Résumé: The photo-fragmentation spectrum of protonated fluorene has been recorded in the visible spectral region, evidencing an absorption that appears largely red shifted in comparison to that of the neutral molecule fluorene. The spectrum shows two different vibrational progressions, separated by 0.19 eV. As in the case of protonated linear polycyclic aromatic hydrocarbons (PAHs), comparison of the measured spectra with ab initio calculations allows to associate the observed absorption shift with the charge transfer character of the excited state. The spectra can be properly simulated by geometry optimization of the ground and excited states, followed by Franck Condon analysis. The two vibrational bands progressions observed are assigned, with relatively good confidence, to the existence of two different conformers. (C) 2011 Elsevier B. V. All rights reserved.

Résumé: This paper deals with our on-going work on the electronic spectroscopy of tetrapyrrole molecules embedded in cryogenic hosts. Under nanosecond laser excitation of free-base tetrabenzoporphin molecules in a nitrogen matrix at 8 K, increasing laser pulse energy results in essential enhancement of the intensity of one or several vibronic emission lines in the fine-structure fluorescence spectrum. Some characteristics of stimulated radiation are realized in the observed intense monochromatic: radiation as a result of a transition from the purely electronic S-1 level to a vibrational sublevel of the S-0 state, corresponding to the excitation of C-C stretching vibrations of tetrapyrrole methane bridges. We also report on persistent spectral hole-burning effects. Hole and antihole formation analysis gives some insight on the photochemical properties of tetrabenzoporphin in a nitrogen matrix. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4746794]

Résumé: Context. High-resolution rotational spectroscopy of the imidogen radical has been limited to the (NH)-N-14 and (ND)-N-14 isotopologues. Imidogen is an important intermediate in the astronomical synthesis of ammonia. Recently, the N-14/N-15 isotopic ratio in ammonia has been obtained in cold, dense molecular clouds. Aims. We conducted a laboratory search for rotational transitions of (NH)-N-15 to investigate in more detail the N-14/N-15 ratio in the interstellar medium. Methods. (NH)-N-15 was generated in a positive column discharge in a flowing (NH3)-N-15 – He (SOLEIL synchrotron) or (NH3)-N-15 – Ar (PhLAM) mixture. High-resolution spectroscopic study of the (NH)-N-15 isotopologue of imidogen in its ground electronic and vibrational state ((X) over tilde (3)Sigma(-)) was carried out in the THz range (up to 225 cm(-1)) with the AILES beamline of the SOLEIL synchrotron and subsequently with the PhLAM spectrometer (around 942 GHz). The observed fine and hyperfine structures were analysed, yielding an accurate set of rotational, fine, and hyperfine parameters. Results. The reported frequencies and molecular constants are suitable for radioastronomical searches of this key species and for N-14/N-15 isotopic ratio astronomical determination.

Résumé: In a recent article generalization of the binomial distribution associated with a sequence of positive numbers was examined. The analysis of the nonnegativeness of the formal probability distributions was a key point to allow to give them a statistical interpretation in terms of probabilities. In this article we present an approach based on generating functions that solves the previous difficulties. Our main theorem makes explicit the conditions under which those formal probability distributions are always non-negative. Therefore, the constraints of non-negativeness are automatically fulfilled giving a complete characterization in terms of generating functions. A large number of analytical examples becomes available. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4757601]

Résumé: We report the first results of molecular frame photoelectron emission for dissociative photoionization (DPI) of H-2 and D-2 molecules induced by a spectrally filtered single high harmonic of a few femtosecond duration, using coincident electron-ion velocity vector correlation techniques. For the studied photon energies around 32 eV, where the resonant excitation of the Q(1) and Q(2) doubly excited states occurs, autoionization and nuclear dynamics are coupled on a few femtosecond timescale, giving rise to quantum interferences. Molecular frame photoelectron angular distributions (MFPADs), traced as a function of the kinetic energy release of the atomic fragments, provide the most sensitive observables for such complex dynamics. These results compare well with recent spectrally resolved experiments using synchrotron radiation which are also reported. As a novel XUV light source running at multi-kHz repetition rate and synchronized with laser pulses, high-order harmonic generation (HHG) opens new possibilities for extending these investigations to time-resolved studies at the femtosecond scale.

Résumé: We consider bipartite quantum systems characterized by a continuous angular variable., representing, for instance, the position of a particle on a circle or in a periodic lattice. For this type of system, we show how to violate inequalities similar to Clauser-Horne-Shimony-Holt (CHSH) ones, originally derived for bipartite spin-1/2-like systems. Such inequalities involve correlated measurement of continuous angular functions and are equivalent to the continuous superposition of CHSH inequalities acting on bidimensional subspaces of the infinite dimensional Hilbert space. As an example, we discuss in detail one application of our results, and we derive inequalities based on orientation correlation measurements. The introduced Bell-type inequalities open the perspective of new and simpler experiments to test the assumptions of locality, realism, or freedom of choice in a variety of quantum systems described by continuous variables. We discuss in detail a possible implementation using the orbital angular momentum of photons.

Résumé: We present a density-functional theory (DFT) study combined with scanning tunneling microscopy (STM) experiments of the chemisorption of the N,N'-bis(1-hexylheptyl) perylene-3,4:9,10-bis(dicarboximide) molecule, noted here as DHH-PTCDI, on the SiC(0001)-3 x 3 surface. Five possible adsorption configurations have been investigated in which molecular adsorption occurs on two adjacent Si adatoms via different pairs of atoms of the molecule. We have calculated the energies, structures, density of states, local density of states, and a calculated STM image and show that chemisorption via two oxygen atoms located on the same side of the molecule on two adjacent Si adatoms is the most favorable, in agreement with the experimental STM images. A comparison between the PTCDI and the adsorption of another large organic molecule (phthalocyanine) on the SiC(0001) surface completes this work.

