Browsing by Author "Vrakking, M.J.J."
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- ItemAttosecond control of electron-ion recollision in high harmonic generation(Bristol : IOP, 2011) Gademann, G.; Kelkensberg, F.; Siu, W.K.; Johnsson, P.; Gaarde, M.B.; Schafer, K.J.; Vrakking, M.J.J.We show that high harmonic generation driven by an intense nearinfrared (IR) laser can be temporally controlled when an attosecond pulse train (APT) is used to ionize the generation medium, thereby replacing tunnel ionization as the first step in the well-known three-step model. New harmonics are formed when the ionization occurs at a well-defined time within the optical cycle of the IR field. The use of APT-created electron wave packets affords new avenues for the study and application of harmonic generation. In the present experiment, this makes it possible to study harmonic generation at IR intensities where tunnel ionization does not give a measurable signal.
- ItemAttosecond electron spectroscopy using a novel interferometric pump-probe technique(College Park, Md. : APS, 2010) Mauritsson, J.; Remetter, T.; Swoboda, M.; Klünder, K.; L'Huillier, A.; Schafer, K.J.; Ghafur, O.; Kelkensberg, F.; Siu, W.; Johnsson, P.; Vrakking, M.J.J.; Znakovskaya, I.; Uphues, T.; Zherebtsov, S.; Kling, M.F.; Lépine, F.; Benedetti, E.; Ferrari, F.; Sansone, G.; Nisoli, M.We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration. © 2010 The American Physical Society.
- ItemAttosecond streaking in a nano-plasmonic field(Bristol : IOP, 2012) Kelkensberg, F.; Koenderink, A.F.; Vrakking, M.J.J.A theoretical study of the application of attosecond streaking spectroscopy to time-resolved studies of the plasmonic fields surrounding isolated, resonantly excited spherical nanoparticles is presented. A classification of the different regimes in attosecond streaking is proposed and identified in our results that are derived from Mie calculations of plasmon fields, coupled to classical electron trajectory simulations. It is shown that in an attosecond streaking experiment, the electrons are almost exclusively sensitive to the component of the field parallel to the direction in which they are detected. This allows one to probe the different components of the field individually by resolving the angle of emission of the electrons. Finally, simulations based on fields calculated by finite-difference time-domain (FDTD) are compared with the results obtained using Mie fields. The two are found to be in good agreement with each other, supporting the notion that FDTD methods can be used to reliably investigate non-spherical structures.
- ItemAttosecond transient absorption spectroscopy without inversion symmetry(Bristol : IOP Publ., 2020) Drescher, L.; Vrakking, M.J.J.; Mikosch, J.Transient absorption is a very powerful observable in attosecond experiments on atoms, molecules and solids and is frequently used in experiments employing phase-locked few-cycle infrared and XUV laser pulses derived from high harmonic generation. We show numerically and analytically that in non-centrosymmetric systems, such as many polyatomic molecules, which-way interference enabled by the lack of parity conservation leads to new spectral absorption features, which directly reveal the laser electric field. The extension of attosecond transient absorption spectroscopy (ATAS) to such targets hence becomes sensitive to global and local inversion symmetry. We anticipate that ATAS will find new applications in non-centrosymmetric systems, in which the carrier-to-envelope phase of the infrared pulse becomes a relevant parameter and in which the orientation of the sample and the electronic symmetry of the molecule can be addressed. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemCarrier-envelope phase-tagged imaging of the controlled electron acceleration from SiO 2 nanospheres in intense few-cycle laser fields(Bristol : IOP, 2012) Zherebtsov, S.; Süßmann, F.; Peltz, C.; Plenge, J.; Betsch, K.J.; Znakovskaya, I.; Alnaser, A.S.; Johnson, N.G.; Kübel, M.; Horn, A.; Mondes, V.; Graf, C.; Trushin, S.A.; Azzeer, A.; Vrakking, M.J.J.; Paulus, G.G.; Krausz, F.; Rühl, E.; Fennel, T.; Kling, M.F.Waveform-controlled light fields offer the possibility of manipulating ultrafast electronic processes on sub-cycle timescales. The optical lightwave control of the collective electron motion in nanostructured materials is key to the design of electronic devices operating at up to petahertz frequencies. We have studied the directional control of the electron emission from 95 nm diameter SiO 2 nanoparticles in few-cycle laser fields with a well-defined waveform. Projections of the three-dimensional (3D) electron momentum distributions were obtained via single-shot velocity-map imaging (VMI), where phase tagging allowed retrieving the laser waveform for each laser shot. The application of this technique allowed us to efficiently suppress background contributions in the data and to obtain very accurate information on the amplitude and phase of the waveform-dependent electron emission. The experimental data that are obtained for 4 fs pulses centered at 720 nm at different intensities in the range (1-4)×10 13Wcm -2 are compared to quasi-classical mean-field Monte-Carlo simulations. The model calculations identify electron backscattering from the nanoparticle surface in highly dynamical localized fields as the main process responsible for the energetic electron emission from the nanoparticles. The local field sensitivity of the electron emission observed in our studies can serve as a foundation for future research on propagation effects for larger particles and field-induced material changes at higher intensities.
