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International audience; Two-color air plasma based broadband terahertz (THz) generation and subsequent air biased coherent detection (ABCD) of the THz field are presented. Both source and detection system are characterized experimentally and numerically, yielding excellent agreement of measured and simulated signals. We reveal that it is crucial to model the whole optical setup and include the various pump distortions, like temporal and spatial walk-off as well as ellipticity of polarization, in order to interpret the experimental measurements correctly. Moreover, it turns out that geometrical effects in the ABCD scheme shape the recorded THz spectra and need to be taken into account. We confirm that THz electric fields with peak amplitude in the MV/cm range and large spectral bandwidth are produced, allowing us to demonstrate THz spectroscopy with molecular samples.

Following ionization by an extreme ultraviolet (XUV) attosecond pulse train, a polyatomic molecule can be promoted to more-than-one excited states of the residual ion. The ensuing relaxation dynamics is often facilitated by several reaction coordinates, making them difficult to disentangle by the usual spectroscopic means. Here, we show that in atto-chemistry isotope labeling can be an efficient tool for unraveling the relaxation pathways in highly excited photoionized molecules. Employing an XUV pump pulse and a near-infrared probe pulse, we found the nuclear as well as coupled electron-nuclear dynamics in ethylene to be almost 40% faster compared to that of its deuterated counterpart. The findings, which are supported by advanced nonadiabatic dynamics calculations, led to the identification of the relevant nuclear coordinates controlling the relaxation. Our experiment highlights the relevance of ultrashort XUV pulses to capture the isotopic effect in few-femtosecond molecular photodynamics.

"We present an interferometric HHG-2 scheme and compare it to the usual XUV-IR RABBIT method that is widely used in attosecond science. Both methods are able to reconstruct the properties of an attosecond pulse train and can be used to measure attosecond ionization time delays in atoms and molecules. While they have several similarities, they also have conceptual differences. Here, we present some particularities of the HHG-2 method and its advantages and drawbacks, which would help to define situations where it can provide information inaccessible by other technics. Keywords: Attosecond, Photoionization, RABBIT "

Imaging in real time the complete dynamics of a process as fundamental as photoemission has long been out of reach due to the difficulty of combining attosecond temporal resolution with fine spectral and angular resolutions. Here, we achieve full decoding of the intricate angle-dependent dynamics of a photoemission process in helium, spectrally and anisotropically structured by twophoton transitions through intermediate bound states. Using spectrally- and angularly-resolved attosecond electron interferometry, we characterize the complex-valued transition probability amplitude towards the photoelectron quantum state. This allows reconstructing in space, time and energy the complete formation of the photoionized wavepacket., Comment: 7 pages, 4 figures

International audience; Quantized nano-objects offer a myriad of exciting possibilities for manipulating electrons and light that impact photonics, nanoelectronics, and quantum information. In this context, ultrashort laser pulses combined with nanotips and field emission have permitted renewing nano-characterization and control electron dynamics with unprecedented space and time resolution reaching femtosecond and even attosecond regimes. A crucial missing step in these experiments is that no signature of quantized energy levels has yet been observed. We combine in situ nanostructuration of nanotips and ultrashort laser pulse excitation to induce multiphoton excitation and electron emission from a single quantized nano-object attached at the apex of a metal nanotip. Femtosecond induced tunneling through well-defined localized confinement states that are tunable in energy is demonstrated. This paves the way for the development of ultrafast manipulation of electron emission from isolated nano-objects including stereographically fixed individual molecules and high brightness, ultrafast, coherent single electron sources for quantum optics experiments.

International audience; Unraveling ultrafast processes induced by energetic radiation is compulsory to understand the evolution of molecules under extreme excitation conditions. To describe these photo-induced processes, one needs to perform time-resolved experiments to follow in real time the dynamics induced by the absorption of light. Recent experiments have demonstrated that ultrafast dynamics on few tens of femtoseconds are expected in such situations and a very challenging task is to identify the role played by electronic and nuclear degrees of freedom, charge, energy flows and structural rearrangements. Here, we performed time-resolved XUV-IR experiments on diamondoids carbon cages, in order to decipher the processes following XUV ionization. We show that the dynamics is driven by two timescales, the first one is associated to electronic relaxation and the second one is identified as the redistribution of vibrational energy along the accessible modes, prior to the cage opening that is involved in all fragmentation mechanisms in this family of molecules.

With the advent of attoscience, it becomes possible to measure the ionization time delay of an electron that escapes its original quantum system after absorbing an XUV photon. It is well established that RABBIT (Re-construction of Attosecond Beating By Interference of Two-photon transitions) protocol is sensitive to ionization time delays with an attosecond accuracy. This technique offers a dual spectral and temporal resolution with a simple analysis when the channels are well separated [1] . It has mainly been applied to systems with few active electronic states. Only recently, RABBIT has successfully been applied to a gaseous/liquid water mixture where strong spectral congestion takes place [2] .