Résumé: In biomedical and food industry, surface colonization by bacteria is harmful: it leads to biofilm formation, a microbial consortia more resistant to antibiotics than planktonic bacteria. In order to design materials able to limit the biofilm formation, the effect of bacteria on materials has to be well characterized. In this work, a well-defined surface composed of a self-assembled monolayer (SAM) of octadecanethiol (ODT) onto a gold surface is probed in situ. The SAM conformation is obtained using the femtosecond vibrational sum frequency generation (SFG) spectroscopy. This technique provides selectively the molecular vibrational signature of the interface. The behaviour of the ODT SAM is studied in different environments: in air, in water, and upon exposure to hydrophilic or hydrophobic Lactococcus lactis bacteria. Modelling the experimental SFG spectra reveals a measurable change of the SAM conformation which depends on the environment, especially on the hydrophilic-hydrophobic character.

Résumé: The vibrational predissociation spectrum of the argon-tagged ionic cluster (NaCl)(32) due to exposure to a chirped infrared laser was theoretically calculated in a time multiscale framework. The response to the laser field is modeled explicitly by molecular dynamics simulations, and statistical rate theories are used to evaluate the long-time dissociation probability. Due to anharmonicities and multiphoton processes, the predissociation spectrum shows some small but systematic red shift in the main absorption line. Negative chirping toward lower frequencies further enhances this effect, and also alters the band intensity and the band width. Positive chirping attenuates the dissociation efficiency.

Résumé: Context. Carbonaceous cosmic dust is observed through infrared spectroscopy either in absorption or in emission. The details of the spectral features are believed to shed some light on its structure and finally enable the study of its life cycle. Aims. The goal is to combine several analytical tools in order to decipher the intimate nanostructure of some soot samples. Such materials provide interesting laboratory analogues of cosmic dust. In particular, spectroscopic and structural characteristics that help to describe the polyaromatic units embedded into the soot, including their size, morphology, and organisation are explored. Methods. Laboratory analogues of the carbonaceous interstellar and circumstellar dust were produced in fuel-rich low-pressure, premixed and flat flames. The soot particles were investigated by infrared absorption spectroscopy in the 2-15 μm spectral region. Raman spectroscopic measurements and high-resolution transmission electron microscopy were performed, which offered complementary information to better delineate the intimate structure of the analogues. Results. These laboratory analogues appeared to be mainly composed of sp(2) carbon, with a low sp(3) carbon content. A cross relation between the positions of the aromatic C=C bands at about 6.2 micron and the band at about 8 micron is shown to trace differences in shapes and structures of the polyaromatic units in the soot. Such effects are due to the defects of the polyaromatic structures in the form of non-hexagonal rings and/or aliphatic bridges. The role of these defects is thus observed through the 6.2 and 7.7 μm aromatic infrared band positions, and a distinction between carriers composed of curved aromatic sheets and more planar ones can be inferred. Based on these nanostructural differences, a scenario of nanograin growth and evolution is proposed.

Résumé: Context. Sticking of H and D atoms on interstellar dust grains is the first step in molecular hydrogen formation, which is a key reaction in the interstellar medium. Isotopic properties of the sticking can have an incidence on the observed HD molecule. Aims. After studying the sticking coefficients of H-2 and D-2 molecules on amorphous silicate surfaces experimentally and theoretically, we extrapolate the results to the sticking coefficient of atoms and propose a formulae that gives the sticking coefficients of H and D on both silicates and icy dust grains. Methods. In our experiments, we used the King and Wells method for measuring the sticking coefficients of H-2 and D-2 molecules on a silicate surface held at 10 K. It consists of measuring with a QMS (quadrupole mass spectrometer) the signals of H-2 and D-2 molecules reflected by the surface during the exposure of the sample to the molecular beam at a temperature ranging from 20 K to 340 K. We tested the efficiency of a physical model, developed previously for sticking on water-ice surfaces. We applied this model to our experimental results for the sticking coefficients of H-2 and D-2 molecules on a silicate surface and estimated the sticking coefficient of atoms by a single measurement of atomic recombination and propose an extrapolation. Results. Sticking of H, D, HD, H-2, and D-2 on silicates grains behaves the same as on icy dust grains. The sticking decreases with the gas temperature, and is dependent on the mass of the impactor. The sticking coefficient for both surfaces and impactors can be modeled by an analytical formulae S (T) = S-0(1 + beta T/ T-0)/(1 + T/T-0)(beta), which describes both the experiments and the thermal distribution expected in an astrophysical context. The parameters S-0 and T-0 are summarized in a table. Conclusions. Previous estimates for the sticking coefficient of H atoms are close to the new estimation; however, we find that, when isotopic effects are taken into account, the sticking coefficient variations can be as much as a factor of 2 at T = 100 K.

Résumé: The failure of antibiotics to inactivate in vivo pathogens organized in biofilms has been shown to trigger chronic infections. In addition to mechanisms involving specific genetic or physiological cell properties, antibiotic sorption and/or reaction with biofilm components may lessen the antibiotic bioavailability and consequently decrease their efficiency. To assess locally and accurately the antibiotic diffusion-reaction, we used for the first time a set of advanced fluorescence microscopic tools (fluorescence recovery after photobleaching, fluorescence correlation spectroscopy, and fluorescence lifetime imaging) that offer a spatiotemporal resolution not available with the commonly used time-lapse confocal imaging method. This set of techniques was used to characterize the dynamics of fluorescently labeled vancomycin in biofilms formed by two Staphylococcus aureus human isolates. We demonstrate that, at therapeutic concentrations of vancomycin, the biofilm matrix was not an obstacle to the diffusion-reaction of the antibiotic that can reach all cells through the biostructure.

