Thanh-Tung Nguyen-Dang

• Université Laval

Single ionization of H2 molecules exposed to strong and short laser pulses is investigated by a semi-classical method. Three laser characteristics are considered: (i) The carrier-wave frequency corresponds to wavelengths covering and bridging the two ionization regimes: From tunnel ionization (TI) at 800 nm to multiphoton ionization (MPI) at 266 nm...

Single ionization of H$_2$ molecules exposed to strong and short laser pulses is investigated by a semi-classical method. Three laser characteristics are considered: i) The carrier-wave frequency corresponds to wavelengths covering and bridging the two ionization regimes: From tunnel ionization (TI) at 800 nm to multiphoton ionization (MPI) at 266...

The present work explores quantitative limits to the Single-Active Electron (SAE) approximation, often used to deal with strong-field ionization and subsequent attosecond dynamics. Using a time-dependent multi\-configuration approach, specifically a Time-Dependent Configuration Interaction (TDCI) method, we solve the time-dependent Schr{\"o}dinger...

The present work explores quantitative limits to the Single-Active Electron (SAE) approximation, often used to deal with strong-field ionization and subsequent attosecond dynamics. Using a time-dependent multiconfiguration approach, specifically a Time-Dependent Configuration Interaction (TDCI) method, we solve the time-dependent Schrödinger equati...

The cover art is based on the Research Paper: ATP synthase: A moonlighting enzyme with unprecedented functions, https://doi.org/10.1039/D1CC06793A.

A recently developed methodology is used to study the dynamics of electronic excitations and ionization of the linear molecule CO_2 in an intense Infrared laser pulse. Emphasis is placed on many-electron, many-orbital effects on channel-resolved momentum distributions of the photoelectron, in both the LIED (Laser-Induced Electron Diffraction) and t...

The fringe pattern that allows geometrical and orbital structure information to be extracted from LIED spectra of symmetric molecules is shown to reflect a symmetry conservation principle. We show that under a field polarization which preserves certain symmetry elements of the molecule, the symmetry character of the initial wave function is conserv...

In this paper, we discuss the possibility of imaging molecular orbitals from Photoelectron spectra obtained via Laser Induced Electron Diffraction (LIED) in linear molecules. This is an extension of our work published recently in Physical Review A 94, 023421 (2016) to the case of the HOMO-1 orbital of the carbon dioxide molecule. We show that such...

The fringe pattern that allows geometrical and orbital structure information to be extracted from LIED spectra of symmetric molecules is shown to reflect a symmetry conservation principle. We show that under a field polarization which preserves certain symmetry elements of the molecule, the symmetry character of the initial wave function is conserv...

In this paper, we discuss the possibility of imaging molecular orbitals from photoelectron spectra obtained via Laser Induced Electron Diffraction (LIED) in linear molecules. This is an extension of our work published recently in Physical Review A \textbf{94}, 023421 (2016) to the case of the HOMO-1 orbital of the carbon dioxide molecule. We show t...

Imaging structures at the molecular level is a fast developing interdisciplinary research field that spans across the boundaries of physics and chemistry. High spatial resolution images of molecules can be obtained with photons or ultrafast electrons. In addition, images of valence molecular orbitals can be extracted via tomographic techniques base...

Imaging structures at the molecular level is a fast developing interdisciplinary research field that spans across the boundaries of physics and chemistry. High spatial resolution images of molecules can be obtained with photons or ultrafast electrons. In addition, images of valence molecular orbitals can be extracted via tomographic techniques base...

A Time-Dependent Configuration Interaction approach using multiple Feshbach partitionings, corresponding to multiple ionization stages of a laser-driven molecule, has recently been proposed [T.-T. Nguyen-Dang and J. Viau-Trudel, J. Chem. Phys. 139, 244102 (2013)]. To complete this development toward a fully ab-initio method for the calculation of t...

To describe successive ionization steps of a many-electron atom or molecule driven by an ultrashort, intense laser pulse, we introduce a hierarchy of successive two-subspace Feshbach partitions of the N-electron Hilbert space, and solve the partitioned time-dependent Schrödinger equation by a short-time unitary algorithm. The partitioning scheme al...

We discuss how a recent pump-probe study [Kelkensberg et al., Phys. Rev. Lett. 103, 123005 (2009)] of the dissociative ionization of H_{2}, under the combined effect of a single extreme ultraviolet attosecond pulse and an intense near-infrared pulse, actually represents a transition-state spectroscopy of the strong-field dissociation step, i.e., of...

