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A prerequisite for advancing hybrid solar light harvesting systems is a comprehensive understanding of the spatiotemporal dynamics of photoinduced interfacial charge separation. Here, we demonstrate access to this transient charge redistribution for a model hybrid system of nanoporous zinc oxide (ZnO) and ruthenium bipyridyl chromophores. The site-selective probing of the molecular electron donor and semiconductor acceptor by time-resolved X-ray photoemission provides direct insight into the depth distribution of the photoinjected electrons and their interaction with the local band structure on a nanometer length scale. Our results show that these electrons remain localized within less than 6 nm from the interface, due to enhanced downward band bending by the photoinjected charge carriers. This spatial confinement suggests that light-induced charge generation and transport in nanoscale ZnO photocatalytic devices proceeds predominantly within the defect-rich surface region, which may lead to enhanced surface recombination and explain their lower performance compared to titanium dioxide (TiO2)-based systems.

Zinc oxide (ZnO) nanomaterials are promising components for chemical and biological sensors and photocatalytic conversion and operate as electron collectors in photovoltaic technologies. Many of th...

While extensive work has been dedicated to the measurement of the demagnetization time following an ultra-short laser pulse, experimental studies of its underlying microscopic mechanisms are still scarce. In transition metal ferromagnets, one of the main mechanism is the spin-flip of conduction electrons driven by electron-phonon scattering. Here, we present an original experimental method to monitor the electron-phonon mediated spin-flip scattering rate in nickel through the stringent atomic symmetry selection rules of x-ray emission spectroscopy. Increasing the phonon population leads to a waning of the 3d → 2p3/2 decay peak intensity, which reflects an increase of the angular momentum transfer scattering rate attributed to spin-flip. We find a spin relaxation time scale in the order of 50 fs in the 3d-band of nickel at room temperature, while consistantly, no such peak evolution is observed for the diamagnetic counterexample copper, using the same method.

The layered dichalcogenide MoS$$_{2}$$ 2 is relevant for electrochemical Li adsorption/intercalation, in the course of which the material undergoes a concomitant structural phase transition from semiconducting 2H-MoS$$_{2}$$ 2 to metallic 1T-Li$$_{\mathrm{x}}$$ x MoS$$_{2}$$ 2 . With the core hole clock approach at the S L$$_{1}$$ 1 X-ray absorption edge we quantify the ultrafast directional charge transfer of excited S3p electrons in-plane ($$\parallel$$ ‖ ) and out-of-plane ($$\perp$$ ⊥ ) for 2H-MoS$$_{2}$$ 2 as $$\tau _{2H,\parallel } = 0.38 \pm 0.08$$ τ 2 H , ‖ = 0.38 ± 0.08 fs and $$\tau _{2H,\perp } = 0.33 \pm 0.06$$ τ 2 H , ⊥ = 0.33 ± 0.06 fs and for 1T-Li$$_{\mathrm{x}}$$ x MoS$$_{2}$$ 2 as $$\tau _{1T,\parallel } = 0.32 \pm 0.12$$ τ 1 T , ‖ = 0.32 ± 0.12 fs and $$\tau _{1T,\perp } = 0.09 \pm 0.07$$ τ 1 T , ⊥ = 0.09 ± 0.07 fs. The isotropic charge delocalization of S3p electrons in the semiconducting 2H phase within the S-Mo-S sheets is assigned to the specific symmetry of the Mo-S bonding arrangement. Formation of 1T-Li$$_{\mathrm{x}}$$ x MoS$$_{2}$$ 2 by lithiation accelerates the in-plane charge transfer by a factor of $$\sim 1.2$$ ∼ 1.2 due to electron injection to the Mo-S covalent bonds and concomitant structural repositioning of S atoms within the S-Mo-S sheets. For excitation into out-of-plane orbitals, an accelerated charge transfer by a factor of $$\sim 3.7$$ ∼ 3.7 upon lithiation occurs due to S-Li coupling.

