New insights into the laser-assisted photoelectric effect from solid-state surfaces

Photoemission from a solid surface provides a wealth of information about the electronic structure of the surface and its dynamic evolution. Ultrafast pump-probe experiments are particularly useful to study the dynamic interactions of photons with surfaces as well as the ensuing electron dynamics induced by these interactions. Time-resolved laser-assisted photoemission (tr-LAPE) from surfaces is a novel technique to gain deeper understanding of the fundamentals underlying the photoemission process. Here, we present the results of a femtosecond time-resolved soft X-ray photoelectron spectroscopy experiment on two different metal surfaces conducted at the X-ray Free-Electron Laser FLASH in Hamburg. We study photoemission from the W 4f and Pt 4f core levels using ultrashort soft X-ray pulses in combination with synchronized infrared (IR) laser pulses. When both pulses overlap in time and space, laser-assisted photoemission results in the formation of a series of sidebands that reflect the dynamics of the laser-surface interaction. We demonstrate a qualitatively new level of sideband generation up to the sixth order and a surprising material dependence of the number of sidebands that has so far not been predicted by theory. We provide a semi-quantitative explanation of this phenomenon based on the different dynamic dielectric responses of the two materials. Our results advance the understanding of the LAPE process and reveal new details of the IR field present in the surface region, which is determined by the dynamic interplay between the IR laser field and the dielectric response of the metal surfaces.Comment: 18 pages, 3 figure

Electronic Quantum Coherence in Glycine Molecules Probed with Ultrashort X-ray Pulses in Real Time

Structural changes in nature and technology are driven by charge carrier motion. A process such as charge-directed reactivity that can be operational in radiobiology is more efficient, if energy transfer and charge motion proceeds along well-defined quantum mechanical pathways keeping the coherence and minimizing dissipation. The open question is: do long-lived electronic quantum coherences exist in complex molecules? Here, we use x-rays to create and monitor electronic wave packets in the amino acid glycine. The outgoing photoelectron wave leaves behind a positive charge formed by a superposition of quantum mechanical eigenstates. Delayed x-ray pulses track the induced electronic coherence through the photoelectron emission from the sequential double photoionization processes. The observed sinusoidal modulation of the detected electron yield as a function of time clearly demonstrates that electronic quantum coherence is preserved for at least 25 femtoseconds in this molecule of biological relevance. The surviving coherence is detected via the dominant sequential double ionization channel, which is found to exhibit a phase shift as a function of the photoelectron energy. The experimental results agree with advanced ab-initio simulations.Comment: 54 pages, 11 figure

Linear Array Detector for Online Diagnostics of Spectral Distributions at MHz Repetition Rates

Free-electron lasers (FELs) based on superconducting accelerator technology and storage ring facilities operate with bunch repetition rates in the MHz range, and the need arises for bunch-by-bunch electron and photon diagnostics. For photon-pulse-resolved measurements of spectral distributions, fast one-dimensional profile monitors are required. The linear array detector KALYPSO (KArlsruhe Linear arraY detector for MHz-rePetition rate SpectrOscopy) has been developed for electron bunch or photon pulse synchronous read-out with frame rates of up to 2.7 MHz. At the FLASH facility at DESY, a current version of KALYPSO with 256 pixels has been installed at a grating spectrometer as online diagnostics to monitor the pulse-resolved spectra of the high-repetition-rate FEL pulses. Application-specific front-end electronics based on MicroTCA standard have been developed for data acquisition and processing. Continuous data read-out with low latency in the microsecond range enables the integration into fast feedback applications. In this paper, pulse-resolved FEL spectra recorded at 1.0 MHz repetition rate for various operation conditions at FLASH are presented, and the first application of an adaptive feedback for accelerator control based on photon beam diagnostics is demonstrated

Incentive system and its evaluation in automotive industry

Devletler ekonomik ve sosyal sorunların çözümüne yönelik olarak sektörlere yönelik yardımlarda bulunurlar. Devletler bu yardımları, sanayi ve ticaret politikaları doğrultusunda bölgesel adaletsizliklerin azaltılması, yatırımın desteklenmesi, rekabet gücünün korunması ve sosyal hedeflere ulaşılması amacıyla, teşvik araçlarını kullanarak gerçekleştirirler. Otomotiv sektöründeki gibi, projelerin etkin işlediği, iş gücü gereksiniminin yüksek olduğu firmalar için proje yönetiminin önemi büyüktür. Proje yönetimi, projenin hedeflerine ulaşılması için gerekli ihtiyaçları karşılamak üzere ilgili tüm bilgi, beceri, araç ve tekniklerin proje faaliyetlerine uygulanmasıdır. Otomotiv sektörü gibi ekonomide büyük yeri olan sektörler için, yatırım yapılacak uygun projenin seçilmesi açısından proje yönetiminin önemi giderek artmaktadır.States are involved in sectoral aid to address economic and social problems. States use these aids for the reduction of regional injustices towards industrial and commercial policies, investment support, protection of competitive power and to reach social goals using incentive tools. As seen in the automotive sector, project management is a great asset for firms which projects are processed efficiently and that require high workforce. The purpose of project management is to meet the needs to reach the project objectives requiring the application of all relevant knowledge, skills, tools and techniques on project activities. The prominence of project management is increasing in terms of selecting the appropriate project to be invested in sectors such as the automotive sector that have a great deal in the economy

Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS.