Résumé: Background and aims Multidrug resistance (MDR) limits effectiveness in treating malignancy by modifying internalisation and/or externalisation of drugs through cancer cell membranes. In this study we describe an assay to monitor patients' responses to chemotherapy. Methods The assay is based on the fluorescent properties of doxorubicin alone as well as in combination with methotrexate, vinblastine, doxorubicin and cisplatin (MVAC). The slide-based cell imaging technique was first optimised using a panel of breast and urothelial cancer cell lines and then extended to fine needle breast aspiration biopsy and urine cytology. Results The drug fluorescence behaviour observed on smears of clinical specimens is identical to that obtained using fixed cultured cells. The fluorescence of sensitive cells to chemotherapy is mainly localised in the nucleus, whereas resistant cells show a weak fluorescence signal localised in the cytoplasm. The difference in terms of fluorescence intensity is also highlighted through fluorescence spectra. Conclusions The results suggest that the assay provides clinically valuable information in predicting responses to doxorubicin and/or MVAC therapy. Originally set up on a confocal microscope, the assay was also effective using a standard epifluorescence microscope; as such it is technically simple, reliable and inexpensive.

Résumé: We report on experimental evidence for the formation of a two dimensional Si/Au(110) surface alloy. In this study, we have used a combination of scanning tunneling microscopy, low energy electron diffraction, Auger electron spectroscopy, and ab initio calculations based on density functional theory. A highly ordered and stable Si-Au surface alloy is observed subsequent to growth of a sub-monolayer of silicon on an Au(110) substrate kept above the eutectic temperature. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4735310]

Résumé: Electromagnetic coupling between plasmonic resonances in metallic nanoparticles allows for engineering of the optical response and generation of strong localized near-fields. Classical electrodynamics fails to describe this coupling across sub-nanometer gaps, where quantum effects become important owing to non-local screening and the spill-out of electrons. However, full quantum simulations are not presently feasible for realistically sized systems. Here we present a novel approach, the quantum-corrected model (QCM), that incorporates quantum-mechanical effects within a classical electrodynamic framework. The QCM approach models the junction between adjacent nanoparticles by means of a local dielectric response that includes electron tunnelling and tunnelling resistivity at the gap and can be integrated within a classical electrodynamical description of large and complex structures. The QCM predicts optical properties in excellent agreement with fully quantum mechanical calculations for small interacting systems, opening a new venue for addressing quantum effects in realistic plasmonic systems.

Résumé: The infrared optical response of amyloid fibrils A beta(1-40) is investigated. Simulations of two models corresponding to different protonation states are compared with experiment. The simulations reveal that vibrational frequency distributions inside the fibrils are dominated by side chain fluctuations. We further confirm earlier suggestions based on 2D-IR measurements that water molecules can be trapped inside the fibrils.

Résumé: Calculations of the half-width, its temperature dependence, and the line shift are made for the rotational states J = 0-120 for two of the Fermi-tetrad bands (30012 <- 00001 and 30013 <- 00001) of CO2 perturbed by N-2. The calculations employ the semi-classical complex Robert-Bonamy method with no ad hoc scaling, J-dependent or otherwise, and an intermolecular potential (IP) comprised of an electrostatic part, an atom-atom part, and an isotropic London dispersion part. The averaging over the impact parameter b and relative speed v are explicitly carried out. Many interesting features about CO2 as the radiating molecule are elucidated. Effects of the trajectory model, the order of the expansion of the atom-atom component of the potential, and the inclusion of the imaginary terms are studied. It is shown that the results are very sensitive to the intermolecular potential. The final IP parameters give results that demonstrate excellent agreement with measurement for the three line shape parameters studied in this work. (C) 2012 Elsevier Ltd. All rights reserved.

Résumé: The advent of milli-kelvin scanning tunneling microscopes (STM) with inbuilt magnetic fields has opened access to the study of magnetic phenomena with atomic resolution at surfaces. In the case of single atoms adsorbed on a surface, the existence of different magnetic energy levels localized on the adsorbate is due to the breaking of the rotational invariance of the adsorbate spin by the interaction with its environment, leading to energy terms in the meV range. These structures were revealed by STM experiments in IBM Almaden in the early 2000s for atomic adsorbates on CuN surfaces. The experiments consisted in the study of the changes in conductance caused by inelastic tunneling of electrons (IETS, inelastic electron tunneling spectroscopy). Manganese and Iron adatoms were shown to have different magnetic anisotropies induced by the substrate. More experiments by other groups followed up, showing that magnetic excitations could be detected in a variety of systems: e.g. complex organic molecules showed that their magnetic anisotropy was dependent on the molecular environment, piles of magnetic molecules showed that they interact via intermolecular exchange interaction, spin waves were excited on ferromagnetic surfaces and in Mn chains, and magnetic impurities have been analyzed on semiconductors. These experiments brought up some intriguing questions: the efficiency of magnetic excitations was very high, the excitations could or could not involve spin flip of the exciting electron and singular-like behavior was sometimes found at the excitation thresholds. These facts called for extended theoretical analysis: perturbation theories, sudden-approximation approaches and a strong coupling scheme successfully explained most of the magnetic inelastic processes. In addition, many-body approaches were also used to decipher the interplay between inelastic processes and the Kondo effect. Spin torque transfer has been shown to be effective in changing spin orientations of an adsorbate in theoretical works, and soon after it was shown experimentally. More recently, the previously mentioned strong coupling approach was extended to treat the excitation of spin waves in atomic chains and the ubiquitous role of electron-hole pair creation in de-exciting spins on surfaces has been analyzed. This review article expounds these works, presenting the theoretical approach by the authors while trying to thoroughly review parallel theoretical and experimental works. (C) 2012 Elsevier Ltd. All rights reserved.

