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Francois Mauger

Publications and source records attributed to Francois Mauger.

At least 19 recordsLinked to original sources
arXiv2026

Variable coherence model for free-electron laser pulses

We introduce the variable coherence model (VCM) for simulating free-electron laser (FEL) pulses generated through self-amplified spontaneous emission. Building on the established partial coherence model of [T. Pfeifer et. al, Opt. Lett. 35, 3441 (2010)], we demonstrate that the implementation of a variable coherence width allows for continuous control over the pulses' characteristic noise, while keeping the average pulse parameters such as the bandwidth fixed. We demonstrate this through systematic statistical analyses of the intensity and number of sub-pulses in VCM pulses, in both time and frequency. In particular, we analyze how the sub-pulse statistics are affected by the coherence width parameter. We perform our analyses across three distinct regimes of FEL parameters and demonstrate how the VCM can generate pulses that range from maximally random to fully coherent. Finally, we illustrate the effect of the VCM variable coherence width on an absorption simulation.

physics.optics↗
arXiv2025

Extended phase-space symplectic integration for electron dynamics

We investigate the use of extended phase-space symplectic integration for simulating two different classes of electron dynamics. The first one, with one and a half degrees of freedom, comes from plasma physics and describes the classical dynamics of a charged particle in a strong, constant, and uniform magnetic field perturbed by a turbulent electrostatic potential. The second one, with an infinite number of degrees of freedom, comes from physical chemistry and corresponds to Kohn-Sham time-dependent density-functional theory. For both we lay out the extension procedure and stability condition for numerical integration of the dynamics using high-order symplectic split-operator schemes. We also identify a computationally inexpensive metric that can be used for on-the-fly estimation of the accuracy of simulations. Our work paves the way for broad application of symplectic split-operator integration of classical and quantum Hamiltonian systems with finite and infinite number of degrees of freedom by comparing different modes of implementation of extended phase space integration.

physics.comp-ph↗
arXiv2025

Configuration-interaction calculations with density-functional theory molecular orbitals for modeling valence- and core-excited states in molecules

We investigate configuration-interaction (CI) calculations on a basis of molecular orbitals generated by preliminary density-functional theory (DFT) calculations. We use this CI/DFT framework to improve the modeling of core-excited states by exploiting the flexibility and account for electron correlation of DFT orbitals compared to the canonical Hartree-Fock analogs. We assess the performance of our approach on the valence- and core-excited electronic states of three molecules with increasing levels of electron-correlation complexity: the singly bonded CH4, doubly bonded CO2, and triply bonded N2. For molecules with strong electron correlation effects, such as CO2 and N2, the inclusion of double excitations is important to model the core-hole excited states with reasonable accuracy. CI/DFT outperforms standard single-reference CI on Hartree-Fock molecular orbitals and competes with multi-reference CI calculations with multi-configuration self-consistent field orbitals in the modeling of molecules with strong electron-correlation effects, but weak multi-reference nature of their wave function such as CO2. In contrast, the choice of the molecular-orbital basis is irrelevant when modeling systems with negligible electron-correlation effects like CH4 or important multi-reference nature of their wave function like N2.

physics.chem-ph↗
arXiv2025

Orbital distortion and parabolic channel effects transform minima in molecular ionization probabilities into maxima

In the tunneling regime and at sufficiently low field amplitudes, the shape of orientation-dependent molecular ionization rate curves usually resembles the shape of the ionized orbital. As the ionizing field strength increases, the shape of the ionization rate can deviate from this pattern. The oft-cited explanation is that the increasing contribution of excited states relative to the ground state modifies the distribution. In this paper, we show that orbital distortion and parabolic channel effects, which are independent of excited-state effects, can also significantly modify the angular dependence of the yields of widely studied molecules where excited state effects are negligible. For example, we find that in CH$_3$Br, the interplay between orbital distortion and parabolic channel effects transforms a local minimum in the orientation-dependent ionization rate to a local maximum as the ionizing field strength increases. To simulate orbital distortion and parabolic channel effects, we use the one-electron weak-field asymptotic theory including the first-order correction (OE-WFAT(1)) in the integral representation. Since OE-WFAT(1) incurs expensive computations when the number of orientation angles is large, we also reformulate the original OE-WFAT(1) algorithm into a partial-wave expansion form, which greatly enhances the efficiency of the method.

physics.atom-ph↗
arXiv2024

Quantitative comparison of TDDFT-calculated HHG yields in ring-shaped organic molecules

We compare the high-harmonic-generation (HHG) yield driven by a mid-infrared laser in three organic ring-shaped molecules, calculated using time-dependent density-functional theory (TDDFT). We average the yield over the relative orientation of the molecules and the linearly-polarized, 1825 nm driving laser pulse in order to compare to experimental spectra obtained by Alharbi et al., Phys. Rev. A 92, 041801 (2015). We find that the raw TDDFT-calculated HHG yield in cyclohexane (CHA) is strongly overestimated compared to those of benzene and cyclohexene, and that this can be attributed to unphysically large contributions from CHA orbitals lying well below the highest-occupied molecular orbital. We show that implementing a simple orbital-resolved scaling factor, which corrects the yield of the tunneling ionization contribution to the first step in the HHG process, leads to much better comparisons with experimental results. Our results are encouraging for the use of TDDFT in systematic computations of HHG in large molecules.