Visible light is shown to create a transient metallic S Mo S surface layer on bulk semiconducting p doped indirect bandgap 2H MoS2. Optically created electron hole pairs separate in the surface band bending region of the p doped semiconducting crystal causing a transient accumulation of electrons in the surface region. This triggers a reversible 2H semiconductor to 1T metal phasetransition of the surface layer. Electron phonon coupling of the indirect bandgap p doped 2H MoS2 enables this efficient pathway even at a low density of excited electrons with a distinct optical excitation threshold and saturation behavior. This mechanism needs to be taken into consideration when describing the surface properties of illuminated p doped 2H MoS2. In particular, light induced increased charge mobility and surface activation can cause and enhance the photocatalytic and photoassisted electrochemical hydrogen evolution reaction of water on 2H MoS2. Generally, it opens up for a way to control not only the surface of p doped 2H MoS2 but also related dichalcogenides and layered systems. The findings are based on the sensitivity of time resolved electron spectroscopy for chemical analysis with photon energy tuneable synchrotron radiation

We report measurements of the temporal dynamics of the valence band photoemission from the magnesium (0001) surface across the resonance of the Γ surface state at 134 eV and link them to observations of high-resolution synchrotron photoemission and numerical calculations of the time-dependent Schrödinger equation using an effective single-electron model potential. We observe a decrease in the time delay between photoemission from delocalized valence states and the localized core orbitals on resonance. Our approach to rigorously link excitation energy-resolved conventional steady-state photoemission with attosecond streaking spectroscopy reveals the connection between energy-space properties of bound electronic states and the temporal dynamics of the fundamental electronic excitations underlying the photoelectric effect., We thank F. Siegrist for experimental assistance, and we acknowledge financial support by the Munich Centre for Advanced Photonics (MAP). R. K. acknowledges an ERC Consolidator Grant “AEDMOS” (ERC-2014-CoG AEDMOS). D. P. acknowledges support from the “MEDEA” (H2020- MSCA-ITN-2014-641789-MEDEA).

Faraday discussions 216, 414 - 433 (2019). doi:10.1039/C8FD00236C, Electronic and lattice contributions to picosecond time-resolved X-ray absorption spectra (trXAS) of CuO at the oxygen K-edge are analyzed by comparing trXAS spectra, recorded using excitation wavelengths of 355 nm and 532 nm, to steady-state, temperature-dependent XAS measurements. The trXAS spectra at pump-probe time-delays ≥150 ps are dominated by lattice heating effects. Insight into the temporal evolution of lattice temperature profiles on timescales up to 100s of nanoseconds after laser excitation are reported, on an absolute temperature scale, with a temporal sensitivity and a spatial selectivity on the order of 10s of picoseconds and 10s of nanometers, respectively, effectively establishing an “ultrafast thermometer”. In particular, for the 532 nm experiment at ∼5 mJ cm−2 fluence, both the initial sample temperature and its dynamic evolution are well captured by a one-dimensional thermal energy deposition and diffusion model. The thermal conductivity k = (1.3 ± 0.4) W m−1 K−1 derived from this model is in good agreement with the literature value for CuO powder, kpowder = 1.013 W m−1 K−1. For 355 nm excitation, a quantitative analysis of the experiments is hampered by the large temperature gradients within the probed sample volume owing to the small UV penetration depth. The impact of the findings on mitigating or utilizing photoinduced lattice temperature changes in future X-ray free electron laser (XFEL) experiments is discussed., Published by Soc., Cambridge [u.a.]

Photo-induced charge carrier dynamics and transient interfacial fields at the interface between N3 polypyridine complexes and films of nanocrystalline ZnO are probed by picosecond time-resolved X-ray photoelectron spectroscopy.