Modern ultrashort X-ray/XUV (extreme ultraviolet) sources provide unique opportunities to investigate the primary reactions of matter upon energetic excitation. Understanding these processes in molecules on ultrafast timescales is required to improve bespoke high-energy radiation detectors, nanomedicine schemes or to study the molecular composition of interstellar media. However, current experiments struggle to provide a general framework because of the uniqueness and complexity of each system. Here we show the universal role of correlation bands—features created by electron correlation. This is done by studying ultrafast energy relaxation of size-scalable two-dimensional molecules following ionization by an ultrashort XUV pulse. We observed long lifetimes that nonlinearly increase with the number of valence electrons. A general law based on solid-like electron–phonon scattering is proposed, which explains both our results and previously reported measurements. This offers new opportunities in attosecond science and high-energy photophysics. The size-dependent lifetimes observed in the ultrafast molecular relaxation dynamics of an entire class of polycyclic aromatic hydrocarbons can be explained by correlation bands and electron–phonon scattering, reminiscent of solid-state systems.

We have investigated photoionization delays in N2 by combining an extreme ultraviolet (XUV) attosecond pulse train generated by high harmonic generation (HHG) and a second harmonic femtosecond pulse with angularly resolved photoelectron spectroscopy. While photoionization delay measurements are usually performed by using a standard XUV-infrared scheme, here we show that the present approach allows us to separate electronic states that otherwise would overlap, thus avoiding the spectral congestion found in most molecules. We have found a relative delay between the X and A ionic molecular states as a function of the photon energy of up to 40 attoseconds, which is due to the presence of a shape resonance in the X channel. This approach can be applied to other small quantum systems with few active electronic states.

International audience; This paper presents a software solution to automatically lock and optimize the carrier envelope phase (CEP) of a femtosecond titanium:sapphire (Ti:Sa) based oscillator following the standard optical arrangement. The solution also mitigates the usual limited-range actuator problem of the slow feedback loop. The principle is to control the relevant stabilization parameters with a single computer to mimic the manual action. After calibration of the piezoelectric actuator, four preliminary measurements are required to locate the CEP-locking point. Then, the algorithm optimizes the CEP performance. Only a few seconds are required to automatically lock and optimize the CEP.

Synopsis We have investigated the photoionization dynamics of several molecular systems using attosecond in-terferometric techniques based on ultrashort extreme ultraviolet (XUV) pulses. For diatomic molecules such as N2, we can get a detailed picture of the photoionization process by measuring the attosecond delays. For polyatomic molecules such as naphthalene, such measurements can provide the average timescale over which photoionization processes at different electron energies can occur in complex quantum systems.

International audience; Femtosecond time-resolved velocity map ion imaging experiments are reported on the second absorption band (B-band) of ethyl iodide at 201.19 and 200.08 nm, corresponding to the 000 and 1810 transitions, i.e., the origin of the band and the first most intense vibronic state assigned to one quantum of excitation in the methyl torsion mode. Electronic predissociation lifetimes and the temporal evolution of the anisotropy have been determined by time-resolved resonance-enhanced multiphoton ionization of iodine and ethyl fragment images. A shorter lifetime measured at the origin of the band in comparison with methyl iodide indicates that predissociation in ethyl iodide is more favorable due to a stronger coupling between the initial Rydberg state and the valence repulsive state correlating with the dissociation fragments. Moreover, vibrational activity in the methyl torsion in the Rydberg state seems to enhance the probability of transfer of population to the valence repulsive state leading to a faster dissociation. The perpendicular character of the transition at early times and the loss of anisotropy as a function of time have been determined from the time-resolved angular distributions of the iodine and ethyl ion images. The initial anisotropy value is consistent with a purely perpendicular transition compatible with the excitation of the [6A′′, 7A′] states with a minor parallel component to the C–I bond. The loss of initial anisotropy over time highlights the parent molecular rotation during predissociation and is compatible with a rotational temperature of the parent molecule of 100 K.