We explore the laser-induced ionization dynamics of N2 and CO2 molecules subjected to a few-cycle, linearly polarized, 800 nm laser pulse using effective two-dimensional single-active-electron time-dependent quantum simulations. We show that the electron recollision process taking place after an initial tunnel ionization stage results in quantum in...

We address the feasibility of imaging geometric and orbital structure of a polyatomic molecule on an attosecond time-scale using the laser induced electron diffraction (LIED) technique. We present numerical results for the highest molecular orbitals of the CO2 molecule excited by a near infrared few-cycle laser pulse. The molecular geometry (bond-l...

The separability of a dressed molecule, a composite molecule + quantized radiation field system, at high field intensities is examined. Various forms of the Hamiltonian describing the dressed molecule are reviewed and are used to assess the zeroth-order separability of the dressed system. A new high-order adiabatic separation between the strongly c...

Electromagnetic field induced imaging and control of ionization or dissociation processes are better understood while referring to basic mechanisms which are molecular non-linear responses to intense and eventually ultrashort laser pulses. We are reviewing some of such mechanisms as bond softening, barrier lowering, vibrational trapping, dynamical...

Many aspects of intense-field molecular dynamics rely on the concept of res-onances. The chapter gives a thorough review of these aspects, bringing out the specificity of laser-induced resonances, in particular those defined in the Floquet or dressed molecule picture. The role of these resonances in the time-resolved dynamics of molecules subjected...

A non-variational Time-Dependent Multi-Configuration Self-Consistent Field (TDMCSCF) scheme has been developed [16] to describe, by an ab-initio approach, the time-resolved electron dynamics of a laser-driven many-electron atomic or molecular system. Its implementation within a typical quantum chemical package is discussed and its use demonstrated...

We present a combined theoretical and experimental study of ultrafast wave-packet dynamics in the dissociative ionization of H_{2} molecules as a result of irradiation with an extreme-ultraviolet (XUV) pulse followed by an infrared (IR) pulse. In experiments where the duration of both the XUV and IR pulses are shorter than the vibrational period of...

A non-variational Time-Dependent Multiconfiguration Self-Consistent Field (TDMCSCF) scheme has been developed [Nguyen-Dang T.T., Peters M., Wang S.-M., Sinelnikov E., and Dion F., J. Chem. Phys. 127, 174107 (2007)] to describe, by an ab-initio approach, the time-resolved electron dynamics of a laser-driven many-electron atomic or molecular system....

An efficient and selective laser-induced molecular stabilization with respect to dissociation is obtained by an appropriate shaping of an intense ultraviolet-visible laser pulse which ensures the adiabatic transport of the system onto an infinitely long-lived, zero-width Floquet resonance. Such a resonance is obtained for a frequency-dependent crit...

A time-dependent multiconfiguration self-consistent field (TDMCSCF) scheme is developed to describe the time-resolved electron dynamics of a laser-driven many-electron atomic or molecular system, starting directly from the time-dependent Schrodinger equation for the system. This nonvariational formulation aims at the full exploitations of concepts,...

A perturbative series is derived for the systematic construction of a dynamical invariant (Lewis invariant) for a time-dependent Hamiltonian which is characterized by a time-scale parameter {tau}, as appears in the usual formulation of the adiabatic theorem. The derivations make efficient use of the quantum averaging method, and the perturbative se...

We show how efficient control can be exerted on molecular dissociation by adiabatic shaping of an intense, single laser pulse in a very simplified way, involving a few parameters (e.g., peak amplitude, carrier-wave, and modulation phases and frequencies). The strategy takes advantage of an appropriate combination of basic multiphoton mechanisms: bo...

Using a recent reformulation of Floquet theory [ S. Guérin and H. R. Jauslin Adv. Chem. Phys. 125 1 (2003)], we discuss the dynamical role of the absolute phase in the photofragmentation of molecules subjected to laser pulses. We show how the dependence of Floquet states on an absolute phase is related to the complexity of the dressed molecular sch...

Some new properties of the Lagrange multiplier μ introduced through the normalization constraint on ρ in the variations of energy density functionals are determined. Through arguments concerning the homogeneity properties of these functionals with respect to μ, it is demonstrated that at the point of variation μ = μ0 = E0/N, where E0 is the ground...

This chapter discusses the theory of intense laser-induced molecular dissociation. Intense laser-induced molecular dissociation is definitely one of the most challenging applications of Quantum Mechanics where theoretical models and their computer-based numerical simulations may help in a quantitative understanding and prediction of experimental di...