Physical review / B 99(2), 020303 (2019). doi:10.1103/PhysRevB.99.020303, The success of many emerging molecular electronics concepts hinges on an atomistic understanding of the underlying electronic dynamics. We employ picosecond time-resolved x-ray photoemission spectroscopy (tr-XPS) to elucidate the roles of singlet and triplet excitons for photoinduced charge generation at a copper-phthalocyanine–C$_{60}$ heterojunction. Contrary to common belief, fast intersystem crossing to triplet excitons after photoexcitation is not a loss channel but contributes to a significantly larger extent to the time-integrated interfacial charge generation than the initially excited singlet excitons. The tr-XPS data provide direct access to the diffusivity of the triplet excitons D$_{CuPc}$=(1.8±1.2) × 10$^{−5}$cm$^2$/s (where CuPc is copper-phthalocyanine) and their diffusion length L$_{diff}$=(8±3)nm., Published by Inst., Woodbury, NY

Electronic and lattice contributions to transient X-ray absorption spectra of CuO are analyzed using picosecond time-resolved and temperature-dependent measurements. Super-bandgap excitation with 355 nm and 532 nm laser pulses leads to significantly different trends.

X-ray photoelectron spectroscopy (XPS) is one of the most powerful techniques to quantitatively analyze the chemical composition and electronic structure of surfaces and interfaces in a non-destructive fashion. Extending this technique into the time domain has the exciting potential to shed new light on electronic and chemical dynamics at surfaces by revealing transient charge configurations with element- and site-specificity. Here, we describe prospects and challenges that are associated with the implementation of picosecond and femtosecond time-resolved X-ray photoelectron spectroscopy at third-generation synchrotrons and X-ray free-electron lasers, respectively. In particular, we discuss a series of laser-pump/X-ray-probe photoemission experiments performed on semiconductor surfaces, molecule-semiconductor interfaces, and films of semiconductor nanoparticles that demonstrate the high sensitivity of time-resolved XPS to light-induced charge carrier generation, diffusion and recombination within the space charge layers of these materials. Employing the showcase example of photo-induced electronic dynamics in a dye-sensitized semiconductor system, we highlight the unique possibility to probe heterogeneous charge transfer dynamics from both sides of an interface, i.e., from the perspective of the molecular electron donor and the semiconductor acceptor, simultaneously. Such capabilities will be crucial to improve our microscopic understanding of interfacial charge redistribution and associated chemical dynamics, which are at the heart of emerging energy conversion, solar fuel generation, and energy storage technologies.

Die Einsatze der Luftrettung reichen in Deutschland von der Bergrettung uber die Primarrettung bis hin zum komplexen Intensivtransport. Die differenzierten Anforderungen der Einsatzarten erfordern somit spezielle Technik und qualifiziertes Personal.

Time-resolved core-level spectroscopy using laser pulses to initiate and short X-ray pulses to trace photoinduced processes has the unique potential to provide electronic state- and atomic site-specific insight into fundamental electron dynamics in complex systems. Time-domain studies using transient X-ray absorption and emission techniques have proven extremely valuable to investigate electronic and structural dynamics in isolated and solvated molecules. Here, we describe the implementation of a picosecond time-resolved X-ray photoelectron spectroscopy (TRXPS) technique at the Advanced Light Source (ALS) and its application to monitor photoinduced electron dynamics at the technologically pertinent interface formed by N3 dye molecules anchored to nanoporous ZnO. Indications for a dynamical chemical shift of the Ru3d photoemission line originating from the N3 metal centre are observed ∼30 ps after resonant HOMO–LUMO excitation with a visible laser pump pulse. The transient changes in the TRXPS spectra are accompanied by a characteristic surface photovoltage (SPV) response of the ZnO substrate on a pico- to nanosecond time scale. The interplay between the two phenomena is discussed in the context of possible electronic relaxation and recombination pathways that lead to the neutralisation of the transiently oxidised dye after ultrafast electron injection. A detailed account of the experimental technique is given including an analysis of the chemical modification of the nano-structured ZnO substrate during extended periods of solution-based dye sensitisation and its relevance for studies using surface-sensitive spectroscopy techniques.

