Group seminars

Probing protein dynamics by intrinsic tryptophan

Prof. Dongping Zhong
Department of Chemistry and Biochemistry, Ohio State university

Tryptophan has the longest wavelength absorption among other amino acids and has recently been used as an optical probe of protein dynamics on ultrafast time scales. Here, by combing femtosecond spectroscopy and site-directed mutagenesis, we have carried out a series of studies including ultrafast protein electron transfer, resonance energy transfer, conformation fluctuations, and water-protein coupled motions, and observed various nonequilibrium protein dynamics with femtosecond temporal and single-residue spatial resolutions. These studies demonstrated that tryptophan is a powerful molecular probe and can be widely used for probing ultrafast protein dynamics on the most fundamental level.

Monday, 26 November 2012, at 15:00 PM in room CH G1 495

Spin-state photo-switching dynamics in molecular crystals investigated by time-resolved optical and x-ray techniques

Prof. Eric Collet
Institut de Physique de Rennes, UMR 6251 University Rennes 1-CNRS, Rennes, France

The current challenge in control science is not only to observe matter on ever smaller scale but also to direct its functionality at the relevant length, time and energy scales. It is the aim of light-driven transformations, to force the matter towards a new state far from thermal equilibrium. Photoinduced transition occurring in multi-stable materials involves different degrees of freedom at the molecular level as well as macroscopic propagating or diffusive effects (shockwave, thermal heating…), each of them being associated with a typical timescale. Understanding the physical mechanisms involved in the transformation induced by an ultra-short femtosecond laser pulse in solids is only emerging. Recent investigations performed by femtosecond optical spectroscopy, x-ray diffraction or XANES will be presented. In addition to the sub-picosecond molecular transformation, other slower effects of elastic or thermal nature should be considered for describing the macroscopic response, as different processes spanning from molecular to material scales are linked together.

Jeudi 8 novembre 2012 à 13 heures 30, en salle G1 495

Computational studies of energy transfer in dendrimers

Prof. Adrian Roitberg,
University of Florida

Excited state nonadiabatic molecular dynamics simulations are used to study the nature of the energy transfer in different model dendritic molecules built from linear poly-phenylene ethynylene (PPE) in different architectures. Dendrimers built from these building blocks have been experimentally shown to undergo highly efficient and ultrafast unidirectional energy transfer. We have recently introduced a highly efficient method to compute non- adiabatic excited-state dynamics, including analytically computed gradients and non- adiabatic couplings. We observe ultrafast and mostly one-directional electronic energy transfer, concomitant with an also ultrafast vibrational energy transfer. The energy gaps and nonadiabatic couplings are strongly influenced by the different nuclear motions in the different potential energy surfaces. This behavior guarantees the successful unidirectional energy transfer associated to the efficient energy funneling in light-harvesting dendrimers.

Tuesday, 9 October 2012, at 16:30 PM in room CH G1 495


CARS Microscopy Made Simple: Label-free, Molecule-Specific Nonlinear Microscopy

Prof. Dr Albert Stolow

Steacie Institute for Molecular Sciences, National Research Counci

We discuss a very simple yet high performance CARS microscopy, recently commercialized by Olympus, based on a single fs Ti:Sapphire oscillator combined with a photonic crystal fibre. By optimally chirping the fs pump and Stokes laser pulses, we achieve high quality multi-modal imaging (simultaneous CARS, two-photon fluorescence, and second harmonic generation) of live cells and tissues with a Raman tuning range from below 900 cm-1 to over 4000 cm-1. Chirp as a control parameter permits simultaneous optimization of the spectral resolution and signal levels of these imaging modalities. We discuss recent technical developments such as all-fibre CARS microscopy, Hilbert Transform CARS, and time-correlated photon counting FLIM-CARS microscopy. We illustrate with applications of multi-modal CARS microscopy to label-free, molecule-specific biomedical and geological imaging.

