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- ItemStrong-field ionization of clusters using two-cycle pulses at 1.8 μm([London] : Macmillan Publishers Limited, part of Springer Nature, 2016) Schütte, Bernd; Ye, Peng; Patchkovskii, Serguei; Austin, Dane R.; Brahms, Christian; Strüber, Christian; Witting, Tobias; Ivanov, Misha Yu; Tisch, John W. G.; Marangos, Jon P.The interaction of intense laser pulses with nanoscale particles leads to the production of high-energy electrons, ions, neutral atoms, neutrons and photons. Up to now, investigations have focused on near-infrared to X-ray laser pulses consisting of many optical cycles. Here we study strong-field ionization of rare-gas clusters (103 to 105 atoms) using two-cycle 1.8 μm laser pulses to access a new interaction regime in the limit where the electron dynamics are dominated by the laser field and the cluster atoms do not have time to move significantly. The emission of fast electrons with kinetic energies exceeding 3 keV is observed using laser pulses with a wavelength of 1.8 μm and an intensity of 1 × 1015 W/cm2, whereas only electrons below 500 eV are observed at 800 nm using a similar intensity and pulse duration. Fast electrons are preferentially emitted along the laser polarization direction, showing that they are driven out from the cluster by the laser field. In addition to direct electron emission, an electron rescattering plateau is observed. Scaling to even longer wavelengths is expected to result in a highly directional current of energetic electrons on a few-femtosecond timescale.
- ItemSimple, accurate, and efficient implementation of 1-electron atomic time-dependent Schrödinger equation in spherical coordinates(Amsterdam : North Holland Publ. Co., 2015) Patchkovskii, Serguei; Müller, Harm GeertModelling atomic processes in intense laser fields often relies on solving the time-dependent Schrödinger equation (TDSE). For processes involving ionisation, such as above-threshold ionisation (ATI) and high-harmonic generation (HHG), this is a formidable task even if only one electron is active. Several powerful ideas for efficient implementation of atomic TDSE were introduced by H.G. Muller some time ago (Muller, 1999), including: separation of Hamiltonian terms into tri-diagonal parts; implicit representation of the spatial derivatives; and use of a rotating reference frame. Here, we extend these techniques to allow for non-uniform radial grids, arbitrary laser field polarisation, and non-Hermitian terms in the Hamiltonian due to the implicit form of the derivatives (previously neglected). We implement the resulting propagator in a parallel Fortran program, adapted for multi-core execution. Cost of TDSE propagation scales linearly with the problem size, enabling full-dimensional calculations of strong-field ATI and HHG spectra for arbitrary field polarisations on a standard desktop PC.
- ItemMultidimensional high harmonic spectroscopy of polyatomic molecules: detecting sub-cycle laser-driven hole dynamics upon ionization in strong mid-IR laser fields(Cambridge [u.a.] : Soc., 2016) Bruner, Barry D.; MaÅ¡Ãn, ZdenÄ›k; Negro, Matteo; Morales, Felipe; Brambila, Danilo; Devetta, Michele; Faccialà , Davide; Harvey, Alex G.; Ivanov, Misha; Mairesse, Yann; Patchkovskii, Serguei; Serbinenko, Valeria; Soifer, Hadas; Stagira, Salvatore; Vozzi, Caterina; Dudovich, Nirit; Smirnova, OlgaHigh harmonic generation (HHG) spectroscopy has opened up a new frontier in ultrafast science, where electronic dynamics can be measured on an attosecond time scale. The strong laser field that triggers the high harmonic response also opens multiple quantum pathways for multielectron dynamics in molecules, resulting in a complex process of multielectron rearrangement during ionization. Using combined experimental and theoretical approaches, we show how multi-dimensional HHG spectroscopy can be used to detect and follow electronic dynamics of core rearrangement on sub-laser cycle time scales. We detect the signatures of laser-driven hole dynamics upon ionization and reconstruct the relative phases and amplitudes for relevant ionization channels in a CO2 molecule on a sub-cycle time scale. Reconstruction of channel-resolved complex ionization amplitudes on attosecond time scales has been a long-standing goal of high harmonic spectroscopy. Our study brings us one step closer to fulfilling this initial promise and developing robust schemes for sub-femtosecond imaging of multielectron rearrangement in complex molecular systems.