Résumé: Lateral phase separations in biological membranes are of great interest, making electron spin resonance (ESR) spectroscopy combined with spin labeling a nondestructive and sensitive technique for the study of lipid rafts. It is currently accepted that the spin probe is localized on the plasma membrane. However, no study confirms this hypothesis. Herein, we report, for the first time, an accurate multispectral method for the quantification of the lipid spin label presence in every subcellular fraction. Cells were incubated with the 5-DOXYL stearic acid derivative and then subfractionated. Results of our multimodal spectroscopy approach ubiquitously demonstrate that the ESR spin label presence only sets in the plasma membranes.

Résumé: DFT/B3LYP calculations are used to analyse the occurrence of reverse isotope shift ratios (ISR) in H/D substitution of the free-base tetrapyrroles, in situations where the frequency ratio v(H)/v(D) is less than 1. The reverse ISR effect is found to be most evident in the out-of-plane bending modes (b(2g) and b(3u), symmetry) involving some N-H motion for the four molecules studied, viz., porphine (H2P), tetraaza-porphine (H(2)TAP), tetrabenzo-porphine (H2TBP), and phthalocyanine (H2Pc). It was analysed by following the evolution of the normal mode frequencies with incremental variation of the H atom masses from 1 to 2 amu. This method allows direct, unambiguous mode correlations to be established between the light and the heavy isotopologues. When the NH(D) motion is predominant, the H to D frequency evolution decreases in a continuous manner for a particular normal mode. In the case of two modes of the same symmetry and whose frequencies are similar, their frequency evolutions could cross, depending on the extent of NH(D) motion involved in them. The evolution diagrams may show avoided crossings of various extents, which thereby reflects the degree of the NH(D) motion in the modes. The reverse ISR effect is directly correlated to these avoided crossings. Because the isotope shifts are quite small (<10 cm(-1)) and occur in the congested 1500-500 cm(-1) spectral region, high-resolution methods yielding narrow line transitions are required for experimental analysis. The matrix isolation technique is particularly well suited for this work and is proposed for use in a search for this effect.

Résumé: The rotation-translation thermalization of CO2 gas is investigated 500 ps after its preheating by a nonresonant short and intense laser pulse. The temperature of thermalization is optically determined with two additional short laser pulses enabling a field-free molecular alignment process and its probing, respectively. The measurements are performed for various intensities of the preheat pulse, leading to the observation of different temperatures which are in very good agreement with classical molecular dynamics simulations. The results can be regarded as a step towards real-time tracking of ultrafast relaxation pathways in molecular motion.

Résumé: We report on the first (to our knowledge) demonstration of nonlinear dual-frequency-comb spectroscopy. In multi-heterodyne femtosecond Raman-induced Kerr-effect spectroscopy, the Raman gain resulting from the coherent excitation of molecular vibrations by a spectrally narrow pump is imprinted onto the femtosecond laser frequency comb probe spectrum. The birefringence signal induced by the nonlinear interaction of these beams and the sample is heterodyned against a frequency comb local oscillator with a repetition frequency slightly different from that of the comb probe. Such time-domain interference provides multiplex access to the phase and amplitude Raman spectra over a broad spectral bandwidth within a short measurement time. (C) 2012 Optical Society of America

Résumé: Wave-particle complementarity is one of the most intriguing features of quantum physics. To emphasize this measurement apparatus–dependent nature, experiments have been performed in which the output beam splitter of a Mach-Zehnder interferometer is inserted or removed after a photon has already entered the device. A recent extension suggested using a quantum beam splitter at the interferometer’s output; we achieve this using pairs of polarization-entangled photons. One photon is tested in the interferometer and is detected, whereas the other allows us to determine whether wave, particle, or intermediate behaviors have been observed. Furthermore, this experiment allows us to continuously morph the tested photon’s behavior from wavelike to particle-like, which illustrates the inadequacy of a naive wave or particle description of light.

Résumé: Magnetic inelastic electron tunneling spectroscopy (IETS) shows sharp increases in conductance when a new conductance channel associated with a change in magnetic structure is open. Typically, the magnetic moment carried by an adsorbate can be changed by collision with a tunneling electron; in this process the spin of the electron can flip or not. A previous one-electron theory [Phys. Rev. Lett. 103, 176601 (2009)] successfully explained both the conductance thresholds and the magnitude of the conductance variation. The elastic spin flip of conduction electrons by a magnetic impurity leads to the well-known Kondo effect. In the present work, we compare the theoretical predictions for inelastic magnetic tunneling obtained with a one-electron approach and with a many-body theory including Kondo-like phenomena. We apply our theories to a singlet-triplet transition model system that contains most of the characteristics revealed in magnetic IETS. We use two self-consistent treatments (noncrossing approximation and self-consistent ladder approximation). We show that, although the one-electron limit is properly recovered, new intrinsic many-body features appear. In particular, sharp peaks appear close to the inelastic thresholds; these are not localized exactly at thresholds and could influence the determination of magnetic structures from IETS experiments. Analysis of the evolution with temperature reveals that these many-body features involve an energy scale different from that of the usual Kondo peaks. Indeed, the many-body features perdure at temperatures much larger than the one given by the Kondo energy scale of the system.