Friday, 5.10.2012 11:00, CH G1 495

Shedding light on ultrafast electron dynamics and excited states with real-time time-dependent density functional theory

Dr. Kenneth A. Lopata
Pacific Northwest National Laboratory, Richland WA, USA

Excited states and ultrafast electron dynamics underpin a wide range of important physical and chemical processes such as spectroscopy, light harvesting, photochemical reactions, and charge transfer. I will discuss real-time time-dependent density functional theory (RT-TDDFT), a powerful first principles approach for modeling electron dynamics in time and space at the sub-femtosecond time scale, and show how RT-TDDFT gives unparalleled insight into the nature of a variety of excited state processes including: valence excitations in polyacenes, excitations in nitrogen-doped titania, core-level near-edge x-ray absorption in ruthenium complexes including scalar relativistic effects, and charge injection in a simple dye-sensitized solar cell analog.

Vendredi 6 juillet 2012 à 15 heures 30, en salle G1 495
Friday, 6 July 2012, at 15:30 PM in room G1 495

Time-resolved photoelectron spectroscopy of aqueous solutions

Dr. Andrea Luebcke
Max-Born Institute for Nonlinear Optics and Short Puls Spectroscopy, Berlin, Germany

We use fs-time resolved photoelectron spectroscopy to study the excited state dynamics of liquid water and of molecules dissolved in water. In my talk, I will concentrate on mainly two systems: 1st liquid water at the liquid-vacuum interface of salt solutions and, 2nd DNA bases in solution.
To excite liquid water at the liquid-vacuum interface, sub-20 fs pulses at 160 nm are employed.
I will show, that cations present in the solution give rise to a charge transfer from a water molecule to the cation which leads to the formation of a hydrated cation-electron complex.
In the 2nd part of my talk, I will concentrate on the excited state dynamics of DNA bases, nucleosides and some derivatives in solution. Molecules are excited by sub-100 fs UV pulses (typically 266nm). The relaxation from the first excited state back to the ground state is extremely fast and seems to be the key for the photostability of DNA. Deviating from optical methods, time-resolved photoelectron spectroscopy shows a long-lived ground state bleach in addition to the short-lived excited state.

Vendredi 6 juillet 2012 à 11 heures 30, en salle G1 495
Friday, 6 July 2012, at 11:30 AM in room G1 495

The SwissFEL X-Ray Laser Project

Dr. Rafael Abela
Project Leader SwissFEL, Paul Scherrer Institute, Villigen

The Paul Scherrer Institute is planning the construction of a X-Ray Free Electron Laser (SwissFEL) facility, which will produce 20 fsec pulses of coherent x-rays in the wavelength range 0.1 to 7 nm, with extremely high peak brightness. These characteristics will provide opportunities for new experiments in chemistry, solid state physics, biochemistry and materials science. The presentation will focus on novel applications, the description of the fundamental aspects of the planned facility, and last but not least the milestones towards the planned operation.

Lundi 2 juillet 2012 à 16 heures, en salle CH G1 495
Monday, 2 July 2012, at 16 PM in room CH G1 495


Probing Electronic Processes in Molecular Complexes by Coherent Multidimensional Visible, UV and X-ray Spectroscopy

Prof. Shaul Mukamel
Department of Chemistry, University of California, Irvine, USA

Energy- transfer and charge-separation pathways in the bacterial reaction center may be revealed by coherent two-dimensional optical spectroscopy. The excited state dynamics and relaxation of electrons and holes are simulated using a two-band tight-binding model. The dissipative exciton and charge carrier motions are calculated using a transport theory, which includes a strong coupling to a harmonic bath with experimentally determined spectral density, and reduces to the Redfield, the Förster, and the Marcus expressions in the proper parameter regimes. Direct spectroscopic signatures of the charge separation are identified.
Time-domain experiments that employ sequences of attosecond x-ray pulses in order to probe electronic and nuclear dynamics in molecules are made possible by newly developed bright coherent ultrafast sources of soft and hard x-rays. By creating multiple core holes at selected atoms and controlled times it should be possible to study the dynamics and correlations of valence electrons as they respond to these perturbations. The stimulated x-ray Raman spectra of trans-N-methylacetamide and Cysteine at the Nitrogen, Sulfur and the Oxygen K-edges in response to two soft x-ray pulses are calculated. The signals are interpreted in terms of the dynamics of valence electronic wave packets prepared and detected in the vicinity of (either the nitrogen or the oxygen) atom. The evolving electronic charge density and electronic coherences are visualized using a basis set of time-dependent natural orbitals. Effects of orbital relaxation upon core excitations are resolved. A two-dimensional extension of the technique that involves a sequence of three resonant Raman pulses will be presented.

