Andre Bandrauk - Academia.edu (original) (raw)

Papers by Andre Bandrauk

Physical Review A, Jun 24, 2004

$Research paper thumbnail of Laser-induced electron diffraction inH2with linear and circular polarization ultrashort XUV laser pulses$

Physical Review Letters, 2005

We study enhanced ionization (EI) in asymmetric molecules by solving the 3D time-dependent Schröd... more We study enhanced ionization (EI) in asymmetric molecules by solving the 3D time-dependent Schrödinger equation for HeH 2 driven by a few-cycle laser pulse linearly polarized along the molecular axis. We find that EI is much stronger when the laser's carrier-envelope phase is such that the electric field at the peak of the pulse is antiparallel to the permanent dipole of the molecule (PDM). This phase dependence is explained by studying the molecule in the presence of a static electric field. When this field is antiparallel to the PDM, the energy of the dressed ground state moves up (with increasing internuclear distance R) to cross with excited states, leading to a stronger ionization via intermediate state resonances and via tunneling. We predict analytically the laser and molecular parameters at which these crossings are expected to occur in any asymmetric molecule.

Physical Review Letters, 2007

Exact (Born-Oppenheimer) 3-D numerical solutions of the time-dependent Schrödinger equation are o... more Exact (Born-Oppenheimer) 3-D numerical solutions of the time-dependent Schrödinger equation are obtained for the one electron linear H-H 2 atom-molecule system at large internuclear distance R in interaction with two-cycles intense (I > 10 14 W cm ÿ2) 800 nm laser pulses. High-order harmonic generation (HHG) spectra are obtained with an energy cutoff larger than the atomic maximum of I p 3U p , where I p is the ionization potential and U p is the ponderomotive energy. At large R, this extended cutoff is shown to be related to the nature of electron transfer, whose direction is shown to depend critically on the carrier-envelope phase (CEP) of the ultrashort pulse. Constructive and destructive interferences in the HHG spectrum resulting from coherent superpositions of electronic states in the H-H 2 system are interpreted in terms of multiple electron trajectories extracted from a time profile analysis.

Physical Review A, 2010

We report correlated two-electron ab initio calculations for the hydrogen molecule H 2 in interac... more We report correlated two-electron ab initio calculations for the hydrogen molecule H 2 in interaction with intense ultrashort laser pulses, via a solution of the full three-dimensional time-dependent Schrödinger equation. Our results for ionization and excitation probabilities (at 800 and 400 nm) as a function of internuclear distance R show strong evidence of enhanced ionization, in both single and double ionization, as well as enhanced excitation, in single and double excitation, as the internuclear distance R increases from the equilibrium value R e. The enhancement of all these molecular processes exhibits a maximum at a critical distance R c , which can be predicted from simple electrostatic and recollision models.

Physical Review A, 2007

The ionization of nonsymmetric heteronuclear diatomic molecules by intense few-cycle laser pulses... more The ionization of nonsymmetric heteronuclear diatomic molecules by intense few-cycle laser pulses linearly polarized along the internuclear axis has been investigated. It is found that enhanced ionization ͑EI͒ occurs in nonsymmetric molecules and is accompanied by enhanced excitation ͑EE͒. We show that the nonsymmetric distribution of the electron cloud between the two nuclei leads to a strong dependence of EI and EE on the carrier envelope phase of few-cycle pulses, and on the orientation of the molecule parallel or antiparallel with the peak electric field of the pulse. This effect is as strong as the pulse duration is short and disappears with increasing pulse duration. The field ionization model, and mainly the "energy level crossing" mechanism, are used to explain these phase effects. The newly proposed energy level crossing mechanism, which is relevant to nonsymmetric molecules, occurs as the driving field moves the dressed ground and excited states closer to each other until their energy levels cross, leading to an enhancement of excitation and ionization. A semiclassical nonadiabatic model derived to interpret the level crossing mechanism also predicts the critical internuclear distance R c at which EI, EE, and energy crossings occur as a function of charge asymmetry and laser intensity, in good agreement with quantum-mechanical simulations.

Physical Review A, 2005

We describe a numerical method used previously ͓Phys. Rev. A 70, 011404͑R͒ ͑2004͔͒ for solving th... more We describe a numerical method used previously ͓Phys. Rev. A 70, 011404͑R͒ ͑2004͔͒ for solving the three-dimensional time-dependent Schrödinger equation for H 2 + ͑with fixed nuclei͒ in interaction with an intense, arbitrary oriented laser pulse. In this approach, we use the prolate spheroidal coordinate system, and expand the time-dependent wave function in a complex basis of Laguerre polynomials and Legendre functions. Our results indicate that ionization, excitation, and harmonic generation are strongly influenced by the orientation of the molecular axis with respect to the laser polarization axis. We evaluate the contribution of each nucleus to harmonic generation, as this permits a quantitative and nonambiguous assessment of interference effects as a function of molecular orientation. A time-profile analysis, using a Gabor transform of the harmonic spectrum around certain harmonics, shows that every half-cycle high-order harmonics are emitted by each nucleus when the electron wave packet returns for a recollision with the molecular core, thus confirming the strong field recollision model in molecules. In general, each nucleus emits both odd and even harmonics, but even harmonics are destroyed by interferences between contributions of each nucleus. These interferences are shown to be maximum at certain harmonic orders as a function of molecular orientation. A comparison of acceleration and dipole formulations of the harmonic emission process is made in order to assess the use of high-order harmonic generation for electron wave-function imaging.