The Reconstruction of Attosecond Harmonic Beating By Interference of Two-photon Transition (RABBITT) method [1–3] is nowadays routinely used in attosecond physics to retrieve the temporal profile of an attosecond pulse train (APT) generated by high harmonic generation. It is employed to measure the ionization/Wigner time [4] of an escaping electron that scatters in the potential of its parent atom or molecule [5]. The RABBITT signal is obtained by measuring the time-resolved photoelectron spectrum of a given target that interacts with an APT (composed of odd harmonics of the fundamental laser frequency (ω 0 ) dressed by a femtosecond pulse at the frequency ω 0 . In the spectral domain, several quantum paths lead to the same electron kinetic energy (Fig. 1.(c)) these paths interfere leading to an oscillation of the electron yield at twice the fundamental frequency with a phase that depends on the (i) phase of the harmonic, (ii) the phase of the IR transition (so called continuum-continuum phase) and (iii) the atomic or molecular phase that corresponds to the transition induced by the XUV. In the temporal domain, one attosecond pulse appears for each half optical cycle of the dressing pulse as shown in Fig. 1(a).

Field Electron Emission (FEE) is one of the earliest tool for surface science characterization and occurs when a strong negative electric field is applied. Field Ion Emission, obtained with opposite bias voltage, was even the first process allowing the direct observation of individual atoms in 1955, more than 25 years before the scanning tunneling microscope. FEE is at the heart of modern characterization instruments in nanoscience such as scanning electron microscopy, transmission electron microscopy or far field scanning tunneling microscopy. Recently, several experiments on sharp metallic nano-tips irradiated by femtosecond laser have shown the great potential of femtosecond laser assisted FEE (FLAFEE) to access the ultra-fast electronic dynamics of nano-objects [1–4].

International audience; Understanding processes induced by XUV excitation of Polycyclic Aromatic Hydrocarbons (PAHs) is at the heart of molecular astrophysics, which aims at understanding molecular evolution in interstellar media. We used ultrashort XUV pulses to produce highly excited PAHs cations. The photo-induced dynamics is probed using a pump-probe XUV-IR spectroscopy. By studying PAH from small (naphthalene) to large (hexabenzocoronene) PAHs, we show that the dynamic is governed by the large density of states, in which many-body quantum effects are dominant.

International audience; Rapid energy transfer from electronic to nuclear degrees of freedom underlies many biological processes and astrophysical observations. The efficiency of this energy transfer depends strongly on the complex interplay between electronic and nuclear motions. In this study, we report two-color pump–probe experiments that probe the relaxation dynamics of highly excited cationic states of naphthalene, a prototypical polycyclic aromatic hydrocarbon molecule, which are produced using wavelength-selected, ultrashort extreme ultraviolet pulses. Surprisingly, the relaxation lifetimes increase with the cationic excitation energy. We postulate that the observed effect is the result of a population trapping that leads to delayed relaxation.

The many-body quantum nature of molecules determines their static and dynamic properties, but remains the main obstacle in their accurate description. Ultrashort extreme ultraviolet pulses offer a means to reveal molecular dynamics at ultrashort timescales. Here, we report the use of time-resolved electron-momentum imaging combined with extreme ultraviolet attosecond pulses to study highly excited organic molecules. We measure relaxation timescales that increase with the state energy. High-level quantum calculations show these dynamics are intrinsic to the time-dependent many-body molecular wavefunction, in which multi-electronic and non-Born−Oppenheimer effects are fully entangled. Hints of coherent vibronic dynamics, which persist despite the molecular complexity and high-energy excitation, are also observed. These results offer opportunities to understand the molecular dynamics of highly excited species involved in radiation damage and astrochemistry, and the role of quantum mechanical effects in these contexts., The many-body quantum nature of molecules determines their static and dynamic properties, but remains the main obstacle in their accurate description. Here, the authors employ ultrafast spectroscopic methods to explore the dynamics of highly excited organic molecules, revealing many-body effects and hints of coherent vibronic dynamics which persist despite their molecular complexity.

Synopsis The efficiency of energy transfer in ultrafast electronic relaxation of molecules depends strongly on the complex interplay between electronic and nuclear motion. In this study we use wavelength-selected XUV pulses to induce relaxation dynamics of highly excited cationic states of naphthalene. Surprisingly, the observed relaxation lifetimes increase with the cationic excitation energy. We propose that this is a manifestation of a quantum mechanical population trapping that leads to delayed relaxation of molecules in the regions with a high density of excited states.

Synopsis We found that the XUV excited caffeine undergoes nondadabatic cascade via more than 100 shake-up states within the time constant of 40 fs by using femtosecond XUV-NIR pump-probe photo-ion spectroscopy and the SAC-CI level of ab-initio many-body theory. This nonadiabatic cascade results in the photofragmentation to specific large fragments. Furether ionization by NIR probe during the cascade increases the small fragments CH3 +, CO+ etc. This results show that both vibronic coupling and electron correlation are the keys for the femtochemistry in the highly-correlated electronic excited states.