Lundi 4 juin 2012 à 12 heures 15, en salle CE100
Monday, 4 June 2012, at 12:15 PM in room CE100

Dr. Pavlo Maksyutenko
Combustion Research Laboratory
Paul Scherrer Institut, Villigen, Switzerland

I will present an overview of my research activities in the last few years. The presentation will be structured according to the experimental techniques involved. Polarization-resolved degenerate four-wave mixing spectroscopy gives insight into the photodissociation dynamics of hydrogen peroxide. Multiple-resonance vibrational excitation followed by photofragment detection provides access to chemically important vibrational states of water and methanol and reveals intramolecular vibrational energy redistribution dynamics in frequency domain. Angular-resolved mass-selective time-of-flight measurements of the products of CH+C2H2 crossed-beam reaction elucidate the reaction mechanisms relevant to soot formation.

Jeudi 10 mai 2012 à 11 heures, en salle CH H1 625
Thursday, 10 May 2012, at 11:00 AM in room CH H1 625

Entanglement and photon-photon interactions for quantum communication

Dr. Enrico Pomarico
University of Geneva, GAP-Optique, Geneva

Quantum communication deals with the distribution of entangled photons at a distance.Sources of photonic entanglement suited to this task need to be developed.
I will present a source, based on Spontaneous Parametric Down Conversion in an integrated cavity-waveguide, that produces narrowband telecom entangled photons and that can be engineered to achieve extremely low losses.
I will also report the progress towards implementation of an experiment of faithful entanglement swapping adopting a nonlinear interaction between two single photons. Sources of photons with high coupling efficiencies have been realized for this task. Nonlinear optics photon – photon interactions can provide competitive alternatives to linear optics based quantum communication.

Mercredi 9 mai 2012 à 14 heures, en salle CH G1 495
Wednesday, 9 May 2012, at 14:00 PM in room CH G1 495

Where Do We Come From? What Are We? Where Are We Going? Can TiO2 be the answer to all this?

Dr. Jacinto Sa
Research Department Synchrotron Radiation and Nanotechnology
Paul Scherrer Institut, Villigen, Switzerland

The increase of atmospheric CO2 to levels, which threatens human existence forced mankind to immediately address the production of carbon-neutral, renewable and storable energy. In nature, plants and some bacteria convert CO2 and H2O into sugars and O2 via photosynthesis, and many research groups are exploring the prospect of performing photosynthesis artificially by means of stable, inorganic photocatalysts. A promising route is to use metal-doped TiO2 to split water and photoreduce CO2 to CO and hydrocarbons under UV irradiation (Eexcitation ≥ Eg TiO2 (anatase) = 3.2 eV), but the yield is low, and many details of the process are poorly understood. To further the fundamental understanding of catalytic processes that occur during artificial photosynthesis and to guide the development of new materials with improved properties, we propose to study the generation of electron hole pairs as a precursor to the formation of CO and hydrocarbons, from the ultrafast to the kinetic domain. Several methodologies that can address each time-dependent step have or are being developed. When understood, the time-critical considerations will help provide answers to the major questions of artificial photosyntesis, namely:
· What is the rate-determining step?
· Is the reaction site specific?
· What is the true quantum yield?

Mercredi 2 mai 2012 à 10 heures 30, en salle CH G1 495
Wednesday, 2 May 2012, at 10:30 AM in room CH G1 495


Imaging Single Molecules: are new insights and features still available in optical microscopy?

Dr Filippo Lusitani
Zernike Institute for Advanced Materials
University of Groningen (The Netherlands)

Optical microscopy is among the oldest techniques developed and implemented for modern science; the first microscope dating back to the end of the sixteenth century. Five centuries of theoretical and technological improvements have provided scientists the possibility to study matter at the single molecule level.

The imaging of a single fluorescent dipole brings the observer into a singular “zone” where the light source is two orders of magnitude smaller than the emitted wavelength giving access to the fundamental phenomena on which the imaging process is based. This subject, and its relevance, will be discussed in the seminar. It is proposed that a detailed characterization and analysis of the image of a point-like source, the Point Spread Function (PSF), can be used to overcome the optical resolution limit imposed by the diffraction of light and even to extract information on the orientation of the emitting molecule.