Structural Dynamics, 2015

We present molecular photoionization processes by intense attosecond ultraviolet laser pulses fro... more We present molecular photoionization processes by intense attosecond ultraviolet laser pulses from numerical solutions of time-dependent Schrödinger equations. Simulations preformed on a single electron diatomic H2+ show minima in molecular photoelectron energy spectra resulting from two center interference effects which depend strongly on molecular alignment. We attribute such sensitivity to the spatial orientation asymmetry of the photoionization process from the two nuclei. A similar influence on photoelectron kinetic energies is also presented.

Physical Review A, 2009

We investigate harmonic generation from H 2 + molecules driven by intense few-cycle laser pulses ... more We investigate harmonic generation from H 2 + molecules driven by intense few-cycle laser pulses whose linearly polarization axis makes an arbitrary angle with respect to the molecular axis. The H 2 + molecule is considered initially in various orbitals with nodal planes. It is found that a strong enhancement of high-order harmonics ͑HOHs͒ occurs when the laser polarization axis overlaps with major axes of electron distribution in the active orbital, while broad suppression of HOHs occurs when the laser polarization axis is parallel to a nodal plane of the active molecular orbital. We show that this harmonic suppression is enhanced by destructive interferences when the nodal and the laser polarization axes are parallel to the internuclear axis, which leads to a shortening of the harmonic cutoff. It follows that the orientation dependence of HOHs intensities mimics the electronic density in active orbitals through the angular dependence of ionization and recombination processes.

Journal of Physics B: Atomic, Molecular and Optical Physics, 2014

Using numerical simulations, we show that atomic high order harmonic generation, HHG, with a circ... more Using numerical simulations, we show that atomic high order harmonic generation, HHG, with a circularly polarized laser field offers an ideal framework for quantum-classical correspondence in strong field physics. With an appropriate initialization of the system, corresponding to a superposition of ground and excited state(s), simulated HHG spectra display a narrow strip of strong harmonic radiation preceded by a gap of missing harmonics in the lower part of the spectrum. In specific regions of the spectra, HHG tends to lock to circularly polarized harmonic emission. All these properties are shown to be closely related to a set of key classical periodic orbits that organize the recollision dynamics in an intense, circularly polarized field.

Physical review letters, Jan 7, 2014

Molecular high-order harmonic generation (MHOHG) in a non-Born-Oppenheimer treatment of H(2)(+), ... more Molecular high-order harmonic generation (MHOHG) in a non-Born-Oppenheimer treatment of H(2)(+), D(2)(+), is investigated by numerical simulations of the corresponding time-dependent Schrödinger equations in full dimensions. As opposed to previous studies on amplitude modulation of intracycle dynamics in MHOHG, we demonstrate redshifts as frequency modulation (FM) of intercycle dynamics in MHOHG. The FM is induced by nuclear motion using intense laser pulses. Compared to fixed-nuclei approximations, the intensity of MHOHG is much higher due to the dependence of enhanced ionization on the internuclear distance. The width and symmetry of the spectrum of each harmonic in MHOHG encode rich information on the dissociation process of molecules at the rising and falling parts of the laser pulses, which can be used to retrieve the nuclear dynamics. Isotope effects are studied to confirm the FM mechanism.

High-order harmonic generation (HHG) is a nonlinear nonperturbative process in ultrashort intense... more High-order harmonic generation (HHG) is a nonlinear nonperturbative process in ultrashort intense laser-matter interaction. It is the main source of coherent attosecond (1 as = 10 −18 s) laser pulses to investigate ultrafast electron dynamics. HHG has become an important table-top source covering a spectral range from infrared to extreme ultraviolet (XUV). One way to extend the cutoff energy of HHG is to increase the intensity of the laser pulses. A consequence of HHG in such intense short laser fields is the characteristic nonadiabatic red and blue shifts of the spectrum, which are reviewed in the present work. An example of this nonperturbative light-matter interaction is presented for the oneelectron nonsymmetric molecular ion HeH 2+ , as molecular systems allow for the study of the laser-molecule orientation dependence of such new effects including a four-step model of MHOHG (Molecular High-order Harmonic Generation).