- ItemCoulomb explosion of diatomic molecules in intense XUV fields mapped by partial covariance(Bristol : Institute of Physics Publishing, 2013) Kornilov, O.; Eckstein, M.; Rosenblatt, M.; Schulz, C.P.; Motomura, K.; Rouzée, A.; Klei, J.; Foucar, L.; Siano, M.; Lübcke, A.; Schapper, F.; Johnsson, P.; Holland, D.M.P.; Schlathölter, T.; Marchenko, T.; Düsterer, S.; Ueda, K.; Vrakking, M.J.J.; Frasinski, L.J.Single-shot time-of-flight spectra for Coulomb explosion of N2 and I2 molecules have been recorded at the Free Electron LASer in Hamburg (FLASH) and have been analysed using a partial covariance mapping technique. The partial covariance analysis unravels a detailed picture of all significant Coulomb explosion pathways, extending up to the N 4+-N5+ channel for nitrogen and up to the I 8+-I9+ channel for iodine. The observation of the latter channel is unexpected if only sequential ionization processes from the ground state ions are considered. The maximum kinetic energy release extracted from the covariance maps for each dissociation channel shows that Coulomb explosion of nitrogen molecules proceeds much faster than that of the iodine. The N 2 ionization dynamics is modelled using classical trajectory simulations in good agreement with the outcome of the experiments. The results suggest that covariance mapping of the Coulomb explosion can be used to measure the intensity and pulse duration of free-electron lasers.
- ItemDelayed relaxation of highly excited naphthalene cations(Bristol : IOP Publ., 2020) Reitsma, G.; Hummert, J.; Dura, J.; Loriot, V.; Vrakking, M.J.J.; Lépine, F.; Kornilov, O.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. © 2019 Published under licence by IOP Publishing Ltd.
- ItemFew-femtosecond passage of conical intersections in the benzene cation([London] : Nature Publishing Group UK, 2017) Galbraith, M.C.E.; Scheit, S.; Golubev, N.V.; Reitsma, G.; Zhavoronkov, N.; Despré, V.; Lépine, F.; Kuleff, A.I.; Vrakking, M.J.J.; Kornilov, O.; Köppel, H.; Mikosch, J.Observing the crucial first few femtoseconds of photochemical reactions requires tools typically not available in the femtochemistry toolkit. Such dynamics are now within reach with the instruments provided by attosecond science. Here, we apply experimental and theoretical methods to assess the ultrafast nonadiabatic vibronic processes in a prototypical complex system - the excited benzene cation. We use few-femtosecond duration extreme ultraviolet and visible/near-infrared laser pulses to prepare and probe excited cationic states and observe two relaxation timescales of 11 ± 3 fs and 110 ± 20 fs. These are interpreted in terms of population transfer via two sequential conical intersections. The experimental results are quantitatively compared with state-of-the-art multi-configuration time-dependent Hartree calculations showing convincing agreement in the timescales. By characterising one of the fastest internal conversion processes studied to date, we enter an extreme regime of ultrafast molecular dynamics, paving the way to tracking and controlling purely electronic dynamics in complex molecules.
- ItemGeneration and characterization of isolated attosecond pulses for coincidence spectroscopy at 100 kHz repetition rate(Bristol : IOP Publ., 2020) Witting, T.; Furch, F.; Osolodkov, M.; Schell, F.; Menoni, C.; Schulz, C.P.; Vrakking, M.J.J.An attosecond pump-probe beamline with 100 kHz repetition rate for coincidence experiments has been developed. It is based on non-collinear optical parametric chirped pulse ampli-cation and delivers 100 µJ sub-4 fs to an high-harmonic generation source. Details on the generation and characterization of isolated attosecond pulses will be presented. © 2019 Published under licence by IOP Publishing Ltd.