- ItemFull-dimensional treatment of short-time vibronic dynamics in a molecular high-order-harmonic-generation process in methane(College Park, Md : APS, 2017) Patchkovskii, Serguei; Schuurman, Michael S.We present derivation and implementation of the multiconfigurational strong-field approximation with Gaussian nuclear wave packets (MC-SFA-GWP) - a version of the molecular strong-field approximation which treats all electronic and nuclear degrees of freedom, including their correlations, quantum mechanically. The technique allows realistic simulation of high-order-harmonic emission in polyatomic molecules without invoking reduced-dimensionality models for the nuclear motion or the electronic structure. We use MC-SFA-GWP to model isotope effects in high-order-harmonic-generation (HHG) spectroscopy of methane. The HHG emission in this molecule transiently involves the strongly vibronically coupled F22 electronic state of the CH4+ cation. We show that the isotopic HHG ratio in methane contains signatures of (a) field-free vibronic dynamics at the conical intersection (CI); (b) resonant features in the recombination cross sections; (c) laser-driven bound-state dynamics; as well as (d) the well-known short-time Gaussian decay of the emission. We assign the intrinsic vibronic feature (a) to a relatively long-lived (≥4 fs) vibronic wave packet of the singly excited ν4 (t2) and ν2 (e) vibrational modes, strongly coupled to the components of the F22 electronic state. We demonstrate that these physical effects differ in their dependence on the wavelength, intensity, and duration of the driving pulse, allowing them to be disentangled. We thus show that HHG spectroscopy provides a versatile tool for exploring both conical intersections and resonant features in photorecombination matrix elements in the regime not easily accessible with other techniques.
- ItemGeneral Time-Dependent Configuration-Interaction Singles II: The Atomic Case(Woodbury, NY : Inst., 2022-10-10) Carlström, Stefanos; Bertolino, Mattias; Dahlström, Jan Marcus; Patchkovskii, SergueiWe present a specialization of the grid-based implementation of the time-dependent configuration-interaction singles described in the preceding paper [S. Carlström et al., preceding paper, Phys. Rev. A 106, 043104 (2022)]. to the case of spherical symmetry. We describe the intricate time propagator in detail and conclude with a few example calculations. Among these, of note are high-resolution photoelectron spectra in the vicinity of the Fano resonances in photoionization of neon and spin-polarized photoelectrons from xenon, in agreement with recent experiments.
- ItemGeneral Time-Dependent Configuration-Interaction Singles I: The Molecular Case(Woodbury, NY : Inst., 2022-10-10) Carlström, Stefanos; Spanner, Michael; Patchkovskii, SergueiWe present a grid-based implementation of the time-dependent configuration-interaction singles method suitable for computing the strong-field ionization of small gas-phase molecules. After outlining the general equations of motion used in our treatment of this method, we present example calculations of strong-field ionization of He, LiH, H2O, and C2H4 that demonstrate the utility of our implementation. The following paper [S. Carlström et al., following paper, Phys. Rev. A 106, 042806 (2022)] specializes to the case of spherical symmetry, which is applied to various atoms.