Physical Review A, 2011

Molecular high-order harmonic generation (MHOHG) from the polar diatomic molecule HeH 2+ in short... more Molecular high-order harmonic generation (MHOHG) from the polar diatomic molecule HeH 2+ in short intense laser fields is studied numerically. Due to the nonadiabatic response of the molecular dipole to the rapid change of laser intensity, a spectral redshift is predicted in high-intensity and ultrashort laser pulses, contrary to the blueshift observed in the harmonics generated from atoms in long laser pulses. The MHOHG temporal structures are investigated by a wavelet time-frequency analysis, which shows that the enhanced excitation of localized long lifetime excited states shifts the harmonic generation spectrum in the falling part of short laser pulses, due to the presence of a permanent dipole moment, and thus is unique to polar molecules.

Canadian Journal of Chemistry, 2012

The nonlinear nonperturbative response of OCS and CS2 to ultrashort (few cycles) intense laser pu... more The nonlinear nonperturbative response of OCS and CS2 to ultrashort (few cycles) intense laser pulses was studied numerically by time-dependent density functional theory (TDDFT) methods to understand molecular ionization as a function of laser–molecule orientation. A time-dependent electron localization function(TDELF) was used to visualize the nonlinear nonperturbative electron transfer occurring during the laser pulse. It was found that, for intensities I > 3.5 × 1014 W/cm2, the inner shell Kohn–Sham (KS) molecular orbitals contribute significantly to the ionization, whereas for the intensity I < 3.5 × 1014 W/cm2, the highest occupied molecular orbital (HOMO) shows the dominant response to the field. In general, the ionization rate maxima correspond to the alignment of maximum KS orbital densities with the laser pulse polarization instead of orbital ionization potentials (IP). These findings are corroborated through analysis of the TDELF images, where the ionization occurs f...

Physical Review A

We investigate the dynamics of relativistic electrons interacting with intense laser fields in a ... more We investigate the dynamics of relativistic electrons interacting with intense laser fields in a linear or circular polarization. First, we study the momentum distributions of a single spatially localized wave packet. We find that these distributions are squeezed in the polarization plane (y-z) as well as along the laser propagation (x) direction. In a chosen gauge, the squeezing direction is controlled by the laser vector potential A and the electron initial momentum. For the case when the electron initial momentum is zero the squeezing occurs directly along the direction of A. We obtain analytical expressions within linear momentum approximation that explain the squeezing features very well by defining a squeezing vector and rotational angle of the squeezed momentum distribution. We analyze the symmetric properties of the momentum distributions viewed in different momentum planes and discuss the effects of different helicity of circular laser polarizations and the direction of the spin quantization. An unexpected feature of bending of momentum distribution is found for very intense laser fields. We extend our investigation to the momentum distribution of two spatially separated wave packets, particularly the orientations of two crossing distributions. It is found that the absolute phase of the initial laser field affects the orientation of the electron momentum distributions while quantum superposition of two states with the same spin gives interference in the momentum distributions that depends on the quantum phase of the electron.

Bulletin of the American Physical Society, Jun 5, 2013

Submitted for the DAMOP13 Meeting of The American Physical Society Resonantly enhanced electron-p... more Submitted for the DAMOP13 Meeting of The American Physical Society Resonantly enhanced electron-positron pair production in ultraintense laser-matter interaction FRANCOIS FILLION-GOURDEAI, Centre de recherches mathematiques, EMMANUEL LORIN, Carleton University, ANDRE BANDRAUK, Universite de Sherbrooke-A new mechanism for pair production from the interaction of a laser with two nuclei is presented. The latter takes advantage of the Stark effect in diatomic molecules and the presence of molecular resonances in the negative and positive energy continua. Both move in the complex energy plane as the interatomic distance and the electric field strength are varied. We demonstrate that there is an enhancement of pair production at the crossing of these resonances. This mechanism is studied in a very simple one-dimensional model where the nuclei are modelled by delta function potential wells and the laser by a constant electric field. The position of resonances is evaluated by using the Weyl-Titchmarch-Kodaira theory, which allows to treat singular boundary value problems and to compute the spectral density. The rate of producing pairs is also computed. It is shown that this process yields a positron production rate which is approximately an order of magnitude higher than in the single nucleus case and a few orders of magnitudes higher than Schwinger's tunnelling result in a static field.

Bulletin of the American Physical Society, Jun 8, 2007

Proceedings of SPIE, Jun 15, 2007

ABSTRACT This paper is devoted to the dynamics of attosecond pulses created during the high order... more ABSTRACT This paper is devoted to the dynamics of attosecond pulses created during the high order harmonic generation process. In this goal we study Ti:sapphir laser pulses propagating in a H2+ gas. The dynamics and propagation of the incident pulse is obtained by solving the macroscopic Maxwell equations. The molecular gas reaction on the electric field, the polarization, is derived from TDSE's following the model presented in [9], [10]. We are especially interested in this work, in the attosecond pulse dynamics and the intensity of the first harmonics dependently of the propagation length inside the gas, on the attosecond pulse generation and propagation and the energy of return graphs in function of the driver phase.