physics.atom-ph↗
arXiv2024

All-Electron Molecular Tunnel Ionization Based on the Weak-Field Asymptotic Theory in the Integral Representation

Tunnel ionization (TI) underlies many important ultrafast processes, such as high-harmonic generation and strong-field ionization. Among the existing theories for TI, many-electron weak-field asymptotic theory (ME-WFAT) is by design capable of accurately treating many-electron effects in TI. An earlier version of ME-WFAT relied on an accurate representation of the asymptotic tail of the orbitals, which hindered its implementation in Gaussian-basis-set-based quantum chemistry programs. In this work, we reformulate ME-WFAT in the integral representation, which makes the quality of the asymptotic tail much less critical, hence greatly facilitating its implementation in standard quantum chemistry packages. The integral reformulation introduced here is therefore much more robust when applied to molecules with arbitrary geometry. We present several case studies, among which is the CO molecule where some earlier theories disagree with experiments. Here, we find that ME-WFAT produces the largest ionization probability when the field points from C to O, as experiments suggest. An attractive feature of ME-WFAT is that it can be used with various types of multielectron methods whether of density functional [Phys. Rev. A 106, 052211 (2022)] or multiconfiguration types, this in turn facilitates tunnel ionization calculation in systems exhibiting a strong multireference character.

physics.chem-ph↗
arXiv2024

Strong-field ionization with few-cycle, mid-infrared laser pulses induces a localized ionization followed by long-lasting charge migration in halogenated organic molecules

We show that applying a few-femtosecond mid-infrared laser pulse parallel to the backbone of a halogenated, conjugated organic molecule induces localized ionization followed by long-lasting field-free charge migration (CM). Through time-dependent density-functional theory calculations, we find that this CM is robust with respect to the parameters of the applied laser pulse. We document the spatial and temporal origin of the localized hole, which is initiated via strong-field tunnel ionization in the halogen end of the molecule, and near the peak of the laser electric field. In bromobutadiyne we find a range of wavelengths around $1500~\text{nm}$ that consistently leads to the initiation of high-contrast CM. In addition, we show that the inclusion of Ehrenfest nuclear dynamics does not disrupt the creation of the localized hole, nor the subsequent CM motion, in bromobutadiyne or \emph{para}-bromoaniline.

physics.atom-ph↗
arXiv2024

QMol-grid: A MATLAB package for quantum-mechanical simulations in atomic and molecular systems

The QMol-grid package provides a suite of routines for performing quantum-mechanical simulations in atomic and molecular systems, currently implemented in one spatial dimension. It supports ground- and excited-state calculations for the Schrodinger equation, density-functional theory, and Hartree-Fock levels of theory as well as propagators for field-free and field-driven time-dependent Schrodinger equation (TDSE) and real-time time-dependent density-functional theory (TDDFT), using symplectic-split schemes. The package is written using MATLAB's object-oriented features and handle classes. It is designed to facilitate access to the wave function(s) (TDSE) and the Kohn-Sham orbitals (TDDFT) within MATLAB's environment.

physics.comp-ph↗
arXiv2023

Tracking Charge Migration with Frequency-Matched Strobo-Spectroscopy

We present frequency-matched strobo-spectroscopy (FMSS) of charge migration (CM) in bromobutadiyne, simulated with time-dependent density-functional theory. CM+FMSS is a pump-probe scheme that uses a frequency-matched HHG-driving laser as an independent probe step following the creation of a localized hole on the bromine atom that induces CM dynamics. We show that the delay-dependent harmonic yield tracks the phase of the CM dynamics through its sensitivity to the amount of electron density on the bromine end of the molecule. FMSS takes advantage of the intrinsic attosecond time resolution of the HHG process, in which different harmonics are emitted at different times and thus probe different locations of the electron hole. Finally, we show that the CM-induced modulation of the HHG signal is dominated by the recombination step of the HHG process, with negligible contribution from the ionization step.

physics.atom-ph↗
arXiv2023

Hamiltonian formulation and symplectic split-operator schemes for time-dependent density-functional-theory equations of electron dynamics in molecules

We revisit Kohn-Sham time-dependent density-functional theory (TDDFT) equations and show that they derive from a canonical Hamiltonian formalism. We use this geometric description of the TDDFT dynamics to define families of symplectic split-operator schemes that accurately and efficiently simulate the time propagation for certain classes of DFT functionals. We illustrate these with numerical simulations of the far-from-equilibrium electronic dynamics of a one-dimensional carbon chain. In these examples, we find that an optimized 4th order scheme provides a good compromise between the numerical complexity of each time step and the accuracy of the scheme. We also discuss how the Hamiltonian structure changes when using a basis set to discretize TDDFT and the challenges this raises for using symplectic split-operator propagation schemes.