Lundi 2 avril 2012 à 11 heures 30, en salle CH G1 495
Monday, 2 April 2012, at 11:30 AM in room CH G1 495

Physics of organic-organic interfaces

Dr Dorota Jarząb
Zernike Institute for Advanced Materials
University of Groningen (The Netherlands)

Organic-organic interfaces are inhered for plastic electronics, which is promising alternatives to epitaxially grown inorganic electronics. However, before organic-based electronics become widespread in daily life, much effort is required. Not all organic semiconductors-based devices perform sufficiently to be of commercial interest. Thus, the understanding and the control of the processes determining the properties of organic semiconductors is crucial to the further development of organic electronics.
Very often, organic electronic devices use as an active layer heterojunctions formed by dissimilar materials. The quality and quantity of the interfaces determines to a large extend a device’s performance and efficiency. Because of the intermolecular interactions in these interfaces, novel phenomena and various types of excited states like Forster energy transfer, charge transfer, charge transfer exciton, excimers emerge. Insights of the phenomena occurring at the organic interfaces are crucial for the further development of plastic electronics. Because organic semiconductor shows strict correlation between electronic and optical properties, and supra-molecular arrangements and a device’s performance, there is a need to study and correlate all these aspects by means of a multidisciplinary study, combining spectroscopic and morphological analysis techniques.
I will present results of investigation of the working mechanism of a variety of organic heterojunctions having application in light emitting diodes, field effect transistors and solar cells. Knowledge and understanding of the nature of the phenomena occurred are obtained by applying measurements techniques such as: time-resolved photoluminescence, scanning near-field microscopy (SNOM), atomic force microscopy (AFM) and confocal microscopy.
Lundi 26 mars 2012 à 13 heures 30, en salle CH G1 495
Monday, 26 March 2012, at 13:30 PM in room CH G1 495

Extending Attosecond Measurements

Prof. Jonathan P. Marangos
Blackett Laboratory, Department of Physics, Imperial College, London

The seminar will introduce some of the challenges that attosecond measurement methods need to address in science, e.g. charge migration in molecules and solids, ultrafast electronic motion at surfaces. The two basic approaches to attosecond measurements, both based on high order harmonic generation, attosecond pump-probe measurements and HHG spectroscopy will be outlined. Recent developments in the HHG spectroscopy method, aimed at extending the technique to a wider range of molecules, will be described. These methods utilise the intrinsic time to frequency mapping in the HHG spectrum to unravel ultrafast dynamics (both nuclear and electronic) in the molecular cation. Longer laser wavelengths, to increase the mapping interval and access molecules with lower ionization potential, and the use of long and short trajectrory groups are both being explored in our laboratory. We have also used multi-colour fields to control the temporal mapping in HHG. Finally some future directions that we plan to explore: XUV initiated HHG, pump-probe fragmentation studies and attosecond transient absorption will be discussed.

Vendredi 23 mars 2012 à 13 heures 30, en salle CH G1 495
Friday, 23 March 2012, at 13:30 PM in room CH G1 495

Three-Dimensional Electronic Spectroscopy of the Photosystem II subunits

Dr Valentyn I. Prokhorenko
Max-Planck Research Department for Structural Dynamics,
University of Hamburg, Germany

An overview of recent experimental results of multidimensional coherent electronic spectroscopy of the Photosystem II major subunits (LHCII and D1D2RC) will be given together with their analysis. These experiments were conducted at room temperature and under aerobic conditions using a 2D- spectrometer described in detail in with high temporal resolution. To create 3D-spectra, a large amount of 2D-spectra was collected by scanning the “waiting time” in different delay windows. Analysis of 3D-arrays of data, created in such a manner, was performed using a global fit approach, though in three dimensions (for the first time). Some technical introduction into three-dimensional global fitting will be provided during the talk. This treatment leads to the so-called two-dimensional decay-associated spectra (2DDAS), from which the energy transfer pathways (or coherence transfer pathways?) can be recovered. In particular, the transfer between Chl_b and Chl_a pools in the LHCII light-harvesting complex, the major antenna in the PSII, is clearly observed. We detect an interesting interplay between corresponding absorption peaks related to these pools (located on the diagonal section of 2D-spectra) and its cross-peak located on the off-diagonal section, which is distinctly resolved.