We present a tutorial to realize a multidimensional fitting procedure capable of extracting all the relevant information contained in a sequence of charged particle images acquired as a function of time in femtosecond pump–probe experiments. The images are reproduced using a 3D fitting method, which provides the velocity (or center-of-mass kinetic energy) and angular distributions contained in the images and their time evolution. A detailed example of the method is shown through the analysis of the time-resolved predissociation dynamics of CH3I on the B-band origin (Gitzinger et al., J. Chem. Phys.2010, 133, 234313). We show that the multidimensional approach is essential for the analysis of complex images that contain several overlapping contributions where reduced dimensionality analyses cannot provide a reliable description of the features present in the image sequence. This methodology can be generalized to many types of multidimensional data analysis., This work has been supported by the Spanish Ministry of Economy and Competitiveness (Grants CTQ2015-65033-P and CTQ2016-75880-P). This research has been carried out within the Unidad Asociada Química Física Molecular between Departamento de Química Física of Universidad Complutense de Madrid and CSIC. The facilities provided by the CLUR (Centro de Láseres Ultrarrápidos) are gratefully acknowledged.

International audience; Processes in atoms or molecules on the attosecond timescale have been measured using XUV attosecond and IR femtosecond pulses overlapping in time and controlled with attosecond accuracy. Within this general framework, many strategies have been developed using the harmonics of the fundamental pulse. In this paper, we focus on a specific configuration where the attosecond pulse train is composed by odd harmonics and is dressed by the second harmonic of the fundamental light. Measuring the angularly resolved photoelectron spectrum as a function of the delay between the pulses, a clear oscillation of the anisotropy parameters appears revealing attosecond controlled interferences. This process, is assigned to interferences between two quantum paths involving one XUV photon, on one path, and a XUV+UV photons on the other path. The XUV-UV delay dependent up–down asymmetry can be interpreted following the usual RABBITT formalism, where the dressing photon energy corresponds to the energy separation between the XUV photons of the attosecond pulse train. This approach allows an intuitive analysis of the interference and provides a well suited method for the study of complex valence band systems thanks to the limited congestion of the resulting spectrum.

An original workflow allowing inline FAIMS separation, electrospray ionization, mass analysis and ion spectroscopy (IRMPD: InfraRed Multiple Photon Dissociation) is presented for multidimensional molecular analysis. This new instrument consists of an ultraFAIMS (Owlstone) device interfaced to a linear ion trap (LTQ XL Thermo Scientific) which was modified for IRMPD spectroscopy. Two modes of operation are demonstrated on an isomeric mixture of paracetamol and 2-phenylglycine. In the first mode a FAIMS (high-Field Asymmetric waveform Ion Mobility Spectrometry) separation of the isomers is performed with a static compensation field for mass- and isomer- selective ion spectroscopy. In the second mode, the compensation field is scanned while the ions are irradiated at a fixed wavenumber. The advantages of this workflow as compared to traditional FAIMS-MS and IRMPD spectroscopy are described. The potential of the two modes for molecular spectroscopy and analytical applications, in particular the new “omics” are discussed.

The standard velocity-map imaging (VMI) analysis relies on the simple approximation that the residual Coulomb field experienced by the photoelectron ejected from a neutral or ion system may be neglected. Under this almost universal approximation, the photoelectrons follow ballistic (parabolic) trajectories in the externally applied electric field, and the recorded image may be considered as a 2D projection of the initial photoelectron velocity distribution. There are, however, several circumstances where this approximation is not justified and the influence of long-range forces must absolutely be taken into account for the interpretation and analysis of the recorded images. The aim of this paper is to illustrate this influence by discussing two different situations involving isolated atoms or molecules where the analysis of experimental images cannot be performed without considering long-range Coulomb interactions. The first situation occurs when slow (meV) photoelectrons are photoionized from a neutral system and strongly interact with the attractive Coulomb potential of the residual ion. The result of this interaction is the formation of a more complex structure in the image, as well as the appearance of an intense glory at the center of the image. The second situation, observed also at low energy, occurs in the photodetachment from a multiply charged anion and it is characterized by the presence of a long-range repulsive potential. Then, while the standard VMI approximation is still valid, the very specific features exhibited by the recorded images can be explained only by taking into consideration tunnel detachment through the repulsive Coulomb barrier.

A pulse shaping control experiment on multiphoton ionization of the methyl iodide (CH3I) molecule is reported. A range of phase masks were applied in the Fourier plane of a 4f pulse shaper containing a spatial light modulator, causing spectro-temporal modulation in the 540 nm pulses synthesized in an optical parametric amplifier. The ionization process was studied through velocity map imaging of outgoing photoelectrons. Four main ionization channels were identified, with relative yields that are dependent on the pulse shape applied, indicating that the ionization routes can be steered through light manipulation to some extent. The main mechanism for control seems to derive from peak intensity effects, but some features observed in the photoelectron spectra defy this explanation. The role of resonances at the three-photon level is discussed. © 2014 Taylor and Francis.