Résumé: The control of the dissociative adsorption of individual hydrogen molecules is performed on the silicon surface at the atomic scale. It is achieved using the tip of a low-temperature (9 K) scanning tunneling microscope (STM) exposed to 10(-6) torr of H-2 and by probing the bare Si(100)-2 x 1 surface at a positive bias. This effect is very localized and is induced by the tunnel electrons. The statistical study of this process reveals an activation energy threshold matching the creation of H-2(-) at the surface of the STM tip. Our results are supported by ab inito density functional calculations of a hydrogenated silicon dimer.

Résumé: The complex Robert-Bonamy (CRB) formalism was used to calculate the half-width, its temperature dependence, and the line shift for CO2 for transitions in the 30012 <- 00001 and 30013 <- 00001 bands with O-2 as the perturbing gas. The calculations were done for rotational quantum numbers from J=0 to J=120 with no ad hoc scaling of the line shape equations. The intermolecular potential parameters are adjusted on accurate experimental measurements of the half-widths, its temperature dependence, and the pressure-induced line shifts so that a single intermolecular potential reproduces all three parameters. Using the results of this work and previous results for N-2-broadening, air-broadening line shape parameters were also determined. The comparison of the CRB calculations with the experimental data available in the literature for the three line shape coefficients demonstrates the quality of the present calculations for the both bands under study. (C) 2012 Elsevier Ltd. All rights reserved.

Résumé: Experimental and theoretical results for molecular-frame photoemission are presented for inner-valence shell photoionization of the CO molecule induced by linearly and circularly polarized light. The experimental recoil frame photoelectron angular distributions (RFPADs) obtained from dissociative photoionization measurements where the velocities of the ionic fragment and photoelectron were detected in coincidence, are compared to RFPADs computed using the multichannel Schwinger configuration interaction method. The formalism for including a finite lifetime of the predissociative ion state is presented for the case of general elliptically polarized light, to obtain the RFPAD rather than the molecular frame photoelectron angular distribution (MFPAD), which would be obtained with the assumption of instantaneous dissociation. We have considered photoionization of CO for the photon energies of 26.0 eV, 29.5 eV, and 32.5 eV. A comparison of experimental and theoretical RFPADs allows us to identify the ionic states detected in the experimental studies. In addition to previously identified states, we found evidence for the 2 (2)Delta state with an ionization potential of 25.3 eV and (2)Sigma(+) states with ionization potentials near 32.5 eV. A comparison of the experimental and theoretical RFPADs permits us to estimate predissociative lifetimes of 0.25-1 ps for some of the ion states. Consideration of the MFPADs of a series of (2)Pi ion states indicates the importance of inter-channel coupling at low photoelectron kinetic energy and the limitations of a single-channel analysis based on the corresponding Dyson orbitals. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3681920]

Résumé: Double ionization of nitrogen molecules is investigated in coplanar asymmetric (e,3-1e) and (e,3e) experiments. The measurements are performed at intermediate incident energy (similar to 600 eV) with one fast scattered electron and two slower ejected electrons sharing equally the excess energy. The data are compared with the predictions of a simple kinematical model recently reported by Lahmam-Bennani et al (2010 J. Phys. B: At. Mol. Opt. Phys. 43 105201) which describes the 'two-step 2' (TS2) mechanism as two successive (e,2e) single ionization interactions. The model is extended to include binary as well as recoil scattering during the two successive single ionization events. It is shown to qualitatively predict the correct angular positions for most of the observed structures. Moreover, the (e,3e) data are compared with the predictions of a first Born model which fails to reproduce the experimental angular distributions. These results demonstrate that the molecular double ionization process is largely dominated by the TS2 mechanism, as was the case in our previous works on atomic targets.

Résumé: The photochemistry of the chelated enol form of acetylacetone (AcAc) was investigated by UV excitation of the S2 state at 266 nm in parahydrogen matrices, complemented by experiments in neon and normal hydrogen matrices. Infrared (IR) spectroscopy, combined with theoretical calculations, was used to identify the photoproducts. Isomerization towards various non-chelated forms (no intramolecular H-bond) of AcAc is the dominant channel whereas fragmentation is very minor. The isomerization kinetics is monitored by IR spectroscopy. Among the seven non-chelated conformers of AcAc, only three are formed in parahydrogen matrices, whereas four are observed in normal hydrogen matrices. This difference suggests that an active tunnelling process between conformers occurs in parahydrogen but is quenched in normal hydrogen where guest-host interactions are stronger. Fragmentation and isomerization of excited AcAc are discussed in the light of these new data. The role of the intermediate triplet state in the S-2 -> S-0 relaxation is confirmed, as the importance of phonons in the condensed phase.