The Fourier transformation of residuals (3D-spectra after subtracting decay components) leads to three- dimensional spectra where the coordinates are the “excitation”, “obser­vation”, and “population” frequencies (“dephasing”, “waiting”, and “population” time delays, respectively). Some resolved structures in these 3D-spectra can be attributed to beatings between different excitonic levels (but not all). In the LHCII spectra we observe huge long-lasting (up to 40 ps) oscillations with low frequency; however, they are more pronounced in the imaginary parts of the 2D- and 3D-spectra.

A collection of “2D-movies”, from which these beatings can be clearly observed “by eye” and intuitively understood, will also be presented during the talk.

Lundi 5 mars 2012 à 11 heures, en salle CE 1103
Monday, 5 March 2012, at 11 PM in room CE 1103

On the interaction of low energy electrons with DNA

Dr Tal Markus
Department of Chemical Physics, Weizmann Institute of Science Rehovot (Israel)

When living organisms are exposed to ionization radiation the most pronounced damage is done to the genome. It was found that even low energy electrons, having energy below the bond energy, can still damage DNA.  These low energy electrons are produced efficiently when cells are irradiated with high energy photons or particles, ca. 5*104 electrons per MeV.
Using self assembled monolayers of DNA and applying low energy electron transmission (LEPET) and two photon photo emission (TPPE) spectroscopy, detailed interaction mechanisms between electrons and DNA were revealed. Electron DNA spin-specific interactions were also found and will be discussed.
Mardi 20 décembre 2011 à 14 heures 30, en salle G1 495
Tuesday, 20 December 2011, at 14:30 PM in room G1 495


First coherent radiation from the FERMI@Elettra free electron laser: status report and plan for the first experiments

Prof. Dr Fulvio Parmigiani
Department of Physics, University of Trieste
and FERMI@Elettra Laboratory Sincrotrone Trieste (Italy)

The new free-electron laser (FEL) source FERMI@Elettra seeded at 260 nanometers with an external laser has produced the first coherent emission from the FEL-1 undulator chain tuned at wavelengths of 65 nanometers (fourth harmonic of the seed laser) and 43 nanometers (sixth harmonic). This marks the first successful operation of FERMI@Elettra in its planned configuration, i.e., as a next-generation seeded free-electron laser source. Over the coming months the commissioning team will continue to improve the overall per-formance of the system and the light will be provided to the first experimental programs. In the meantime the construction of the FEL-2 will be completed (August 2011) and the commissioning started.
Here I will present the scientific program for the first experiments along with the strategic road map for the commissioning of the entire facility and future upgrades.

Vendredi 16 décembre 2011 à 11 heures, en salle G1 495
Friday, 16 December 2011, at 11 AM in room G1 495


Demonstrating superconductivity induced modifications of distinctive interband transitions in copper-based superconductors

Prof. Dr Fulvio Parmigiani
Department of Physics, University of Trieste
and FERMI@Elettra Laboratory Sincrotrone Trieste (Italy)

In the last years, a significant theoretical effort has been focused to investigate possible effects relating the interband optical properties of the normal and superconducting states in high-TC superconductors (HTSC). In search of an experimental evidence of such effects, continuous wave (CW) optical spectroscopies have been widely used to measure the high-energy (>1 eV) dielectric function. Unfortunately, these conventional spectroscopies failed to spot the evolution of the dielectric function, in the interband spectral region, across the superconducting transition. Here, by adopting a non-equilibrium approach to the problem, we show how the non-thermal photoinjection of excitations affects the high-energy optical properties, demonstrating a superconductivity induced modification of distinctive interband transitions.