Detailed results of numerical one-dimensional wave-packet simulations are reported for the dynamics of a Dâ{sup +} molecule subjected to separated and combined 400-nm and 800-nm intense laser pulses with their polarization vectors lying either parallel or perpendicular to the internuclear axis of the molecule. Depending on the laser excitation cond...

We establish a rigorous demonstration that the quasi-static picture of molecular dynamics in intense laser fields derives from a high-order adiabatic representation associated with an approximate Lewis dynamical invariant of the laser-driven system. Qualitative changes in molecular structure as reflected in the molecular orbital compositions are di...

In an Infra-red (IR) laser field, synchronizing molecular wavepacket motions and the field oscillations can give interesting dynamical effects, suggesting new control schemes. We demonstrate that these effects are intrinsically due to a local time-asymmetry in the time-dependent molecular structure and force field impressed by the laser field on th...

The dynamical dissociation quenching (DDQ) effect is a new mechanism for laser-induced vibrational trapping of molecules in the infrared (IR) spectral range. Previously demonstrated for one-dimensional, prealigned diatomic molecules [see F. Châteauneuf, T. Nguyen-Dang, N. Ouellet, and O. Atabek, J. Chem. Phys. 108, 3974 (1998)], the effect was show...

A theory of molecular structure is presented. The theory demonstrates that the concepts of atoms and bonds may be rigorously defined and given physical expression in terms of the topological properties of the observable distribution of charge for a molecular system. As a consequence of these definitions, one in turn obtains a definition of structur...

Experiments have been carried out to study the dissociative ionization of a D2 molecule interacting with an intense linearly polarized two-color (ω+2ω) laser field. The response of the molecule to the individual laser fields at 800 and 400 nm indicate that at the shorter wavelength some of the molecules are dissociating in a direction perpendicular...

Dissociative ionization of the D2 molecule was studied using linearly polarized 400- and 800-nm laser pulses. At 800 nm, the D+ ions resulting from the dissociation of the molecule are observed to be parallel to the polarization of the laser field. At the shorter wavelength (400 nm), we observe that a considerable number of D+ ions are ejected perp...

It is shown that, under an intense high-frequency laser field, electronic distributions in molecules exhibit a dichotomy effect just as previously found in atoms. The generalization of the formal demonstration of the dichotomy effect as given in M. Gavrila and J. Shertzer, Phys. Rev. A 41, 477 (1990) to many-electron, polyatomic molecules is consid...

Molecular internal force fields and photofragmentation dynamics may dramatically be affected by very strong radiative interactions. Highly non-linear field induced barrier lowering and stabilization mechanisms, either in the UV–Visible or IR excitation regimes are at the origin of the chemical bond softening and hardening processes, respectively. F...

This article explores the influence of permanent dipole moments, i.e., of direct vibrational excitations, on the dynamical dissociation quenching (DDQ) effect, a mechanism for laser-induced vibrational trapping in the infrared (IR) spectral range which was recently demonstrated for the homonuclear H2+ ion, and was shown to result from a proper sync...

The present paper deals with the development of a Mathematica package of programs for handling quantum mechanical equations involving commutators and unitary transformations. One program implements general commutators handling rules. Another evaluates a series of nested commutators representing a general unitary transformations. We apply these new...

Theoretical simulations of processes leading to the complete Coulomb explosion of H2 under a 150-fs, 800-nm Ti:sapphire laser pulse are made using two approaches. The first approach consists of purely nuclear, multichannel wave-packet calculations while the second approach involves electronic-nuclear, three-body wave-packet propagations on one-dime...

The dynamics of dissociation of the hydrogen molecular ion H2+ in an intense infrared (IR) field is studied by a series of wave packet simulations. In these simulations, the molecular ion is assumed to be instantly prepared at the initial time by a sudden ionization of the ground-state H2 parent molecule, and a variety of frequency and intensity co...

The dynamics of multielectron dissociative ionization (MEDI) of H2 in an intense IR laser pulse are investigated using a wave-packet propagation scheme. The electron tunneling processes corresponding to the successive ionizations of H2 are expressed in terms of field-free Born-Oppenheimer (BO) potential energy surfaces (PES) by transforming the tun...

The description of the wave-packet time-resolved dynamics in a two-channel molecular system driven by a cw laser field is considered within the time-independent Floquet representation. It is shown that, at high field intensity, the wave-packet motions are governed solely by the pair of adiabatic dressed potential-energy surfaces (PES's) associated...

