrier diffusion and recombination.
Hot-electron thermalization by electron-electron scattering is measured by two-photon photoemission spectroscopy at clean Cu(110) and Cu(100) surfaces. Two-photon photoemission spectra are measured as a function crystal face, laser polarization, and photon energy to establish the excitation mechanism. Time-resolved measurements of hot-electrons dynamics using approximately 3.2 eV femtosecond laser excitation light show a two- component decay due to dephasing and population relaxation dynamics. Hot-electron lifetimes in the 1.3 - 3.3 eV energy range decrease from approximately 80 fs to approximately 15 fs as the energy is increased. Energy dependence of the population decay rate is approximately 6.5 times slower than predicted by the Fermi liquid theory and shows a modest dependence on the Cu crystal face. Observation of hot-electron coherence on <EQ 15 fs time scale opens the way for coherent control of hot-electron induced dynamics at metal interfaces by ultrafast lasers.
The structure of W(110) surface covered by submonolayer of C or Ba atoms is studied by thermal He diffractive scattering. For C different preparation methods give (2 X 2) and R(3 X 5) tungsten carbide structures. For Ba a periodic structure with 25 angstrom lattice spacing is observed.
Fluorescence excitation and emission spectra of the S1 and S2 states of model trienes and tetraenes are measured in free jet expansions. The barriers to cis-trans isomerization in the S1 state are < 200 cm-1 for trienes and approximately 2000 cm-1 for tetraenes. < 250 fs nonradiative decay of the S2 state of tetraenes is deduced from the observed Lorentzian linewidths.