Jeudi 15 décembre 2011 à 11 heures, en salle BCH 1103
Thursday, 15 December 2011, at 11:00 AM in room BCH 1103

Structural and dynamical studies of hemeproteins by X-ray absorption near edge structure (XANES) spectroscopy

Prof. Stefano Della Longa
Faculty of Medicine and Surgery, Dpt of Experimental Medicine, University of L’Aquila

XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) are techniques, sensitive to the electronic state and the structure of an atom environment, and are especially useful to probe metal sites in biomolecules.
XANES quantitative analysis in the framework of the multiple scattering (MS) theory was shown to be able to interpret both the static XANES spectra and the dramatic changes observed in carbonmonoxy-myoglobin (MbCO) single crystal, following the photolysis of the Fe-CO bond at T=20K.
Moreover, in a synchrotron multiple anomalous diffraction (MAD) beam-line, fast “in situ” XANES measurements can be carried out during XRD data collection. The analysis of “in situ” XANES spectra has been applied to refine XRD data of one and the same cyanomet-myoglobin (MbCN) single crystal, allowing to obtain the X-ray structure of the hemeprotein with enhanced active site resolution. Such detailed structural data are mandatory in many cases for elucidating the function of metal centres, as structural changes to the metal coordination during redox or substrate-binding reactions are generally <0.1 Å and hence remain unnoticed in standard protein crystallography unless the resolution is exceptionally high.
The local dynamics of the heme sites can be investigated coupling this spectroscopic technique to photolysis experiments to study excited states in the femtosecond to millisecond time-window or low temperature-trapped intermediates; on the other hand, the finding that dissociation of the exogenous ligand from the heme iron can be induced at low temperature by X-ray irradiation itself can be further exploited to investigate intermediate states of the ligand binding process.

Mercredi 16 novembre 2011 à 13 heures 30, en salle CH G1 495
Wednesday, 16 November 2011, at 13:30 PM in room CH G1 495


Ultrafast soft x-ray photoelectron spectroscopy at liquid water micro jets

Dr Manfred Faubel
Max-­Planck-­Institute for Dynamics and Self-­Organization, 37073 Göttingen, Germany

High vacuum x-­ray photoelectron spectroscopy studies of liquid aqueous surfaces use very thin microjets with high streaming velocities of 10 m/s to 100 m/s as a now well established technique for producing clean free vacuum surfaces of volatile liquids. In a recent proof-­of-principles study at Max-­Planck and University-­Chemistry-­Institutes in Göttingen we show that High-­Harmonics-­Generation of 40eV -­‐70eV soft X-­rays is a powerful, competitive, table-­top-­scale alternative to synchrotron radiation based ESCA with the additional benefit of fs time resolution. This is demonstrated in first laser pump-­photoelectron probe measurements for the time evolution of supercritical water photoelectron spectra, and, for the absolute calibration of the solvated electron binding level in liquid water.

Mercredi 19 octobre 2011 à 16 heures, en salle CH G1 495
Wednesday, 19 October 2011, at 16 PM in room CH G1 495


Elucidating excited state dynamics in DNA by time-resolved fluorescence spectroscopy

Dr Thomas Gustavsson,
Laboratoire Francis Perrin, CEA/IRAMIS/SPAM – CNRS URA 2453
91191 Gif-sur-Yvette, France

We focus our research on the directly UV excited states of DNA in order to characterize their structure and dynamics and possibly link these to UV photodamage. To this purpose, we use femtosecond fluorescence spectroscopy. The individual DNA bases are characterized by ultrafast fluorescence lifetimes (<1 ps), which today can be explained in terms of very efficient non-radiative relaxation channels (“conical intersections”) coupling the excited state to the ground state. The organization of bases into single and double strands affects the fluorescence dynamics profoundly. The fluorescence anisotropies of all helices decay on the sub-picosecond timescale, implying ultrafast energy transfer among the constituent bases. The fluorescence lifetimes, on the other hand, are much longer and strongly sequence dependent, showing that the excitation energy is trapped by dark “reservoir” states.

Lundi 25 juillet 2011 à 14 heures, en salle CH G1 495
Monday, 25 July 2011, at 14 PM in room CH G1 495

Recent Advances of the Biophotonics group at the University of Naples

Professor Carlo Altucci
Physics Department, Università degli Studi di Napoli Federico II

Progresses in Biophotonics taking place at the Naples Physics Department are reported. In particular, interactions between biomolecules triggered both in vitro and in living human cells by femotsecond laser UV pulses are investigated. A model system for the DNA-protein interaction is developed. Applications to biosensing are also reported, based on the use of Quartz crystal micro-balances properly functionalized by irradiation with UV femtosecond pulses.