- ItemAll-optical Stückelberg spectroscopy of strongly driven Rydberg states(College Park, MD : APS, 2022) Bengs, Ulrich; Patchkovskii, Serguei; Ivanov, Misha; Zhavoronkov, NickolaiThe AC Stark shift of electronic levels is ubiquitous in the interaction of intense light fields with atoms and molecules. As the light intensity changes on the rising and falling edges of a femtosecond laser pulse, it shifts the Rydberg states in and out of multiphoton resonances with the ground state. The two resonant pathways for transient excitation arising at the leading and the trailing edges of the pulse generate Young's type interference, generally referred to as the Stückelberg oscillations. Here we report the observation of the Stückelberg oscillations in the intensity of the coherent free-induction decay following resonant multiphoton excitation. Moreover, combining the experimental results with accurate numerical simulations and a simple model, we use the Stückelberg oscillations to recover the population dynamics of strongly driven Rydberg states inside the laser pulse by all-optical measurements after the end of the pulse. We demonstrate the potential of this spectroscopy to characterize lifetimes of Rydberg states dressed by laser fields with strengths far exceeding the Coulomb field between the Rydberg electron and the core.
- ItemPopulation transfer to high angular momentum states in infrared-assisted XUV photoionization of helium(Bristol : IOP Publ., 2020) Mayer, Nicola; Peng, Peng; Villeneuve, David M.; Patchkovskii, Serguei; Ivanov, Misha; Kornilov, Oleg; Vrakking, Marc J.J.; Niikura, HiromichiAn extreme-ultraviolet (XUV) laser pulse consisting of harmonics of a fundamental near-infrared (NIR) laser frequency is combined with the NIR pulse to systematically study two-color photoionization of helium atoms. A time-resolved photoelectron spectroscopy experiment is carried out where energy- A nd angle-resolved photoelectron distributions are obtained as a function of the NIR intensity and wavelength. Time-dependent Schrödinger equation calculations are performed for the conditions corresponding to the experiment and used to extract residual populations of Rydberg states resulting from excitation by the XUV + NIR pulse pair. The residual populations are studied as a function of the NIR intensity (3.5 × 1010-8 × 1012 W cm-2) and wavelength (760-820 nm). The evolution of the photoelectron distribution and the residual populations are interpreted using an effective restricted basis model, which includes the minimum set of states relevant to the features observed in the experiments. As a result, a comprehensive and intuitive picture of the laser-induced dynamics in helium atoms exposed to a two-color XUV-NIR light field is obtained. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemStrong-field control and enhancement of chiral response in bi-elliptical high-order harmonic generation: an analytical model(Bristol : IOP Publ., 2018-05-30) Ayuso, David; Decleva, Piero; Patchkovskii, Serguei; Smirnova, OlgaThe generation of high-order harmonics in a medium of chiral molecules driven by intense bi-elliptical laser fields can lead to strong chiroptical response in a broad range of harmonic numbers and ellipticities (Ayuso et al 2018 J. Phys. B: At. Mol. Opt. Phys. 51 06LT01). Here we present a comprehensive analytical model that can describe the most relevant features arising in the high-order harmonic spectra of chiral molecules driven by strong bi-elliptical fields. Our model recovers the physical picture underlying chiral high-order harmonic generation (HHG) based on ultrafast chiral hole motion and identifies the rotationally invariant molecular pseudoscalars responsible for chiral dynamics. Using the chiral molecule propylene oxide as an example, we show that one can control and enhance the chiral response in bi-elliptical HHG by tailoring the driving field, in particular by tuning its frequency, intensity and ellipticity, exploiting a suppression mechanism of achiral background based on the linear Stark effect.
- ItemChiral dichroism in bi-elliptical high-order harmonic generation(Bristol : IOP Publ., 2018-02-28) Ayuso, David; Decleva, Piero; Patchkovskii, Serguei; Smirnova, OlgaThe application of strong bi-elliptically polarized laser fields to the generation of high-order harmonics in organic molecules offers exceptional opportunities for chiral recognition and chiral discrimination. These fields are made by combining an elliptically polarized fundamental, typically in the infrared range, with its counter-rotating second harmonic. Here we present a theoretical study of the harmonic emission from the chiral molecule propylene oxide in bi-elliptical fields. Our calculations include, for the first time in such a complex system, accurate photorecomination matrix elements, evaluated using the static-exchange density functional theory method. We show that bi-elliptical light can induce strong chiral dichroism in the harmonic spectra of chiral molecules in a broad range of harmonic numbers and ellipticities.