In this work, we synthesize Co-doped ZnS nanocrystals by pulsed laser ablation and deposition and investigate the composition and dynamics of the plasma produced in the ablation by time-of-flight mass spectrometry. The first experimental observation of Co-substituted ZnS clusters in the ablation plasma is reported; three series of cationic bimetallic clusters Zn n-mComSn +, Znn-mCo mSn+1 +, and Znn-mCo mSn-1 + are identified upon 1064 nm laser ablation of targets of Co (2%) in ZnS. Neutral clusters produced in the ablation are also observed by F2 laser (λ = 157 nm) postionization. Deposits are collected at the same laser ablation wavelength and fluence at which the above bimetallic clusters are observed in the plasma. Analysis by high-resolution transmission electron microscopy and energy-dispersive X-ray spectrometry obtains that the deposits are composed of Zn0.96Co 0.04S nanocrystals with local structure showing epitaxial growth between the zinc blende and wurtzite politypes. The deposits show paramagnetic behavior, but no magnetic ordering is observed. The results constitute a first step to help to elucidate the participation of cluster assembly processes in the synthesis of Co-doped ZnS nanomaterials by pulsed laser ablation and deposition. © 2013 American Chemical Society., Financial support was provided by the Spanish MINECO (CTQ2010-15680), Russian RFBR and SBRAS (10-03-00441 and Integration Project No. 134), and CSIC-RFBR (2008RU0069/09-02-91291). M.L.A. and M.J. wish to thank the CSIC and ESF (European Social Fund) for contracts under the JAE Fellowship Programme.

Femtosecond lasers, used as tools to investigate the ablation dynamics of solids, can help to develop strategies to control the deposition of nanomaterials by pulsed laser ablation. In this work, Co/ZnS targets, potential candidates for the synthesis of diluted magnetic semiconductor materials, are irradiated by sequences of two femtosecond laser pulses delayed in the picosecond time scale. The ionic composition of the ablation plasma and the dependence of the ion signals on the interpulse delay and relative fluence are determined by time-of-flight mass spectrometry. The results show that, when pulses of different fluence are used, highly asymmetric ion yields are obtained, with more intense ion signals detected when the lower fluence pulse is temporally ahead. The comparison between asymmetric and equal fluence double pulse ablation dynamics provides some understanding of the different processes that modify the properties of the layer irradiated by the first pulse and of the mechanisms affecting the coupling of the delayed pulse into the material. The final outcome of the double pulse irradiation is characterized through the analysis of the deposits produced upon ablation.

International audience; We present a simple upgrade of a time-resolved attosecond extreme ultraviolet and infrared (XUV-IR) setup designed to perform time-resolved or spectrally resolved studies of a target under similar experimental conditions. A flat XUV grating inserted in the path of an attosecond pulse train obtained via high order harmonic generation in gases is used either in the zeroth order of diffraction to follow attosecond dynamics or in the first order of diffraction to study the target spectral response with a temporally stretched single harmonic. Electron momentum measurement is performed with a velocity map imaging spectrometer, and the 10 W femtosecond laser system operating at 5 kHz provides an XUV photon flux compatible with rapid acquisition in both the monochromatic and broadband configurations. The change of experimental configuration between broadband and monochro-matic sources is rapid and performed in situ. We present the experimental implementation applied to krypton atoms and detail the capabilities and limits of this approach when an XUV grating with constant groove density is used with a converging XUV beam.

We investigate the dynamical alignment of jet-cooled OCS molecules induced by a short laser pulse. The alignment is measured through the orientational contribution of the optical Kerr effect using a second weak laser pulse as a probe. Maximum alignment is observed at conditions close to saturation of ionization. The results are analysed with a quantum mechanical model solving for the rotational dynamics.

This book documents the recent vivid developments in the research field of ultrashort intense light pulses for probing and controlling ultrafast dynamics. The recent fascinating results in studying and controlling ultrafast dynamics in ever more complicated systems such as (bio-)molecules and structures of meso- to macroscopic sizes on ever shorter time-scales are presented. The book is written by some of the most eminent experimental and theoretical experts in the field. It covers the new groundbreaking research directions that were opened by the availability of new light sources such as fully controlled intense laser fields with durations down to a single oscillation cycle, short-wavelength laser-driven attosecond pulses and intense X-ray pulses from the upcoming free electron lasers. These light sources allowed the investigation of dynamics in atoms, molecules, clusters, on surfaces and very recently also in nanostructures and solids in new regimes of parameters which, in turn, led to the identification of completely new dynamics and methods for controlling it. Example topics covered by this book include the study of ultrafast processes in large molecules using attosecond pulses, control of ultrafast electron dynamics in solids with shaped femtosecond laser pulses, light-driven ultrafast plasmonic processes on surfaces and in nanostructures as well as research on atomic and molecular systems under intense X-ray radiation. This book is equally helpful for people who would like to step into this field (e.g. young researchers), for whom it provides a broad introduction, as well as for already experienced researchers who may enjoy the exhaustive discussion that covers the research on essentially all currently studied objects and with all available ultrafast pulse sources.