Vendredi 8 juillet 2011 à 14 heures, en salle CH G1 495
Friday, 8 July 2011, at 14 PM in room CH G1 495

Attosecond pulse generation and x-ray photoelectron spectroscopy of metallic surfaces

Dr Christopher Arrell
Attosecond Laboratory, Imperial College, London

I will described the use of hollow fibre pulse compression to produce a few cycle (3.5fs) IR pulse which is subsequently used to produce an isolated attosecond pulse of soft x-ray light through high harmonic generation. By controlling the carrier envelope phase it is shown the ionisation can be restricted to a single half-cycle of the IR field.

I will also describe the use of soft x-rays produced by high harmonic generation in x-ray photoelectron spectroscopy measurements of metallic surfaces, and their use in measuring the thermalisation time of hot electrons in a gold target. Our recent investigations have been studying enhanced localised plasmonic fields on rough Au surfaces. This work has revealed the onset of tunnelling emission from surfaces irradiated with moderate IR field intensities.

Vendredi 29 avril 2011 à 10 heures, en salle CH G1 495
Friday, 29 April 2011, at 10 AM in room CH G1 495


Bio-Imaging from Tissue to Molecule

Professor Theo Lasser
Laboratoire d’Optique biomédicale
EPFL, Lausanne (Switzerland)

Imaging is key for medical diagnosis and provides new insights for the life sciences. Tissue, cell and subcellular structures can all be visualised using optical microscopy and so provide a variety of information with high spatial resolution. Structural information complemented by the functional information made possible by new optical techniques like Fourier Domain Optical Coherence Tomography (FDOCT), Doppler Imaging and extended-focus Optical Coherence Microscopy (xf-OCM) and its latest extension Fluorescence Coherence Tomography (FCT), which allows extending these methods into the molecular dimensions.
We will present selected examples ranging from retina bloodflow, diabetes, stem cells to brain research and include new concepts for superresolution nanoscopy with a strong emphasis on the underlying optical concepts, and conclude with an outlook for imaging in medicine and the life sciences.

Mercredi 27 avril 2011 à 14 heures, en salle CH G1 495
Wednesday, 27 April 2011, at 14 PM in room CH G1 495


Polyatomic Molecules in Laser Fields: Dynamics, Control, Strong Fields

Professor Albert Stolow
Steacie Institute for Molecular Sciences, National Research Council

The most general molecular dynamic processes involve the coupled flow of both valence electronic charge and vibrational energy. Time-Resolved Photoelectron Spectroscopy (TRPES) is a powerful probe of ultrafast non-adiabatic dynamics in polyatomic molecules, as it simultaneously observes both electronic and vibrational dynamics. Time-Resolved Coincidence Imaging Spectroscopy (TRCIS) further permits direct time-resolved imaging of electronic dynamics in the Molecular Frame during a chemical reaction. We can actively control molecular dynamics without any net absorption of light, using the non-perturbative Dynamic Stark Effect. As laser fields get stronger still, a new Nonadiabatic Multi-Electron (NME) dynamics emerges and has important consequences for all strong field processes in polyatomic molecules, including high harmonic generation and attosecond spectroscopy.

Lundi 11 avril 2011 à 14 heures, en salle CH G1 495
Monday, 11 April 2011, at 14 PM in room CH G1 495

Biological nanowires involved in mineral respiration

Dr Tom Clarke
University of East Anglia, School of Biological Sciences
In the absence of oxygen, some bacterial species can couple oxidation of organic matter to reduction of oxidized metals, such as iron [Fe(III)] and manganese [Mn(IV)] (hydr)oxides, via a biological process termed dissimilatory metal reduction (DMR) that can be coupled to energy conservation. It is becoming increasingly apparent that this process is not confined to the reduction of insoluble minerals, but is also used by some bacteria to reduce or oxidise soluble electron acceptors into insoluble pre cipitates. The electron transfer pathway of these bacteria requires a number of multi-heme cytochromes that are assembled in the periplasm and transported to the outside of the cell, but key questions are yet to be resolved, in particular how electrons are passed through the outer membrane to the extracellular multi-heme cytochromes and ultimately to the mineral surface. We propose that a system typically consisting of a three component – protein system containing an integral membrane 24-28 strand porin, a periplasmic decaheme cytochrome and an outer membrane decaheme cytochrome is responsible for electron transfer through the membrane. Our spectro-potentiometric characterisation of these complexes has shown how these systems are organised for efficient electron transfer while the molecular resolution of an outer membrane cytochrome provides molecular insight into how reduction of insoluble substrate (e.g. minerals), soluble substrates (e.g. flavins) and cytochrome redox partners might be possible in tandem at different termini of a electron transport chain on the cell surface.