- ItemHighly non-linear ionization of atoms induced by intense high-harmonic pulses(Bristol : IOP Publishing, 2020) Senfftleben, B.; Kretschmar, M.; Hoffmann, A.; Sauppe, M.; Tümmler, J.; Will, I.; Nagy, T.; Vrakking, M.J.J.; Rupp, D.; Schütte, B.Intense extreme-ultraviolet (XUV) pulses enable the investigation of XUV-induced non-linear processes and are a prerequisite for the development of attosecond pump - attosecond probe experiments. While highly non-linear processes in the XUV range have been studied at free-electron lasers (FELs), high-harmonic generation (HHG) has allowed the investigation of low-order non-linear processes. Here we suggest a concept to optimize the HHG intensity, which surprisingly requires a scaling of the experimental parameters that differs substantially from optimizing the HHG pulse energy. As a result, we are able to study highly non-linear processes in the XUV range using a driving laser with a modest (˜ 10 mJ) pulse energy. We demonstrate our approach by ionizing Ar atoms up to Ar5 + , requiring the absorption of at least 10 XUV photons. © 2020 The Author(s). Published by IOP Publishing Ltd
- ItemImaging of carrier-envelope phase effects in above-threshold ionization with intense few-cycle laser fields(College Park, MD : Institute of Physics Publishing, 2008) Kling, M.F.; Rauschenberger, J.; Verhoef, A.J.; Hasović, E.; Uphues, T.; Milošević, D.B.; Muller, H.G.; Vrakking, M.J.J.Sub-femtosecond control of the electron emission in above-threshold ionization of the rare gases Ar, Xe and Kr in intense few-cycle laser fields is reported with full angular resolution. Experimental data that were obtained with the velocity-map imaging technique are compared to simulations using the strong-field approximation (SFA) and full time-dependent Schrödinger equation (TDSE) calculations. We find a pronounced asymmetry in both the energy and angular distributions of the electron emission that critically depends on the carrier-envelope phase (CEP) of the laser field. The potential use of imaging techniques as a tool for single-shot detection of the CEP is discussed. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
- ItemIntracycle interference in ionization of Ar by a laser assisted XUV pulse(Bristol : IOP Publ., 2017) Arbó, D.G.; López, S. D.; Kubin, M.; Hummert, J.; Vrakking, M.J.J.; Kornilov, O.Synopsis We present a theoretical and experimental study of the subcycle interference in laser assisted XUV ionization of Ar atoms. Averaging over the focal volume happens to blur the intracycle interference, which thus cannot be measured directly. We show that even at these conditions, the intracycle interference can be obtained through the subtraction of two different angle and energy-resolved distributions at slightly different laser intensities.
- ItemMapping the dissociative ionization dynamics of molecular nitrogen with attosecond resolution(Bristol : IOP Publ., 2015) Klinker, M.; Trabattoni, A.; González-Vázquez, J.; Liu, C.; Sansone, G.; Linguerri, R.; Hochlaf, M..; Klei, J.; Vrakking, M.J.J.; Martin, F.; Nisoli, M.; Calegari, F.We wish to understand the processes underlying the ionization dynamics of N2 as experimentally induced and studied by recording the kinetic energy release (KER) in a XUV-pump/IR-probe setup. To this end a theoretical model was developed describing the ionization process using Dyson Orbitals and, subsequently, the dissociation process using a large set of diabatic potential energy surfaces (PES) on which to propagate. From said set of PES, a small subset is extracted allowing for the identification of one and two photon processes chiefly responsible for the experimentally observed features.
- ItemObservation of correlated electronic decay in expanding clusters triggered by near-infrared fields([London] : Nature Publishing Group UK, 2015) Schütte, B.; Arbeiter, M.; Fennel, T.; Jabbari, G.; Kuleff, A.I.; Vrakking, M.J.J.; Rouzée, A.When an excited atom is embedded into an environment, novel relaxation pathways can emerge that are absent for isolated atoms. A well-known example is interatomic Coulombic decay, where an excited atom relaxes by transferring its excess energy to another atom in the environment, leading to its ionization. Such processes have been observed in clusters ionized by extreme-ultraviolet and X-ray lasers. Here, we report on a correlated electronic decay process that occurs following nanoplasma formation and Rydberg atom generation in the ionization of clusters by intense, non-resonant infrared laser fields. Relaxation of the Rydberg states and transfer of the available electronic energy to adjacent electrons in Rydberg states or quasifree electrons in the expanding nanoplasma leaves a distinct signature in the electron kinetic energy spectrum. These so far unobserved electron-correlation-driven energy transfer processes may play a significant role in the response of any nano-scale system to intense laser light.
- ItemPhotoelectron holography in strong optical and dc electric fields(Bristol : Institute of Physics Publishing, 2014) Stodolna, A.; Huismans, Y.; Rouzée, A.; Lépine, F.; Vrakking, M.J.J.The application of velocity map imaging for the detection of photoelectrons resulting from atomic or molecular ionization allows the observation of interferometric, and in some cases holographic structures that contain detailed information on the target from which the photoelecrons are extracted. In this contribution we present three recent examples of the use of photoelectron velocity map imaging in experiments where atoms are exposed to strong optical and dc electric fields. We discuss (i) observations of the nodal structure of Stark states of hydrogen measured in a dc electric field, (ii) mid-infrared strong-field ionization of metastable Xe atoms and (iii) the reconstruction of helium electronic wavepackets in an attosecond pump-probe experiment. In each case, the interference between direct and indirect electron pathways, reminiscent of the reference and signal waves in holography, is seen to play an important role.