International audience; This combined experimental and theoretical study demonstrates that the surface plasmon resonance in C60 alters the valence photoemission quantum phase, resulting in strong effects in the photoelectron angular distribution and emission time delay. Electron momentum imaging spectroscopy is used to measure the photoelectron angular distribution asymmetry parameter that agrees well with our calculations from the time-dependent local density approximation (TDLDA). Significant structure in the valence photoemission time delay is simultaneously calculated by TDLDA over the plasmon active energies. Results reveal a unified spatial and temporal asymmetry pattern driven by the plasmon resonance and offer a sensitive probe of electron correlation. A semiclassical approach facilitates further insights into this link that can be generalized and applied to other molecular systems and nanometer-sized metallic materials exhibiting plasmon resonances.

International audience; XUV induced dynamics in molecules is still largely unexplored while experimental and theoretical tools are becoming available in several labs. In this work, we present a compact XUV beamline designed to induce and probe femtosecond and attosecond dynamics in gas samples (from atoms to complex molecular species). The induced processes are studied with time-resolved photoelectron or/and photoion imaging. The characterization of the performances of the experimental setup is presented. We show experimental results obtained in naphthalene molecule showing femtosecond and attosecond time-resolved photoelectron imaging experiments allowing electron wavepacket phase measurements.

International audience; We perform time-dependent local density functional calculations of the time delay in C60 HOMO and HOMO-1 photoionization at giant plasmon energies. A semiclassical model is used to develop further insights.

Highly excited molecular species are at play in the chemistry of interstellar media and are involved in the creation of radiation damage in a biological tissue. Recently developed ultrashort extreme ultraviolet light sources offer the high excitation energies and ultrafast time-resolution required for probing the dynamics of highly excited molecular states on femtosecond (fs) (1 fs=10−15s) and even attosecond (as) (1 as=10−18 s) timescales. Here we show that polycyclic aromatic hydrocarbons (PAHs) undergo ultrafast relaxation on a few tens of femtoseconds timescales, involving an interplay between the electronic and vibrational degrees of freedom. Our work reveals a general property of excited radical PAHs that can help to elucidate the assignment of diffuse interstellar absorption bands in astrochemistry, and provides a benchmark for the manner in which coupled electronic and nuclear dynamics determines reaction pathways in large molecules following extreme ultraviolet excitation., Extreme UV light sources allow us to study the dynamics of excited molecular stets over remarkably short timeframes. Here, the authors probe polyaromatic hydrocarbons—large organic molecules—and show their electronic excitation and subsequent ultrafast relaxation.

International audience; Hole migration is a fascinating process driven by electron correlation, in which purely electronic dynamics occur on a very short time scale in complex ionized molecules, prior to the onset of nuclear motion. However, it is expected that due to coupling to the nuclear dynamics, these oscillations will be rapidly damped and smeared out, which makes experimental observation of the hole migration process rather difficult. In this Letter, we demonstrate that the instantaneous ionization of benzene molecules initiates an ultrafast hole migration characterized by a periodic breathing of the hole density between the carbon ring and surrounding hydrogen atoms on a subfemtosecond time scale. We show that these oscillations survive the dephasing introduced by the nuclear motion for a long enough time to allow their observation. We argue that this offers an ideal benchmark for studying the influence of hole migration on molecular reactivity.

A new strategy of pulse shaping based on the quasi-direct space-to-time (QDST) conversion principle has been recently proposed theoretically [Mnguez-Vega , Opt. Express 16, 16993 (2008)]. In this letter, the working principle of this pulse shaper has been experimentally verified by using a simple second order cross-correlation setup with spatial resolution. The QDST conversion principle is evidenced through the experimental measurement of the parabolic dependence of the time delay on the focal plane versus the radii of the components of the spatial profile in the far field.

© 2014, Springer-Verlag Berlin Heidelberg. Wavefront aberrations play a major role when focusing an ultrashort laser pulse to a high-quality focal spot. Here, we report a novel method to measure and correct wavefront aberrations of a 30-fs pulsed laser beam. The method only requires a programmable liquid-crystal spatial light modulator and a camera. Wavefront retrieval is based on pupil segmentation with an annular lens array, which allows us to determine the local phase that minimizes focusing errors due to wavefront aberrations. Our method provides accurate results even when implemented with low dynamic range cameras and polychromatic beams. Finally, the retrieved phase is added to a diffractive lens codified onto the spatial light modulator to experimentally demonstrate near-diffraction-limited femtosecond beam focusing without refractive components.