Jeudi 17 mars 2011 à 10 heures, en salle CH G1 495
Thursday, 17 March 2011, at 10 AM in room CH G1 495


High Harmonic spectroscopy of electron dynamics in molecules

Watching Ultrafast Motion:
High Harmonic spectroscopy of electron dynamics in molecules

Professor Olga Smirnova
Max Born Institut, Berlin  

High harmonic emission occurs when an electron, liberated from a molecule by an incident intense laser field, gains energy from the field and recombines with the parent molecular ion. High harmonic spectroscopy carries the potential of combining sub-Å spatial and attosecond temporal resolution of electronic structures and dynamics in molecules. Spatial resolution in high harmonic generation comes from the Angstrom scale of the de-Broglie wavelength of the recombining electron. Temporal resolution comes from the attosecond duration of the recollision event and attosecond temporal correlation between the energy of the emitted photon (harmonic number) and the moments of ionization and recombination. We use high harmonic spectroscopy to characterize the attosecond dynamics of multi-electron re-arrangement during strong-field ionization of molecules. We reconstruct the relative phase between different ionization continua to characterize the hole left upon ionization. The analysis has led us to an unexpected conclusion regarding the hole left in N2 molecules: the initial phase between the dominant channels (X and A states of the cation) appears to be close to p. Theoretical analysis shows that a new channel in electron tunneling from the molecule could be responsible for this surprising result. This new channel includes correlation –induced excitations of the electrons left in the ion.

Vendredi, 28 janvier 2011 à 11 heures en salle CH G1 495
Friday, 28 January 2011 at 11 AM in room CH G1 495


Single-shot Femtosecond Electron Diffraction

le mercredi 17 novembre à 11h00 en salle G1 495

Jom Luiten
Eindhoven University of Technology

Single-shot Femtosecond Electron Diffraction

In 2009 the first hard X-ray Free Electron Laser has become operational – LCLS at Stanford University – which enables recording the full diffraction pattern of a tiny protein crystal in a single, few-femtosecond shot. Why bother about electrons anymore? Electrons and X-rays both enable the study of structural dynamics at atomic length scales, yet the information that can be extracted by probing with either electrons or X-rays is quite different and, in fact, complementary. A pulsed electron source with the X-ray Free Electron Laser capability of performing single-shot, femtosecond diffraction would therefore be highly desirable, if only because of practical considerations of size, costs, and accessibility of the setup.

The primary obstacle facing the realization of such an electron source is the space charge problem: packing the number of electrons required for recording a full diffraction pattern in a single sub-picosecond pulse will inevitably lead to a rapid Coulomb expansion of the pulse and therefore loss of temporal resolution. We have developed a method, based on radio-frequency (RF) techniques, to invert the Coulomb expansion. We will report on the first experiments demonstrating RF compression of 0.25 pC, 100 keV electron bunches to sub-100 fs bunch lengths. We have used these bunches to produce high-quality, single-shot diffraction patterns of poly-crystalline gold.

In all ultrafast electron diffraction experiments up to now electron bunches have been generated by femtosecond photoemission from metal cathodes. The transverse coherence length of the resulting beams is fundamentally limited to ~1 nm for crystal samples of ~100 micron size, and therefore generally does not allow the study of, e.g., protein samples. We have developed a new, ultracold pulsed electron source, based on near-threshold photo-ionization of a laser-cooled gas. The source is characterized by an effective electron temperature of ~10 K, almost three orders of magnitude lower than conventional sources. This should enable coherence lengths of a few tens of nm for crystal samples with a size of ~100 micron. By combining the ultracold electron source with RF acceleration and bunch compression techniques, single-shot, femtosecond studies of the structural dynamics of macromolecular crystals will become possible.

Ultrafast aqueous proton transfer

Mercredi 17 novembre à 15h00
salle CH B 30

Dr Erik Nibbering
Max-Born-Institut, Berlin

Ultrafast aqueous proton transfer