We present a picosecond time-resolved X-ray absorption spectroscopy (tr-XAS) setup designed for synchrotron-based studies of interfacial photochemical dynamics. The apparatus combines a high power, variable repetition rate picosecond laser system with a time-resolved X-ray fluorescence yield detection technique. Time-tagging of the detected fluorescence signals enables the parallel acquisition of X-ray absorption spectra at a variety of pump–probe delays employing the well-defined time structure of the X-ray pulse trains. The viability of the setup is demonstrated by resolving dynamic changes in the fine structure near the O1s X-ray absorption edge of cuprous oxide (Cu2O) after photo-excitation with a 355 nm laser pulse. Two distinct responses are detected. A pronounced, quasi-static, reversible change of the Cu2O O1s X-ray absorption spectrum by up to ∼30% compared to its static line shape corresponds to a redshift of the absorption edge by ∼1 eV. This value is small compared to the 2.2 eV band gap of Cu2O but in agreement with previously published results. The lifetime of this effect exceeds the laser pulse-to-pulse period of 8 μs, resulting in a quasi-static spectral change that persists as long as the sample is exposed to the laser light, and completely vanishes once the laser is blocked. Additionally, a short-lived response corresponding to a laser-induced shift of the main absorption line by ∼2 eV to lower energies appears within

Time-resolved XPS is used to monitor charge carrier dynamics in bare and dye-sensitized ZnO nanoparticle films upon sub-bandgap excitation. Results are discussed in the context of impurity-related bandgap states and electron transfer from the chromophore.

We describe a partial electron yield detector for X-ray absorption studies with a large angle of acceptance of π sr, a well-defined electron cut-off energy due to a spherical retardation optics, and a linear, well reproducible gain. The detector operates without electron multipliers and is assembled from standard replacement parts of conventional LEED optics.

The ordering and conformational properties of dicarbonitrile-para-oligophenyls are studied with complementary methods, namely X-ray structure analysis, low-temperature scanning tunneling microscopy, and near-edge X-ray absorption fi ne-structure spectroscopy. The packing of the functionalized variants differs from their technologically interesting para-oligophenyl counterparts, both in the bulk crystal phase and in thin fi lms grown by organic molecular beam epitaxy (OMBE) under ultra-high vacuum conditions on the Ag(111) surface. In the crystal phase, the conformation depends on the number n of phenyl rings, exhibiting an intriguing screw-like structure in the case of n = 4 at room temperature as well as at 180 K. For OMBE-grown thin fi lms, the whole series acquires the same type of conformation, characterized by alternately twisted phenyl rings, similar to the pure oligophenyl species. However, for all tested molecules, the orientation of the molecular reference plane is uniform within the entire fi lm and coincides with the surface plane. This contrasts with the herringbone ordering adopted by the phenyl backbones without the carbonitrile groups. Our results demonstrate how the functionalization of moieties with extended conjugated electron systems can help to improve the structural homogeneity in technologically relevant organic thin fi lms.

The dynamics of the charge transfer (CT) in alkanethiolate self-assembled monolayers is addressed by resonant Auger spectroscopy using the core hole clock method. The CT pathway was unambiguously defined by resonant excitation of the nitrile tailgroup attached to the alkyl backbone. The length of this backbone was varied to monitor the respective dependence of the CT time. It was found that, similar to the static conductance, this dependence can be coarsely described by an exponential function with an attenuation factor of 0.93 per methylene unit (0.72 A−1; a tunneling along the chain was assumed). As a result, the CT time is quite long even for a relatively short alkyl chain; in particular, it is ca. 100 fs for the chain consisting of only four CH2 units. In contrast, the CT time associated with the thiolate headgroup anchor was found to be quite short, viz. 2.3 fs, which suggests an efficient interfacial electronic coupling between the aliphatic backbone of the CnCN molecules and the substrate over the ...