The correlation between chemical structure and dynamics has been explored in a series of molecules with increasing structural complexity in order to investigate its influence on bond cleavage reaction times in a photodissociation event. Femtosecond time-resolved velocity map imaging spectroscopy reveals specificity of the ultrafast carbon-iodine (C-I) bond breakage for a series of linear (unbranched) and branched alkyl iodides, due to the interplay between the pure reaction coordinate and the rest of the degrees of freedom associated with the molecular structure details. Full-dimension time-resolved dynamics calculations support the experimental evidence and provide insight into the structure-dynamics relationship to understand structural control on time-resolved reactivity. This journal is © the Partner Organisations 2014.

The introduction of time-resolved measurements in the femtosecond time-scale using velocity map imaging techniques of charged particles (ions and photoelectrons) in combination with resonant multiphoton ionization of the fragments for the study of the photodissociation dynamics of small polyatomic molecules is reviewed. A typical experiment consists of the measurement of a sequence of images, whose analysis requires in most cases sophisticated multidimensional fitting methods to extract all the relevant time-resolved information contained in the images. In particular, the application of these techniques to the study of the direct photodissociation (A band) and electronic predissociation (B band) of methyl iodide along with the detection and characterization of transient species and the study of cluster dissociation, as a case example for femtosecond velocity map imaging, are presented and discussed. © Springer International Publishing Switzerland 2014.

Strong field control scenarios are investigated in methyl iodide photochemistry in the UV. Dynamics Stark Control and pump-dump strategies are shown capable of altering key photodissociation dynamical observables of this benchmark molecule. Perspectives for future directions are given.

International audience; We investigate a variant of the d-scan technique, an intuitive pulse characterization method for retrieving the spectral phase of ultrashort laser pulses. In this variant a ramp of quadratic spectral phases is applied to the input pulses and the second harmonic spectra of the resulting pulses are measured for each chirp value. We demonstrate that a given field envelope produces a unique and unequivocal chirp-scan map and that, under some asymptotic assumptions, both the spectral amplitude and phase of the measured pulse can be retrieved analytically from only two measurements. An iterative algorithm can exploit the redundancy of the information contained in the chirp-scan map to discard experimental noise, artifacts, calibration errors and improve the reconstruction of both the spectral intensity and phase. This technique is compared to two reference characterization techniques (FROG and SRSI). Finally, we perform d-scan measurements with a simple grating-pair compressor.

International audience; Spatial shaping of ultrashort laser beams at the focal plane is theoretically analyzed. The description of the pulse is performed by its expansion in terms of Laguerre-Gaussian orthonormal modes. This procedure gives both a comprehensive interpretation of the propagation dynamics and the required signal to encode onto a spatial light modulator for spatial shaping, without using iterative algorithms. As an example, pulses with top-hat and annular spatial profiles are designed and their dynamics analyzed. The interference of top-hat pulses is also investigated finding potential applications in high precision pump-probe experiments (without using delay lines) and for the creation of subwavelength ablation patterns. In addition, a novel class of ultrashort pulses possessing non-stationary orbital angular momentum is also proposed. These exotic pulses provide additional degrees of freedom that open up new perspectives in fields such as laser-matter interaction and micro-machining.

Stark shift of the 3 R 1 Rydberg state of CH3I is measured for different Stark field intensities. Photodissociation products, generated after predissociation of the state, were detected when the molecules were excited with photons resonant with the energy difference between the ground and the shifted 3 R 1 state, allowing the shift to be quantified. Qualitative agreement has been found with a 1D model.

Femtosecond time-resolved velocity map imaging experiments are reported on several vibronic levels of the second absorption band (B-band) of CH 3I, including vibrational excitation in the 2 and 3 modes of the bound 3R 1(E) Rydberg state. Specific predissociation lifetimes have been determined for the 201 and 301 vibronic levels from measurements of time-resolved I*( 2P 12) and CH 3 fragment images, parent decay, and photoelectron images obtained through both resonant and non-resonant multiphoton ionization. The results are compared with our previously reported predissociation lifetime measurements for the band origin 000 [Gitzinger, J. Chem. Phys. 132, 234313 (2010)10.1063/1.3455207]. The result, previously reported in the literature, where vibrational excitation to the C-I stretching mode ( 3) of the CH 3I 3R 1(E) Rydberg state yields a predissociation lifetime about four times slower than that corresponding to the vibrationless state, whereas predissociation is twice faster if the vibrational excitation is to the umbrella mode ( 2), is confirmed in the present experiments. In addition to the specific vibrational state lifetimes, which were found to be 0.85 0.04 ps and 4.34 0.13 ps for the 201 and 301 vibronic levels, respectively, the time evolution of the fragment anisotropy and the vibrational activity of the CH 3 fragment are presented. Additional striking results found in the present work are the evidence of ground state I( 2P 32) fragment production when excitation is produced specifically to the 301 vibronic level, which is attributed to predissociation via the A-band 1Q 1 potential energy surface, and the indication of a fast adiabatic photodissociation process through the repulsive A-band 3A 1(4E) state, after direct absorption to this state, competing with absorption to the 301 vibronic level of the 3R 1(E) Rydberg state of the B-band.