- ItemProbing multiphoton light-induced molecular potentials([London] : Nature Publishing Group UK, 2020) Kübel, M.; Spanner, M.; Dube, Z.; Naumov, A.Yu.; Chelkowski, S.; Bandrauk, A.D.; Vrakking, M.J.J.; Corkum, P.B.; Villeneuve, D.M.; Staudte, A.The strong coupling between intense laser fields and valence electrons in molecules causes distortions of the potential energy hypersurfaces which determine the motion of the nuclei and influence possible reaction pathways. The coupling strength varies with the angle between the light electric field and valence orbital, and thereby adds another dimension to the effective molecular potential energy surface, leading to the emergence of light-induced conical intersections. Here, we demonstrate that multiphoton couplings can give rise to complex light-induced potential energy surfaces that govern molecular behavior. In the laser-induced dissociation of H2+, the simplest of molecules, we measure a strongly modulated angular distribution of protons which has escaped prior observation. Using two-color Floquet theory, we show that the modulations result from ultrafast dynamics on light-induced molecular potentials. These potentials are shaped by the amplitude, duration and phase of the dressing fields, allowing for manipulating the dissociation dynamics of small molecules.
- ItemPropagation-assisted generation of intense few-femtosecond high-harmonic pulses(Bristol : IOP Publishing, 2020) Major, B.; Kretschmar, M.; Ghafur, O.; Hoffmann, A.; Kovács, K.; Varjú, K.; Senfftleben, B.; Tümmler, J.; Will, I.; Nagy, T.; Rupp, D.; Vrakking, M.J.J.; Tosa, V.; Schütte, B.The ongoing development of intense high-harmonic generation (HHG) sources has recently enabled highly non-linear ionization of atoms by the absorption of at least 10 extreme-ultraviolet (XUV) photons within a single atom (Senfftleben et al, arXiv:1911.01375). Here we investigate how the generation of these very intense HHG pulses in our 18-m-long beamline is aided by the reshaping of the fundamental, few-cycle, near-infrared (NIR) driving laser within a 30-cm-long HHG Xe medium. Using an incident NIR intensity that is higher than what is required for phase-matched HHG, signatures of reshaping are found by measuring the NIR blueshift and the fluorescence from the HHG medium along the propagation axis. These results are well reproduced by numerical calculations that show temporal compression of the NIR pulses in the HHG medium. The simulations predict that after refocusing an XUV beam waist radius of 320 nm and a clean attosecond pulse train can be obtained in the focal plane, with an estimated XUV peak intensity of 9 × 1015 W cm-2. Our results show that XUV intensities that were previously only available at large-scale facilities can now be obtained using moderately powerful table-top light sources. © 2020 The Author(s). Published by IOP Publishing Ltd
- ItemSignatures of Light-Induced Potential Energy Surfaces in H2+(Bristol : IOP Publ., 2020) Kübel, M.; Spanner, M.; Dube, Z.; Naumov, A. Yu; Vrakking, M.J.J.; Corkum, P.B.; Villeneuve, D.M.; Staudte, A.Using theory and Cold Target Recoil Ion Momentum Spectroscopy we find signatures of light-induced molecular potential energy surfaces in the 3-dimensional proton momentum distributions of dissociating H+2. © 2020 Journal of Physics: Conference Series. All rights reserved.
- ItemUltrafast relaxation dynamics of highly-excited states in N2 molecules excited by femtosecond XUV pulses(Les Ulis : EDP Sciences, 2013) Lucchini, M.; Seung, Kim, K.; Calegari, F.; Kelkensberg, F.; Siu, W.K.; Sansone, G.; Vrakking, M.J.J.; Hochlaf, M.; Nisoli, M.We used velocity-map-imaging to measure electronic and nuclear dynamics in N2 molecules excited by a train of attosecond pulses. A time-to-space mapping of autoionization channel is demonstrated. It is found that the autoionization becomes energetically allowed when the two nuclei are still very close (~ 3 Å) and that it can be coherently manipulated by a strong femtosecond infrared pulse.
- ItemXUV excitation followed by ultrafast non-adiabatic relaxation in PAH molecules as a femto-astrochemistry experiment([London] : Nature Publishing Group UK, 2015) Marciniak, A.; Despré, V.; Barillot, T.; Rouzée, A.; Galbraith, M.C.E.; Klei, J.; Yang, C.-H.; Smeenk, C.T.L.; Loriot, V.; Nagaprasad Reddy, S.; Tielens, A.G.G.M.; Mahapatra, S.; Kuleff, A.I.; Vrakking, M.J.J.; Lépine, F.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.