Time-domain spectroscopy of methane excited by resonant high-energy mid-IR pulses

We describe the implementation of nonlinear time-domain spectroscopy of rotovibrational IR-active modes in methane through broadband Four-Wave Mixing driven by resonant high-energy mid infrared laser pulses. At high driving pulse intensities we observe an efficient vibrational ladder climbing triggered in the molecules. This study opens the possibility to impulsively and selectively excite molecules of biological interest to high-lying vibrational states and to characterize their dynamics.

Ultrafast relaxation of photoexcited superfluid He nanodroplets

The relaxation of photoexcited nanosystems is a fundamental process of light–matter interaction. Depending on the couplings of the internal degrees of freedom, relaxation can be ultrafast, converting electronic energy in a few fs, or slow, if the energy is trapped in a metastable state that decouples from its environment. Here, we study helium nanodroplets excited resonantly by femtosecond extreme-ultraviolet (XUV) pulses from a seeded free-electron laser. Despite their superfluid nature, we find that helium nanodroplets in the lowest electronically excited states undergo ultrafast relaxation. By comparing experimental photoelectron spectra with time-dependent density functional theory simulations, we unravel the full relaxation pathway: Following an ultrafast interband transition, a void nanometer-sized bubble forms around the localized excitation (He\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}^{* }$$\end{document}*) within 1 ps. Subsequently, the bubble collapses and releases metastable He\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}^{* }$$\end{document}* at the droplet surface. This study highlights the high level of detail achievable in probing the photodynamics of nanosystems using tunable XUV pulses.

There is interest in understanding the relaxation mechanisms of photoexcitation in atoms, molecules and other complex systems. Here the authors unravel the photoexcitation and ultrafast relaxation of superfluid helium nanodroplets using a pump-probe experiment with FEL pulses.

Time-Resolved Measurement of Interatomic Coulombic Decay Induced by Two-Photon Double Excitation of Ne_{2}

The hitherto unexplored two-photon doubly excited states [Ne^{*}(2p^{-1}3s)]_{2} were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD), which predominantly produces singly ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne_{2}^{+} ions was recorded as a function of delay between the extreme ultraviolet pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly excited states, 390(-130/+450)  fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

Circular Dichroism in Multiphoton Ionization of Resonantly Excited He^{+} Ions

Intense, circularly polarized extreme-ultraviolet and near-infrared (NIR) laser pulses are combined to double ionize atomic helium via the oriented intermediate He^{+}(3p) resonance state. Applying angle-resolved electron spectroscopy, we find a large photon helicity dependence of the spectrum and the angular distribution of the electrons ejected from the resonance by NIR multiphoton absorption. The measured circular dichroism is unexpectedly found to vary strongly as a function of the NIR intensity. The experimental data are well described by theoretical modeling and possible mechanisms are discussed.

Slow Interatomic Coulombic Decay of Multiply Excited Neon Clusters

Ne clusters (∼5000  atoms) were resonantly excited (2p→3s) by intense free electron laser (FEL) radiation at FERMI. Such multiply excited clusters can decay nonradiatively via energy exchange between at least two neighboring excited atoms. Benefiting from the precise tunability and narrow bandwidth of seeded FEL radiation, specific sites of the Ne clusters were probed. We found that the relaxation of cluster surface atoms proceeds via a sequence of interatomic or intermolecular Coulombic decay (ICD) processes while ICD of bulk atoms is additionally affected by the surrounding excited medium via inelastic electron scattering. For both cases, cluster excitations relax to atomic states prior to ICD, showing that this kind of ICD is rather slow (picosecond range). Controlling the average number of excitations per cluster via the FEL intensity allows a coarse tuning of the ICD rate.

Probe of Multielectron Dynamics in Xenon by Caustics in High-Order Harmonic Generation

We investigated the giant resonance in xenon by high-order harmonic generation spectroscopy driven by a two-color field. The addition of a nonperturbative second harmonic component parallel to the driving field breaks the symmetry between neighboring subcycles resulting in the appearance of spectral caustics at two distinct cutoff energies. By controlling the phase delay between the two color components it is possible to tailor the harmonic emission in order to amplify and isolate the spectral feature of interest. In this Letter we demonstrate how this control scheme can be used to investigate the role of electron correlations that give birth to the giant resonance in xenon. The collective excitations of the giant dipole resonance in xenon combined with the spectral manipulation associated with the two-color driving field allow us to see features that are normally not accessible and to obtain a good agreement between the experimental results and the theoretical predictions.