A series of biphenylnitrile-based self-assembled monolayers (SAMs) on Au(111) was characterized by several complementary spectroscopic techniques, including resonant Auger electron spectroscopy (RAES). In spite of the disturbing role of nitrile, the SAMs exhibited high-quality and expected structural behavior, including the odd−even effects, even though with a lower extent than in the analogous nonsubstituted or methyl-substituted systems. A characteristic feature of the target films, and presumably also a general feature of oligophenyl-based SAMs, is a significant twist of the molecular backbone, similar to the situation in the analogous molecular crystals. No perceptible signal related to tail group-to-substrate transfer of the resonantly excited electron was recorded in the RAES spectra of the target films, even though such a transfer is energetically allowed in some of the cases. This was tentatively explained by the hypothesis that the charge transfer time is too long and lies beyond the range access...

Whereas static conductance of molecules arranged in self-assembled monolayers has been extensively studied, little is known about the dynamics of the charge transfer in these systems. By the example of alkanethiolate films, we show that the latter phenomena can be successfully addressed by resonant Auger spectroscopy, using the core hole clock method. The charge transfer pathway was unambiguously defined by resonant excitation of the nitrile tailgroup attached to the alkyl backbone. Extension of this approach to other functional groups is possible, leading to new information valuable for molecular electronics and other applications.

Interactions of thermally and electronically stimulated reactions in thin layers on surfaces are investigated. For self-assembled monolayers, thermal activation promotes many processes primarily induced by electronic excitations. We demonstrate that the film temperature is an important parameter for steering these reactions towards different final products. Using chemisorbed water on Ru(001) as an example, we investigate how the products of an irradiation induced reaction catalyze thermally stimulated dissociation of water molecules.

Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation. © 2014 American Chemical Society.

Attosecond light pulses are now available experimentally, enabling ultrafast processes on the atomic scale to be probed; here the free-electron-like propagation of electrons through ultrathin layers of magnesium is observed in real time. The recent availability of attosecond light pulses means that it is now possible to observe ultrafast processes at an atomic scale. So far, such measurements have been carried out in gases, but now Reinhard Kienberger and colleagues use attosecond pulses to probe a fundamental process in the solid state, namely the transport of electrons through a crystal. They use attosecond pulses to launch photoelectron wavepackets inside a tungsten crystal that is covered by a controllable number of magnesium layers. Measuring the time of arrival of the wavepackets at the surface as a function of the number of layers reveals free-electron-like propagation of electrons inside the magnesium layers. The study demonstrates that real-time access to atomic-scale electron transport on the surface is possible. The propagation and transport of electrons in crystals is a fundamental process pertaining to the functioning of most electronic devices. Microscopic theories describe this phenomenon as being based on the motion of Bloch wave packets1. These wave packets are superpositions of individual Bloch states with the group velocity determined by the dispersion of the electronic band structure near the central wavevector in momentum space1. This concept has been verified experimentally in artificial superlattices by the observation of Bloch oscillations2—periodic oscillations of electrons in real and momentum space. Here we present a direct observation of electron wave packet motion in a real-space and real-time experiment, on length and time scales shorter than the Bloch oscillation amplitude and period. We show that attosecond metrology3 (1 as = 10−18 seconds) now enables quantitative insight into weakly disturbed electron wave packet propagation on the atomic length scale without being hampered by scattering effects, which inevitably occur over macroscopic propagation length scales. We use sub-femtosecond (less than 10−15 seconds) extreme-ultraviolet light pulses3 to launch photoelectron wave packets inside a tungsten crystal that is covered by magnesium films of varied, well-defined thicknesses of a few angstroms4. Probing the moment of arrival of the wave packets at the surface with attosecond precision reveals free-electron-like, ballistic propagation behaviour inside the magnesium adlayer—constituting the semi-classical limit of Bloch wave packet motion. Real-time access to electron transport through atomic layers and interfaces promises unprecedented insight into phenomena that may enable the scaling of electronic and photonic circuits to atomic dimensions. In addition, this experiment allows us to determine the penetration depth of electrical fields at optical frequencies at solid interfaces on the atomic scale.