Résumé: The mechanisms of nonradiative deactivation of a phenylalanine residue after near-UV photoexcitation have been investigated in an isolated peptide chain model (N-acetylphenylalaninylamide, NAPA) both experimentally and theoretically. Lifetime measurements at the origin of the first pipi* state of jet-cooled NAPA molecules have shown that (i) among the three most stable conformers of the molecule, the folded conformer NAPA B is approximately 50-times shorter lived than the extended major conformer NAPA A and (ii) this lifetime is virtually insensitive to deuteration at the NH(2) and NH sites. Concurrent time-dependent density functional theory (TDDFT) based nonadiabatic dynamics simulations in the full dimensionality, carried out for the NAPA B conformer, provided direct insights on novel classes of ultrafast deactivation mechanisms, proceeding through several conical intersections and leading in fine to the ground state. These mechanisms are found to be triggered either (i) by a stretch of the N(Phe)H bond, which leads to an H-transfer to the ring, or (ii) by specific backbone amide distortions. The potential energy surfaces of the NAPA conformers along these critical pathways have been characterized more accurately using the coupled cluster doubles (CC2) method and shown to exhibit barriers that can be overcome with moderate excess energies. These results analyzed in the light of the experimental findings enabled us to assign the short lifetime of NAPA B conformer to a number of easily accessible exit channels from the initial pipi* surface, most importantly the one involving a transfer of electronic excitation to an npi* surface, induced by distortions of the backbone peptide bond.

Résumé: Pure rotational transitions of OH and SH radicals have been recorded in the THz spectral range using cw-THz and synchrotron-based FT-FIR techniques. Line lists on these radicals have been completed in the three and two lowest vibrational states for OH and SH, respectively. Furthermore, the hyperfine structure of OH and SH has been observed for the first time using infrared IR FT-spectroscopy, and at frequencies higher than 1 THz, respectively. A combined fit has been made for each of these radicals including v = 0, 1 and 2 for OH and v = 0 and 1 for SH. (c) 2012 Elsevier B.V. All rights reserved.

Résumé: We report an experimental study of the temporal duration of a transient pumping extreme ultraviolet (XUV) laser emitted at 18.9 nm, using an ultrafast x-ray streak camera. We have investigated the shot-to-shot reproducibility of the pulse duration, as well as its behaviour as a function of several pumping parameters. Our results show that the pulse duration increases slowly with the pump pulse duration, in agreement with previous observations performed in a different geometry. The angle of the pump laser relative to the target surface also affects the XUV laser duration in a measurable way. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4757878]

Résumé: The adsorption and island formation of C-60 fullerenes on the hydrogenated C(100)-(2 x 1):H diamond surface is studied using high-resolution noncontact atomic force microscopy in ultrahigh vacuum. At room temperature, C-60 fullerene molecules assemble into monolayer islands, exhibiting a hexagonally close-packed internal structure. Dewetting is observed when raising the substrate temperature above approximately 505 K, resulting in two-layer high islands. In contrast to the monolayer islands, these double-layer islands form extended wetting layers. This peculiar behavior is explained by an increased molecule-substrate binding energy in the case of double-layer islands, which originates from charge transfer doping. Only upon further increasing the substrate temperature to approximately 615 K, the wetting layer desorbs, corresponding to a binding energy of the charge transfer-stabilized film of 1.7 eV.

Résumé: We have investigated the self-assembly of didecyldiselenide on gold containing mercury using X-ray photoelectron spectroscopy, cyclic voltammetry and infrared spectroscopy. The analysis of intensity and chemical shift of selected Se, Hg, and Au photoelectron lines on samples with increasing Hg content, show that didecyldiselenide adsorption strongly contributed to segregation of bulk Hg to the surface. The voltammetry results support this conclusion and suggest the formation of Hg-Au surface amalgam. The Hg surface segregation effect must be related to the restructuring of the surface following initial adsorption, and to the strong selenophilicity of Hg. The reflectance absorbance infrared spectroscopy studies show that the molecular layer on Hg-Au substrates lacks good order.

Résumé: A model calculating the laser fields at a flat structureless surface taking into account the surface photoelectric effect is presented. The photon is p or transverse magnetic linearly polarized, continuous and its wave length is long, i.e. lambda(vac) >= 12.4 nm. The sharp rise of the electron density at the interface generates an atomic scale spatial dependence of the laser field. In real space and in the temporal gauge, the vector potential A of the laser is obtained as a solution of the classical Ampere-Maxwell and the material equations. The susceptibility is a product of the electron density of the material system with the surface and of the bulk tensor and non-local isotropic (TNLI) polarizability. The electron density is obtained quantum mechanically by solving the Schrodinger equation. The bulk TNLI polarizability including dispersion is calculated from a Drude-Lindhard-Kliewer model. In one dimension perpendicular to the surface the components A(x)(z,omega) and A(z)(z,omega) of the vector potential are solutions of the Ampere-Maxwell system of two coupled integro-differential equations. The model, called vector potential from the electron density-coupled integro-differential equations (VPED-CIDE), is used here to obtain the electron escape probability from the power density absorption, the reflectance, the electron density induced by the laser and Feibelman's parameters d(parallel to) and d(perpendicular to). Some preliminary results on aluminium surfaces are given here and in a companion paper the photoelectron spectra are calculated with results in agreement with the experiment.

Résumé: In this work we report on the ionization of a gaseous neon target by combining extreme ultraviolet (XUV) radiation from the Free Electron Laser in Hamburg with an intense synchronized optical laser. Applying energy-resolved photoelectron spectroscopy, the dependence of the electrons ejected from singly charged neon (Ne+) on the relative polarization of the XUV and optical laser fields was investigated. The electron spectra exhibit a strong dependence on the degree of the relative orientation of the linear polarization vectors. A quantitative analysis was undertaken using multichannel time-dependent perturbation theory in order to reproduce the magnitude of the ejected low kinetic energy electrons as a function of the relative polarization directions and explicitly taking into account the influence of the residual Ne2+ cores (D-1, P-3). It is concluded that the variation of the photoelectron spectrum with the deviation of XUV and optical field directions is mainly related to the different responses of the magnetic substates of the ejected electrons in their interaction with the laser field.