Non-collinear high-order harmonic generation by three interfering laser beams

High order harmonic generation (HHG) has shown its impact on several applications in Attosecond Science and Atomic and Molecular Physics. Owing to the complexity of the experimental setup for the generation and characterization of harmonics, as well as to the large computational costs of numerical modelling, HHG is generally performed and modelled in collinear geometry. Recently, several experiments have been performed exploiting non-collinear geometry, such as HHG in a grating of excited molecules created by crossing beams. In such studies, harmonics were observed at propagation directions different from those of the driving pulses; moreover the scattered harmonics were angularly dispersed.In this work we report on a new regime of HHG driven by multiple beams, where the harmonics are generated by three synchronized, intense laser pulses organized in a non-planar geometry. Although the configuration we explore is well within the strong-field regime, the scattered harmonics we observe are not angularly dispersed.

High resolution multiphoton spectroscopy by a tunable free-electron-laser light

Seeded free electron lasers theoretically have the intensity, tunability, and resolution required for multiphoton spectroscopy of atomic and molecular species. Using the seeded free electron laser FERMI and a novel detection scheme, we have revealed the two-photon excitation spectra of dipole-forbidden doubly excited states in helium. The spectral profiles of the lowest (-1,0)(+1) (1)S(e) and (0,1)(0) (1)D(e) resonances display energy shifts in the meV range that depend on the pulse intensity. The results are explained by an effective two-level model based on calculated Rabi frequencies and decay rates.

Novel collective autoionization process observed in electron spectra of He clusters

The ionization dynamics of He nanodroplets irradiated with intense femtosecond extreme ultraviolet pulses of up to 1013  W/cm2 power density have been investigated by photoelectron spectroscopy. Helium droplets were resonantly excited to atomiclike 2p states with a photon energy of 21.4 eV, below the ionization potential (Ip), and directly into the ionization continuum with 42.8 eV photons. While electron emission following direct ionization above Ip is well explained within a model based on a sequence of direct electron emission events, the resonant excitation provides evidence of a new, collective ionization mechanism involving many excited atomiclike 2p states. With increasing power density the direct photoline due to an interatomic Coulombic decay disappears. It indicates that ionization occurs due to energy exchange between at least three excited atoms proceeding on a femtosecond time scale. In agreement with recent theoretical work the novel ionization process is very efficient and it is expected to be important for many other systems.

High-order harmonic spectroscopy for molecular imaging of polyatomic molecules

High-order harmonic generation is a powerful and sensitive tool for probing atomic and molecular structures, combining in the same measurement an unprecedented attosecond temporal resolution with a high spatial resolution of the order of an angstrom. Imaging of the outermost molecular orbital by high-order harmonic generation has been limited for a long time to very simple molecules, like nitrogen. Recently we demonstrated a technique that overcame several of the issues that have prevented the extension of molecular orbital tomography to more complex species, showing that molecular imaging can be applied to a triatomic molecule like carbon dioxide. Here we report on the application of such a technique to nitrous oxide (N₂O) and acetylene (C₂H₂). This result represents a first step towards the imaging of fragile compounds, a category which includes most of the fundamental biological molecules.

Coeliac disease in primary care: evaluation of a case-finding strategy

BACKGROUND

Coeliac disease is still under-diagnosed as a consequence of poor physician awareness of the clinical spectrum of the disease. We evaluated the feasibility and the cost-effectiveness of a case-finding approach for early identification of cases, carried out by primary care practitioners.

METHODS

We developed a case-finding strategy based on testing for anti-tissue transglutaminase IgA antibodies in subjects showing predefined signs and symptoms or belonging to at-risk groups.

RESULTS

Sixty-nine primary care doctors and 60 primary care paediatricians agreed to participate. One thousand forty-one adults and 447 children were selected for anti-tissue transglutaminase testing during the year of the study (2001). Thirty-one (2.08%, 19 adults and 12 children) were ultimately diagnosed as coeliac patients. While no cases of coeliac disease had been diagnosed by the participating doctors in the previous year, 29 subjects were diagnosed as coeliacs in the year after the completion of the study (2002). The prevalence of confirmed coeliac disease in the population under study increased from 1:1,506 to 1:1,073 in adults and from 1:827 to 1:687 in children from year 2000 to 2001. When cases diagnosed in 2002 are included, the prevalence is 1:832 and 1:602, respectively. We calculated a cost of 923.25 euros for each new case diagnosed.

CONCLUSIONS

Case-finding is a feasible and successful strategy for detecting undiagnosed coeliac patients and has the important added value of increasing the awareness of the disease among primary care physicians; it represents a cost-effective alternative to population screening for reducing the burden of undiagnosed coeliac disease.