Picosecond time-resolved in situ X-ray absorption and X-ray photoelectron spectroscopy techniques for atomic site-specific real-time studies of interfacial photoelectrochemistry are developed at the Advanced Light Source (ALS). First experiments monitor electronic dynamics in films of dye-sensitized nanocrystals and at hematite-electrolyte interfaces.

A multitude of physical effects have been proposed to determine the photoemission delay between core and valence electrons in tungsten. We present selected streaking experiments that clarify its origins.

Christian Bressler replied: The short answer is: horribly. SASE XFEL sources (without self-seeding), especially if monochromatized, deliver uctuations up to 100%, while synchrotrons deliver a photon number entirely governed by photon statistics (shot noise). For a physicist SR is the ideal probe with its extreme stability. However, XFEL sources can deliver superior (scientic) information (due to their incredible single-pulse intensities), if you can reliably measure the incident intensity on a shot-by-shot basis. I am not sure about the level of I0 accuracy these days (probably on the percent level, thus improvements can be expected), but overall the data quality resembles what has been achieved earlier in TR X-ray experiments at SR sources with kHz laser systems, but here with femtosecond time resolution. This is a developing eld, and improvements should be expected over time, similar as in the eld of MHz-SR X-ray experiments.

An apparatus for sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy studies with pulsed and constant wave X-ray light sources is presented. A differentially pumped hemispherical electron analyzer is equipped with a delay-line detector that simultaneously records the position and arrival time of every single electron at the exit aperture of the hemisphere with ∼0.1 mm spatial resolution and ∼150 ps temporal accuracy. The kinetic energies of the photoelectrons are encoded in the hit positions along the dispersive axis of the two-dimensional detector. Pump-probe time-delays are provided by the electron arrival times relative to the pump pulse timing. An average time-resolution of (780 ± 20) ps (FWHM) is demonstrated for a hemisphere pass energy Ep = 150 eV and an electron kinetic energy range KE = 503–508 eV. The time-resolution of the setup is limited by the electron time-of-flight (TOF) spread related to the electron trajectory distribution within the analyzer hemisphere and within the electrostatic lens system that images the interaction volume onto the hemisphere entrance slit. The TOF spread for electrons with KE = 430 eV varies between ∼9 ns at a pass energy of 50 eV and ∼1 ns at pass energies between 200 eV and 400 eV. The correlation between the retarding ratio and the TOF spread is evaluated by means of both analytical descriptions of the electron trajectories within the analyzer hemisphere and computer simulations of the entire trajectories including the electrostatic lens system. In agreement with previous studies, we find that the by far dominant contribution to the TOF spread is acquired within the hemisphere. However, both experiment and computer simulations show that the lens system indirectly affects the time resolution of the setup to a significant extent by inducing a strong dependence of the angular spread of electron trajectories entering the hemisphere on the retarding ratio. The scaling of the angular spread with the retarding ratio can be well approximated by applying Liouville's theorem of constant emittance to the electron trajectories inside the lens system. The performance of the setup is demonstrated by characterizing the laser fluence-dependent transient surface photovoltage response of a laser-excited Si(100) sample.

We introduce a viable scheme for measuring the absolute duration of photoemission from solids. It employs an atomic chronograph on the surface during attosecond streaking spectroscopy. First experimental results on a tungsten(110) surface are presented.