The ultrafast dynamics of photon-to-charge conversion in an organic light-harvesting system is studied by femtosecond time-resolved X-ray photoemission spectroscopy (TR-XPS) at the free-electron laser FLASH. This novel experimental technique provides site-specific information about charge separation and enables the monitoring of free charge carrier generation dynamics on their natural timescale, here applied to the model donor-acceptor system CuPc:C60. A previously unobserved channel for exciton dissociation into mobile charge carriers is identified, providing the first direct, real-time characterization of the timescale and efficiency of charge generation from low-energy charge-transfer states in an organic heterojunction. The findings give strong support to the emerging realization that charge separation even from energetically disfavored excitonic states is contributing significantly, indicating new options for light harvesting in organic heterojunctions

The spin and orbital contributions to the total magnetic moments of free Fe, Co, and Ni clusters

We present size dependent spin and orbital magnetic moments of cobalt (Co$_{n}$$^{+}$, 8 ≤ n ≤ 22), iron (Fe$_{n}$$^{+}$, 7 ≤ n ≤ 17), and nickel cluster (Ni$_{n}$$^{+}$, 7 ≤ n ≤ 17) cations as obtained by X-ray magnetic circular dichroism (XMCD) spectroscopy of isolated clusters in the gas phase. The spin and orbital magnetic moments range between the corresponding atomic and bulk values in all three cases. We compare our findings to previous XMCD data, Stern-Gerlach data, and computational results. We discuss the application of scaling laws to the size dependent evolution of the spin and orbital magnetic moments per atom in the clusters. We find a spin scaling law “per cluster diameter,” ~n$^{−1/3}$, that interpolates between known atomic and bulk values. In remarkable contrast, the orbital moments do likewise only if the atomic asymptote is exempt. A concept of “primary” and “secondary” (induced) orbital moments is invoked for interpretation

Photoinduced dynamics at the water/TiO2_2(101) interface

We present a femtosecond time-resolved optical pump-soft x-ray probe photoemission study in which we follow the dynamics of charge transfer at the interface of water and anatase TiO$_2$(101). By combining our observation of transient oxygen O 1s core level peak shifts at submonolayer water coverages with Ehrenfest molecular dynamics simulations we find that ultrafast interfacial hole transfer from TiO$_2$ to molecularly adsorbed water is completed within the 285 fs time resolution of the experiment. This is facilitated by the formation of a new hydrogen bond between an O$_2$c site at the surface and a physisorbed water molecule. The calculations fully corroborate our experimental observations and further suggest that this process is preceded by the efficient trapping of the hole at the surface of TiO$_2$ by hydroxyl species (-OH), that form following the dissociative adsorption of water. At a water coverage exceeding a monolayer, interfacial charge transfer is suppressed. Our findings are directly applicable to a wide range of photocatalytic systems in which water plays a critical role

Differential Measurement of Electron Ejection after Two-Photon Two-Electron Excitation of Helium

We report the measurement of the photoelectron angular distribution of two-photon single-ionization near the $2p^2$ $^1D^e$ double-excitation resonance in helium, benchmarking the fundamental nonlinear interaction of two photons with two correlated electrons. This observation is enabled by the unique combination of intense extreme ultraviolet pulses, delivered at the high-repetition-rate free-electron laser in Hamburg (FLASH), ionizing a jet of cryogenically cooled helium atoms in a reaction microscope. The spectral structure of the intense self-amplified spontaneous emission free-electron laser pulses has been resolved on a single-shot level to allow for post selection of pulses, leading to an enhanced spectral resolution, and introducing a new experimental method. The measured angular distribution is directly compared to state-of-the-art theory based on multichannel quantum defect theory and the streamlined $R$-matrix method. These results and experimental methodology open a promising route for exploring fundamental interactions of few photons with few electrons in general

Ultrafast Real-Time Dynamics of CO Oxidation over an Oxide Photocatalyst

Femtosecond X-ray laser pulses synchronized with an optical laser were employed to investigate the reaction dynamics of the photooxidation of CO on the anatase TiO$_2$(101) surface in real time. Our time-resolved soft X-ray photoemission spectroscopy results provide evidence of ultrafast timescales and, coupled with theoretical calculations, clarify the mechanism of oxygen activation that is crucial to unraveling the underlying processes for a range of photocatalytic reactions relevant to air purification and self-cleaning surfaces. The reaction takes place between 1.2 and 2.8 (±0.2) ps after irradiation with an ultrashort laser pulse leading to the formation of CO$_2$, prior to which no intermediate species were observed on a picosecond time scale. Our theoretical calculations predict that the presence of intragap unoccupied O$_2$ levels leads to the formation of a charge-transfer complex. This allows the reaction to be initiated following laser illumination at a photon energy of 1.6 eV (770 nm), taking place via a proposed mechanism involving the direct transfer of electrons from TiO$_2$ to physisorbed O$_2$