Résumé: A simple approximate trial wavefunction is used to study, within a variational approach and beyond the Born-Oppenheimer approximation, the molecular ion H-2(+) confined by impenetrable spherical surfaces. This wavefunction describes explicitly the motion of the nuclei and the electron of this three-body system in its lowest rovibrational state. Two cut-off functions are introduced to fulfil the Dirichlet boundary conditions, respectively, for the electron and the nuclei. The results show an increase of the energy of the ion when both the confinement radius r(ce) of the electron and R-cN of the nuclei become smaller. The electron constraint is found to be much more efficient to increase the energy than the nuclei confinement. This study shows that a metastable bound state of the ion, above the energy corresponding to the dissociation limit, can be obtained when the electron constraint is present. Expectation values of the internuclear and electron-to-origin distances are also calculated and show a rapid decrease when the confinement is stronger. Finally, the quantum pressure versus the confinement radius is also estimated. A rapid rise of the pressure value when r(ce) is less than 3 au is observed.

Résumé: Using synchrotron-generated vacuum-ultraviolet radiation and multiplexed time-resolved photoionization mass spectrometry we have measured the absolute photoionization cross-section for the propargyl (C(3)H(3)) radical, sigma(propargyl) (ion)(E), relative to the known absolute cross-section of the methyl (CH(3)) radical. We generated a stoichiometric 1:1 ratio of C(3)H(3):CH(3) from 193 nm photolysis of two different C(4)H(6) isomers (1-butyne and 1,3-butadiene). Photolysis of 1-butyne yielded values of sigma(propargyl)(ion)(10.213 eV)=(26.1+/-4.2) Mb and sigma(propargyl)(ion)(10.413 eV)=(23.4+/-3.2) Mb, whereas photolysis of 1,3-butadiene yielded values of sigma(propargyl)(ion)(10.213 eV)=(23.6+/-3.6) Mb and sigma(propargyl)(ion)(10.413 eV)=(25.1+/-3.5) Mb. These measurements place our relative photoionization cross-section spectrum for propargyl on an absolute scale between 8.6 and 10.5 eV. The cross-section derived from our results is approximately a factor of three larger than previous determinations.

Résumé: The dependence of the inelastic lifetime of electrons in the first n = 1 image-potential state of clean and rare-gas covered Ag(111), Cu(111), and Cu(100) surfaces on their momentum parallel to the surface has been studied experimentally by means of time-and angle-resolved two-photon photoemission spectroscopy (2PPE) and theoretically by calculations based on the many-body theory within the self-energy formalism. Similar to the previously studied clean Cu(100) surface, the theoretical results are in excellent agreement with the experiment findings for Cu(111). For Ag(111), the theory overestimates the decay rate and its momentum dependence, which is attributed to the neglect of surface plasmon excitations. With increasing parallel momentum, the n = 1 state shifts out of the projected bulk band gap on both surfaces and turns into an image-potential resonance. This opens an additional decay channel by resonant electron transfer into the bulk, which is theoretically treated by the application of the wave packet propagation approach. The expected stronger increase of the decay rate upon crossing the edge of the band gap, however, is not observed in the experiment. The decoupling of the image-potential states from the metal surface upon adsorption of rare-gas layers results in a decrease of the decay rate as well as of its momentum dependence by a similar factor, which can be successfully explained by the change of interband and intraband contributions to the total decay rate.

Résumé: A highly sensitive integrated optical biosensor based on the cascade of a Fabry-Perot cavity laser and a microring resonator is investigated theoretically and experimentally. The free spectral ranges of the microring and the laser cavity are designed to be about the same in order to apply intensity interrogation. The variation of the refractive index of the fluidic sample results in a large shift in the envelope function of the total transmission spectrum of the sensor, and consequently, a large change in the transmitted power. The simple light-source-integrated sensing scheme achieved a high sensitivity of 1000 dB/RIU in the preliminary experiment, demonstrating its principle and potential for low-cost practical applications.

Résumé: Density functional theory (DFT) calculations are used to investigate the stability on SiC(0001) surfaces of different chemical groups -NH2, -NO2, -CH3, -OH, -SH and -CN. The adsorption stability decreases in the order -NO2 > -OH > -NH2 > -SH > -CN > -CH3. The stability of the single molecule-substrate bond is strongly influenced by the polarizability, which in turn depends on different parameters such as the electronegativity, atomic size and chemical environment. In a further step, methyl (-ACH(3)) and phenyl (-AC(6)H(5)) substituted groups are also considered and similar behaviour is observed. The inductive effect of the -CH3 or -C6H5 groups modifies the polarization of the Si adatom-molecule bond and the steric hindrance due to their size influences the molecular orientation. These two parameters affect the calculated adsorption energy, and are more important for -C6H5 substituent. This study provides clear tendencies that can be applied to more complex systems. Comparison of the adsorption of two large molecules, H2Pc (metal-free phthalocyanine) and PTCDI (perylene tetracarboxylic diimide) on the SiC(0001) surface is presented as an example. (C) 2012 Elsevier B.V. All rights reserved.

Résumé: We perform dynamical calculations on two robust N-2-N-2 potential energy surfaces in order to intercompare pressure broadening coefficients derived from close coupling and coupled states quantum dynamical methods, the semi-classical model of Robert and Bonamy and a full classical method. The coupled states and full classical results compare well with the experimental results or with close coupling values when available. This study confirms that the classical method is a good alternative at room and high temperatures to quantum dynamical methods. The results obtained using the semi-classical method however deviate from the other sets of data at all temperatures considered here (77-2400 K). (C) 2012 Elsevier Ltd. All rights reserved.