We discuss experiments that address the ultrafast dynamics inherent to the photoemission process in condensed matter. In our experimental approach, an extreme ultraviolet attosecond light pulse launches photoelectron wave packets inside a solid. The subsequent emission dynamics of these photoelectrons is probed with the light field of a phase-stabilized near-infrared laser pulse. This technique is capable of resolving subtle emission delays of only a few attoseconds between electron wave packets that are released from different energy levels of the crystal. For simple metals, we show that these time shifts may be interpreted as the real-time observation of photoelectrons propagating through the crystal lattice prior to their escape into vacuum. The impact of adsorbates on the observed emission dynamics is also investigated.

Attosecond science recently celebrated its first decade which brought fascinating new insights into the dynamics of electrons in atoms and simple molecules. Most of these achievements are enabled by implementing attosecond extreme-ultraviolet (XUV) light bursts together with few-cycle near-infrared (NIR) and ultraviolet (UV) laser pulses in different pump-probe configurations. However, the application of these novel experimental tools for time-resolving attosecond and few-femtosecond electron dynamics in the solid state is still very limited. The study of electron dynamics in bulk materials, their surfaces and interfaces is crucial for both advancing our fundamental understanding of these processes and their application in future technological devices. In this chapter, electron dynamics in condensed matter will be reviewed in the light of open questions that can be addressed with state-of-the-art time-resolved spectroscopy. Experimental prerequisites particular to the study of condensed matter systems are discussed. The potential of using ultrashort light pulses to investigate electron dynamics at surfaces on ultrashort timescales is illustrated by few-femtosecond transient UV-reflectivity and attosecond time-resolved XUV-photoemission experiments on metal surfaces, semiconductors, and more complex adsorbate-substrate systems.

We report on laser-assisted attosecond photoemission from single-crystalline magnesium. In strong contrast to the previously investigated transition metal tungsten, photoelectron wave packets originating from the localized core level and delocalized valence-band states are launched simultaneously from the solid within the experimental uncertainty of 20 as. This phenomenon is shown to be compatible with a heuristic model based on free-particle-like propagation of the electron wave packets generated inside the crystal by the attosecond excitation pulse and their subsequent interaction with the assisting laser field at the metal-vacuum interface.

Femtosecond charge transfer (CT) dynamics in a series of self-assembled monolayers with an oligo(phenylenethynylene) and oligo(phenyl) backbone is addressed by resonant Auger spectroscopy using the core hole clock method. The characteristic CT times are found to depend strongly on the character of the molecular orbital (MO) which mediates the CT process. This demonstrates that the efficiency and rate of CT through molecular frameworks can be controlled by resonant injection of the charge carriers into specific MOs.

A lithographic method to generate reactive thiol groups on functionalized synthetic diamond for biosensor and molecular electronic applications is developed. We demonstrate that ultrananocrystalline diamond (UNCD) thin films covalently functionalized with surface-generated thiol groups allow controlled thiol-disulfide exchange surface hybridization processes. The generation of the thiol functional head groups was obtained by irradiating phenylsulfonic acid (PSA) monolayers on UNCD surfaces. The conversion of the functional headgroup of the self-assembled monolayer was verified by using X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and fluorescence microscopy. Our findings indicate the selective generation of reactive thiol surface groups. Furthermore, we demonstrate the grafting of yeast cytochrome c to the thiol-modified diamond surface and the electron transfer between protein and electrode.

Defining Time-Zero When a high-energy photon hits an atom and is absorbed, the result can be the excitation and emission of an electron. This photoemission, or photoelectric effect, is generally assumed to occur instantaneously, and represents the definition of “time-zero” in clocking such ultrafast events. Schultze et al. (p. 1658 , see the cover; see the Perspective by van der Hart ) use ultrafast spectroscopy, with light pulses on the time scale of several tens of attoseconds, to test this assumption directly. They excite neon atoms with 100 eV photons and find that there is a small (20-attosecond) time delay between the emission of electrons from the 2 s and 2 p orbitals of the atoms. These results should have implications in modeling electron dynamics occurring on ultrafast time scales.

