Two-center interference and ellipticity in high-order harmonic generation from H2+_2^+

We present a theoretical investigation into the two-center interference in aligned H$_2^+$. The influence of the laser field on the recombination step is investigated by comparing laser-induced harmonic generation with harmonic generation from field-free collisions of Gaussian wave packets with the core. We find that for different Gaussian wave packets colliding with the molecule, the interference minimum occurs at the same alignment angle. The same result is obtained for the laser-induced spectrum when only a single electronic trajectory per harmonic contributes. When multiple electronic trajectories contribute, we find an effect on the minimum position because the interference between short and long trajectories is alignment-dependent. The two-center interference and the influence of the Coulombic potential are clearly seen not only in the harmonic intensity and phase but also in the polarization direction and ellipticity. We observe significant ellipticity of the emitted radiation around the two-center interference minimum.Comment: 10 pages, 15 figures; v2: clearer figures and other small changes; v3: small correction

Determination of ionization and tunneling times in high-order harmonic generation

From the numerical solution of the time-dependent Schrödinger equation, we obtain the times of ionization and return of the laser-driven electron in high-order harmonic generation by probing the dynamics with a second harmonic field polarized orthogonal to the fundamental field and observing the harmonic emission in dependence on the two-color delay. Our retrieval method using complex-time evolution gives ionization and return times in excellent agreement with the quantum-orbit model, while a retrieval based on real-time classical dynamics can introduce substantial errors. Because of the imaginary parts, the harmonic signal polarized along the probe field is nonzero for any two-color delay. The tunneling time can be retrieved under an assumption for the return time. © 2013 American Physical Society

Extreme-ultraviolet frequency combs from high-order harmonic generation with few-cycle pulse trains

The spectrum of a high-repetition train of laser pulses consists of many equally spaced lines, forming an optical frequency comb which is useful for high-precision spectroscopy. By exposing atoms to a train of strong pulses, frequency combs reaching into the extreme ultraviolet may be produced via high-order harmonic generation. Here, we report a theoretical study of extreme-ultraviolet frequency-comb generation by trains of few-cycle pulses. We analyze the nontrivial comb structure arising from overlapping harmonic orders. The spacings of the comb lines and their dependence on the offset frequency of the incident pulse train are discussed. © 2017 American Physical Society

Analysis of electron trajectories with two-color strong-field ionization

Two-color ionization of atoms with a strong 800-nm fundamental component and a weak 400-nm component with perpendicular polarization gives detailed insight into the ionization dynamics. When the delay between the two colors is varied on a subcycle scale, the slow-electron signal shows an oscillatory structure due to intracycle interference between different ionization times. Using a trajectory-based interference model, we extract the relative strength of the two contributing pathways. Ionization times can be read directly from the delay scan, and the times for the long trajectories agree well with the quantum-orbit model. The fast electrons arise predominantly from long rescattering trajectories. © 2015 American Physical SocietyDF

High-order harmonic generation in the presence of a resonance

We investigate high-order harmonic generation from laser-irradiated systems that support a shape resonance. From the numerical solution of the time-dependent Schrödinger equation, we calculate the harmonic spectra and the time-frequency analysis of the harmonic intensity and phase. The analysis reveals the separate contributions of the short and long trajectories as well as the resonance. A range of harmonics is strongly enhanced by the presence of the resonance irrespective of the pulse length. The signature of the resonance remains significant after coherent summation over intensities as a simple method to simulate macroscopic effects. The time-frequency analysis supports the recently proposed four-step mechanism of the enhanced harmonic generation process. © 2011 American Physical Society

Photoelectron circular dichroism of chiral molecules studied with a continuum-state-corrected strong-field approximation

Motivated by recent experiments on circular dichroism in the photoelectron momentum distributions from strong-field ionization of chiral molecules [C. Lux, Angew. Chem. Int. Ed. 51, 5001 (2012)1433-785110.1002/anie.201109035; C. S. Lehmann, J. Chem. Phys. 139, 234307 (2013)JCPSA60021-960610.1063/1.4844295], we investigate the origin of this effect theoretically. We show that it is not possible to describe photoelectron circular dichroism with the commonly used strong-field approximation due to its plane-wave nature. We therefore apply the Born approximation to the scattering state and use this as a continuum-state correction in the strong-field approximation. We obtain electron distributions for the molecules camphor and fenchone. In order to gain physical insight into the process, we study the contributions of individual molecular orientations. © 2014 American Physical Society.DFG/EXC/QUES

Multiconfiguration time-dependent Hartree approach for electron-nuclear correlation in strong laser fields

The multiconfiguration time-dependent Hartree approach is applied to study the electron-nuclear correlation in the dynamics of molecules subject to strong external laser fields, using the example of a model hydrogen molecular ion. The ground state of the system is well described by as few as two single-particle functions per degree of freedom. A significantly larger but moderate number of configurations is required to predict laser-induced fragmentation probabilities and high-order harmonic generation spectra accurately, showing that the correlation between electronic and nuclear degree of freedom is strongly increased by the presence of the laser field. © 2010 The American Physical Society

Probing Fano resonances with ultrashort pulses

In this paper, autoionizing states in the one-dimensional helium atom are investigated by numerical solution of the time-dependent two-electron Schrodinger equation. The atom is irradiated by an extreme ultraviolet (XUV) attosecond pulse and a time-delayed infrared few-cycle laser pulse. The XUV pulse populates a superposition of doubly excited states, leading to Fano resonances in the photoelectron spectrum. It is demonstrated that the Fano line profile is strongly modified by the presence of the laser field. Laser-induced coupling between the different doubly excited states causes the population of autoionizing states that cannot be reached by absorbing a single XUV photon from the ground state. The resulting additional peaks in the photoelectron spectrum are modulated as a function of time delay. Furthermore, the photoelectron spectrum exhibits a fringe pattern that is determined by the time delay but is independent of the details of the laser pulse.DFG/QUESTChina Scholarship Counci

Signatures of electronic structure in bi-circular high-harmonic spectroscopy

High-harmonic spectroscopy driven by circularly-polarized laser pulses and their counter-rotating second harmonic is a new branch of attosecond science which currently lacks quantitative interpretations. We extend this technique to the mid-infrared regime and record detailed high-harmonic spectra of several rare-gas atoms. These results are compared with the solution of the Schrodinger equation in three dimensions and calculations based on the strong-field approximation that incorporate accurate scattering-wave recombination matrix elements. A quantum-orbit analysis of these results provides a transparent interpretation of the measured intensity ratios of symmetry-allowed neighboring harmonics in terms of (i) a set of propensity rules related to the angular momentum of the atomic orbitals, (ii) atom-specific matrix elements related to their electronic structure and (iii) the interference of the emissions associated with electrons in orbitals co- or counter-rotating with the laser fields. These results provide the foundation for a quantitative understanding of bi-circular high-harmonic spectroscopy.Comment: Accepted in Physical Review Letter