Résumé: A strong visible a(similar to 3)Sigma(+)(u) – X-similar to 1 Sigma(+)(g) luminescence was observed upon UV excitation of cryogenic rare gas (argon, krypton, and xenon) matrices doped with dicyanoacetylene (NC4N). Spectra and life-times of this phosphorescence have been measured. A detailed analysis of resolved vibronic bands is presented. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4745884]

Résumé: We report the measurement of the linewidth and temporal coherence of a lambda=46.9 nm neon-like argon capillary discharge soft-x-ray laser and its variation with plasma column length. A wave-front-division interferometer was used to resolve the 3p S-1(0)-3s P-1(1) laser line, resulting in measured relative linewidths of Delta lambda/lambda = (3-4) x 10(-5). The measurements do not observe saturation rebroadening when this clearly dominantly Doppler-broadened inhomogeneous line is amplified beyond the intensity corresponding to gain saturation. Model simulations indicate this is the result of a comparatively small collisional broadening that sufficiently homogenizes the line profile to practically eliminate inhomogeneous saturation rebroadening. Collisional redistribution is computed to play only a very minor role in homogenizing the line profile.

Résumé: Using density functional theory (DFT) we report results for the electronic structure and vibrational dynamics of hydrogenated silicon carbide (001) (3 x 2) surfaces with various levels of hydrogenation. These results were obtained using density functional theory with a generalized gradient exchange correlation function. The calculations reveal that metallization can be achieved via hydrogen atoms occupying the second silicon layer. Further increase of hydrogen occupation on the second silicon layer sites results in a loss of this metallization. For the former scenario, where metallization occurs, we found a new vibrational mode at 1870 cm(-1), which is distinct from the mode associated with hydrogen atoms on the first layer. Furthermore, we found the diffusion barrier for a hydrogen atom to move from the second to the third silicon layer to be 258 meV. (c) 2012 Elsevier B.V. All rights reserved.

Résumé: Terahertz absorption spectroscopy and far infrared Fourier transform (FIR-FT) emission spectroscopy were employed to measure new rotational and ro-vibrational spectra and validate existing datasets for high temperature water, with the goal to improve the accuracy of predictions for pure rotation in the (0 2 0), (1 0 0) and (0 0 1) vibrational states. A total of 104 new rotational transitions in (0 0 0), (0 1 0), (0 2 0), (1 0 0) and (0 0 1) were observed in the 293-2723 GHz region with MW accuracy and with observed J(Max) = 18, 11, 10, 14, 10; K-a(Max)= 10, 7, 7, 7, 4; E ''(Max)= 4178, 3771, 4996, 5430, 4939 cm(-1), respectively. A total of 4194 new FIR-FT transitions were assigned in the 50-600 cm(-1) region to the same five vibrational states with estimated experimental uncertainty of 0.0008-0.002 cm(-1) and with J(Max) = 27; K-a(Max) = 21, 18, 18, 17, 17, respectively. Together with previous high-resolution observations, these new data were fitted with the Bending-Rotation approach and the Euler Hamiltonian. The new measurements and predictions reported here will support the analyses of astronomical observations by high-resolution spectroscopy telescopes such as Herschel, SOFIA, and ALMA. (c) 2012 Elsevier Inc. All rights reserved.

Résumé: We study theoretically the quantum well states (QWSs) localized in Pb overlayers on Cu(111) surface. Particular emphasis is given to the states with energies close to the vacuum level. Inclusion of the long-range image potential tail into the model potential description of the system allows us to show the effect of hybridization between QWSs and image potential states (ISs). The particle-in-a-box energy sequence characteristic for QWSs evolves into the Rydberg series converging towards the vacuum level. The electron density of the corresponding states is partially moved from inside the metal overlayer into the vacuum. The decay rates due to the inelastic electron-electron scattering decrease with increasing energy, opposite to “conventional” QWSs and similar to the ISs. Many-body and wave packet propagation calculations of the inelastic decay rates are supplemented by simple analysis based on the phase accumulation model and wave-function penetration approximation. This allows an analytical description of the dependence of the QWS/ISs hybridization on different parameters and, in particular, on the overlayer thickness.

Résumé: We compare advantages and drawbacks of the two techniques mainly used for the experimental study of gas phase photoionization processes on ionic targets in combination with synchrotron radiation, the merged beam and ion trap. The comparison will be illustrated by recent results obtained on atomic and small molecular positively charged ions using both techniques at SOLEIL, the French synchrotron radiation facility.

Résumé: K-shell photoionization cross-section measurements for singly ionized atomic nitrogen made at the SOLEIL Light Source for the first time are compared with MCDF and R-matrix calculations.

Résumé: Dual-comb spectroscopy is extended to the visible range. The dense rovibronic spectrum of iodine around 520 nm is measured within 12 ms at Doppler-limited resolution with a scheme that only uses free-running lasers.

Résumé: A new scheme of real-time dual-comb spectroscopy that only uses free-running femtosecond lasers provides distortion-free Fourier spectra with resolved comb lines across the full spectral span of erbium-doped fiber lasers (>80 nm centered around 1550 nm) without a posteriori data processing.

Résumé: We present mid-infrared frequency comb generation from crystalline MgF2 microresonators, with mode spacing of 50-150 GHz around the 2.5 μm-wavelength CW-pump. Low phase-noise is verified by beating the comb modes with a narrow line width CW laser.