We investigated hydroxyalkylphosphonate monolayers as a novel platform for the biofunctionalization of silicon-based field effect sensor devices. This included a detailed study of the thin film properties of organophosphonate films on Si substrates using several surface analysis techniques, including AFM, ellipsometry, contact angle, X-ray photoelectron spectroscopy (XPS), X-ray reflectivity, and current-voltage characteristics in electrolyte solution. Our results indicate the formation of a dense monolayer on the native silicon oxide that has excellent passivation properties. The monolayer was biofunctionalized with 12 mer peptide nucleic acid (PNA) receptor molecules in a two-step procedure using the heterobifunctional linker, 3-maleimidopropionic-acid-N-hydroxysuccinimidester. Successful surface modification with the probe PNA was verified by XPS and contact angle measurements, and hybridization with DNA was determined by fluorescence measurements. Finally, the PNA functionalization protocol was translated to 2 microm long, 100 nm wide Si nanowire field effect devices, which were successfully used for label-free DNA/PNA hybridization detection.

We describe an apparatus for attosecond photoelectron spectroscopy of solids and surfaces, whichcombines the generation of isolated attosecond extreme-ultraviolet (XUV) laser pulses by high har-monic generation in gases with time-resolved photoelectron detection and surface science techniquesin an ultrahigh vacuum environment. This versatile setup provides isolated attosecond pulses withphoton energies of up to 140 eV and few-cycle near infrared pulses for studying ultrafast electrondynamics in a large variety of surfaces and interfaces. The samples can be prepared and characterizedon an atomic scale in a dedicated flexible surface science end station. The extensive possibilities of-fered by this apparatus are demonstrated by applying attosecond XUV pulses with a central photonenergy of $\sim$ 125 eV in an attosecond streaking experiment of a xenon multilayer grown on a Re(0001)substrate.

Photoemission spectroscopy is central to understanding the inner workings of condensed matter, from simple metals and semiconductors to complex materials such as Mott insulators and superconductors1. Most state-of-the-art knowledge about such solids stems from spectroscopic investigations, and use of subfemtosecond light pulses can provide a time-domain perspective. For example, attosecond (10-18 seconds) metrology allows electron wave packet creation, transport and scattering to be followed on atomic length scales and on attosecond timescales2-7. However, previous studies could not disclose the duration of these processes, because the arrival time of the photons was not known with attosecond precision. Here we show that this main source of ambiguity can be overcome by introducing the atomic chronoscope method, which references all measured timings to the moment of light-pulse arrival and therefore provides absolute timing of the processes under scrutiny. Our proof-of-principle experiment reveals that photoemission from the tungsten conduction band can proceed faster than previously anticipated. By contrast, the duration of electron emanation from core states is correctly described by semiclassical modelling. These findings highlight the necessity of treating the origin, initial excitation and transport of electrons in advanced modelling of the attosecond response of solids, and our absolute data provide a benchmark. Starting from a robustly characterized surface, we then extend attosecond spectroscopy towards isolating the emission properties of atomic adsorbates on surfaces and demonstrate that these act as photoemitters with instantaneous response. We also find that the tungsten core-electron timing remains unchanged by the adsorption of less than one monolayer of dielectric atoms, providing a starting point for the exploration of excitation and charge migration in technologically and biologically relevant adsorbate systems.

Ejection of an electron by absorption of an extreme ultraviolet (xuv) photon probes the many-electron response of a solid well beyond the single-particle picture. Photoemission spectra feature complex correlation satellite structures signifying the simultaneous excitation of single or multiple plasmons. The time delay of the plasmon satellites relative to the main line can be resolved in attosecond streaking experiments. Time-resolved photoemission thus provides the key to discriminate between intrinsic and extrinsic plasmon excitation. We demonstrate the determination of the branching ratio between intrinsic and extrinsic plasmon generation for simple metals.