physics.comp-ph↗
arXiv2022

Characterizing Particle-Like Charge Migration Dynamics with High-Harmonic Sideband Spectroscopy

We introduce high-harmonic sideband spectroscopy (HHSS) and show that it can be a robust probe of attosecond charge migration (CM) in a halogenated carbon-chain molecule. We simulate both the CM and harmonic-generation (HHG) dynamics using ab initio time-dependent density-functional theory. We find that CM dynamics initiated along the molecular backbone induces sidebands in the HHG spectrum driven by a delayed laser pulse that is polarized perpendicular to the molecular axis. Monitoring the spectrum as either the HHG laser frequency or the relative delay is scanned allows for the extraction of detailed information about the time-domain characteristics of the CM process.

physics.atom-ph↗
arXiv2022

Charge Migration Manifests as Attosecond Solitons in Conjugated Organic Molecules

Charge migration is the electronic response that immediately follows localized ionization or excitation in a molecule, before the nuclei have time to move. It typically unfolds on sub-femtosecond time scales and most often corresponds to dynamics far from equilibrium, involving multi-electron interactions in a complex chemical environment. While charge migration has been documented experimentally and theoretically in multiple organic and inorganic compounds, the general mechanism that regulates it remains unsettled. In this work we use tools from nonlinear dynamics to analyze charge migration that takes place along the backbone of conjugated hydrocarbons, which we simulate using time-dependent density functional theory. In this electron-density framework we show that charge migration modes emerge as attosecond solitons and demonstrate the same type of solitary-wave dynamics in both simplified model systems and full three-dimensional molecular simulations. We show that these attosecond-soliton modes result from a balance between dispersion and nonlinear effects tied to time-dependent multi-electron interactions.%Our soliton-mode mechanism, and the nonlinear tools we use to analyze it, pave the way for understanding migration dynamics in a broad range of organic molecules.%For instance, we demonstrate the opportunities for chemically steering charge migration via molecular functionalization, which can alter both the initially localized electron perturbation and its subsequent time evolution.

physics.atm-clus↗
arXiv2015

Semi-Classical Wavefunction Perspective to High-Harmonic Generation

We introduce a semi-classical wavefunction (SCWF) model for strong-field physics and attosecond science. When applied to high harmonic generation (HHG), this formalism allows one to show that the natural time-domain separation of the contribution of ionization, propagation and recollisions to the HHG process leads to a frequency-domain factorization of the harmonic yield into these same contributions, for any choice of atomic or molecular potential. We first derive the factorization from the natural expression of the dipole signal in the temporal domain by using a reference system, as in the quantitative rescattering (QRS) formalism [J. Phys. B. 43, 122001 (2010)]. Alternatively, we show how the trajectory component of the SCWF can be used to express the factorization, which also allows one to attribute individual contributions to the spectrum to the underlying trajectories.

physics.atom-ph↗
arXiv2014

Electronic dynamics and frequency-dependent effects in circularly polarized strong-field physics

We analyze, quantum mechanically, the dynamics of ionization with a strong, circularly polarized, laser field. We show that the main source for non-adiabatic effects is connected to an effective barrier lowering due to the laser frequency. Such non-adiabatic effects manifest themselves through ionization rates and yields that depart up to more than one order of magnitude from a static-field configuration. Beyond circular polarization, these results show the limits of standard instantaneous - static-field like - interpretation of laser-matter interaction and the great need for including time dependent electronic dynamics.

physics.atom-ph↗
arXiv2013

Recollision scenario without tunneling : Role of the ionic core potential

The standard model of strong laser physics, the recollision scenario, omits the ionic core potential after tunneling. Strikingly, although the Coulomb interaction drives all stages of recollision, the maximum energy the electrons bring back to the core is found by ignoring it. We resolve this long-standing paradox by showing that this good agreement stems from a fortuitous cancellation at high intensities. Instead of the three step model, we find that the Coulomb interaction can be fully integrated into a purely classical scenario that explains recollisions without invoking tunneling.

physics.atom-ph↗
arXiv2013

Quantum-classical correspondence in circularly polarized high harmonic generation

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.

physics.atom-ph↗
arXiv2013

Recolliding orbits in an intense laser field

We show that a family of key periodic orbits drive the recollision process in a strong circulary polarized laser field. These orbits, coined recolliding periodic orbits, exist for a wide range of parameters and their relative influence changes as the laser and atomic parameters are varied. We find the necessary conditions for recollision-driven nonsequential double ionization to occur. The outlined mechanism is universal in that it applies equally well beyond atoms: The internal structure of the target species plays a minor role in the recollision process.

physics.atom-ph↗