19 results
Search Results
Now showing 1 - 10 of 19
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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
- 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 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.
- 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.