We present the second order cross-correlation of a quasi-direct space-to-time (QDST) pulse shaper with spatial resolution. The acquisition of the phase matching signal by a 2D-CCD camera has allowed the experimental validation that the temporal profile at the focus is controlled through the modulation of the beam profile at the entrance of a kinoform diffractive lens. An example of a temporal shaped pulse produced by the QDST pulse shaper is shown.

This paper shows the results of combined experimental and theoretical work that have unravelled the mechanism of ultrafast ejection of a methyl group from a cluster, the methyl iodide dimer (CH3I)2. Ab initio calculations have produced optimized geometries for the dimer and energy values and oscillator strengths for the excited states of the A band of (CH 3I)2. These calculations have allowed us to describe the blue shift that had been observed in the past in this band. This blue shift has been experimentally determined with higher precision than in all previously reported experiments, since it has been measured through its effect upon the kinetic energy release of the fragments using femtosecond velocity map imaging. Observations of the reaction branching ratio and of the angular nature of the fragment distribution indicate that two main changes occur in A-band absorption in the dimer with respect to the monomer: a substantial change in the relative absorption to different states of the band, and, more importantly, a more efficient non-adiabatic crossing between two of those states. Additionally, time resolved experiments have been performed on the system, obtaining snapshots of the dissociation process. The apparent retardation of more than 100 fs in the dissociation process of the dimer relative to the monomer has been assigned to a delay in the opening of the optical detection window associated with the resonant multiphoton ionization detection of the methyl fragment. © the Owner Societies 2011., This work has been financed by the Spanish MICINN through Grant No. CTQ2008-02578/BQU, and Consolider program “Science and Applications of Ultrafast Ultraintense Lasers” No. CSD2007-00013. We also acknowledge the support from the European Marie Curie Initial Training Network Grant No. CA-ITN-214962-FASTQUAST. JGV acknowledges MICINN for a Juan de la Cierva contract. VL acknowledges CSIC for a JAE-Doc contract and EU for an ER contract.

10 pags, 11 figs, 2 pags, As a contribution to the ongoing controversy about the role of higher-order Kerr effect (HOKE) in laser filamentation, we first provide thorough details about the protocol that has been employed to infer the HOKE indices from the experiment. Next, we discuss potential sources of artifact in the experimental measurements of these terms and show that neither the value of the observed birefringence, nor its inversion, nor the intensity at which it is observed, appear to be flawed. Furthermore, we argue that, independently on our values, the principle of including HOKE is straightforward. Due to the different temporal and spectral dynamics, the respective efficiency of defocusing by the plasma and by the HOKE is expected to depend substantially on both incident wavelength and pulse duration. The discussion should therefore focus on defining the conditions where each filamentation regime dominates. © 2011 Pleiades Publishing, Ltd., This work was supported by the Conseil Régional de Bourgogne, the ANR COMOC, the FASTQUAST ITN Program of the 7th FP and the Swiss NSF (contracts 200021-125315 and 200021-125315-2).

5 pags, 4 figs.-- PACS number(s): 42.65.Jx, 42.65.Tg, 78.20.Ci. -- Publisher error corrected 27 September 2010, Erratum Phys. Rev. A 82, 039905 (2010): https://doi.org/10.1103/PhysRevA.82.033826, Based on numerical simulations, we show that higher-order nonlinear indices (up to n8 and n10, respectively) of air and argon have a dominant contribution to both focusing and defocusing in the self-guiding of ultrashort laser pulses over most of the spectrum. Plasma generation and filamentation are therefore decoupled. As a consequence, ultraviolet wavelength may not be the optimal wavelength for applications requiring to maximize ionization. © 2010 The American Physical Society., This work was supported by the Conseil Régional de Bourgogne, the ANR COMOC, the FASTQUAST ITN Program of the 7th FP, and the Swiss NSF (Contracts No. 200021-116198 and No. 200021-125315).