We show that, on a short time scale, the dynamics of vibrational excitations in multimode ground-state molecular systems, linearly coupled to a laser field, can be expressed as a simple functional of the laser pulse area. The dependence of the vibrational system’s dynamics on a field area leads to simple algebraic equations for this area, in the fo...

We review tunnel ionization of atoms and molecules by an intense CO2 laser. Newest recent results and calculation are also presented concerning dissociative tunnel ionization of diatomic molecules.

Femtosecond laser pulses have been used to monitor chemical dynamics with the finest time- resolved details. They also offer the possibility of actually steering the molecular dynamics of a reactive system by controlling the laser pulses' parameters. Explorations of this possibility follow two main avenues: laser coherent control exploits phase coh...

The properties of the laser-induced first resonance state in the photodissociation of H+2 are obtained from calculations in the radiation-field (RF) gauge, also called the velocity gauge, as a function of intensity and wavelength of the field within an adiabatic electronic-field representation, and the results are compared to the corresponding ones...

The wave packet dynamics of the photodissociation of H2(+) under excitation by laser pulses of short durations at 329.7 nm are studied. The photodissociation process involves essentially two coupled channels, and the detailed mechanism for the formation of fragment kinetic energy spectra is examined by following the evolution of structures in the c...

The formalism developed recently by Nguyen-Dang, Manoli, and Abou-Rachid [Phys. Rev. A 43, 5012 (1991)] describes nonperturbatively the dynamics of N-level systems in the presence of a laser pulse in terms of a generalized adiabatic evolution. This formalism is adapted to multichannel molecular systems, defining a time-dependent electronic represen...

A time‐dependent adiabatic electronic representation is defined by solving the local N‐level electronic time‐dependent Schrödinger equations at each nuclear configuration of a general N‐channel, laser‐driven molecular system. These solutions are eigenstates of a time‐dependent effective Hamiltonian with respect to which the exact time‐evolution of...

Field-induced resonances in H2+ dressed by an intense laser field are calculated in the electric-field gauge (length gauge) in a fully coupled adiabatic electronic-field representation. Distinctions between diabatic and adiabatic resonance states are made. Adiabatic resonances are characterized in terms of shape and Feshbach resonances.

The exact adiabatic representation scheme of Nguyen-Dang [J. Chem. Phys. 90, 2657 (1989)] is applied to time evolution of the general laser-driven two- and three-level systems. It leads to exact formal results that do not assume a specific shape of the pulse and do not invoke the rotating-wave approximation (RWA). These results are a generalization...

We derive the condition for a time dependent quantum system to exhibit an exact or higher order adiabatic time evolution. To this end, the concept of adiabaticity is first analyzed in terms of the transformation properties of the time-dependent Schro¨dinger equation under a general unitary transformation Uˆ(t). The system will follow an adiabatic t...

A generalization of the Numerov three-point recurrence formula is obtained to permit direct numerical integration of non-adiabatically coupled differential equations. The generalized algorithm is tested on one- and two-channel model systems.

In the Bloch-Nordsieck representation of dressed molecules1–4, which is the quantal counterpart of the space-translation semiclassical method5, the interaction between the radiation field and the molecular system is transformed into field-dependent displacements in the molecular potential energy. An adiabatic separation between the field and the mo...

The phase corrected adiabatic approximation [Y. Maréchal, J. Chem. Phys. 83, 247 (1985); T. T. Nguyen‐Dang and A. D. Bandrauk, J. Chem. Phys. 85, 7224 (1986)], is applied to obtain a second‐order adiabatic representation for the generalized Hénon–Heiles system. This representation provides a formal link between nonintegrable Hénon–Heiles systems an...

The phase corrected adiabatic separation method proposed recently by Marechal (J. Chem. Phys. 83, 247 (1985)) is reexamined. We have been able to show that a state independent differential equation exists, which governs the phase coupling function f of this method, if f further satisfies a set of constraints, Eqs. (7) of the present paper. The meth...

The dressed molecular Hamiltonian for molecules in strong electromagnetic fields is derived in the Bloch–Nordsieck (BN) and electric field (EF) representations beyond the dipole approximation. Both representations, which are related by simple unitary transformations generate photon coherent states beyond the dipole approximation. It is shown that a...

The validity of the adiabatic approximation applied to systems of two coupled oscillators–typically represented by the Henon–Heiles’ Hamiltonian–is examined. Through explicit analytical estimates of nonadiabatic couplings, it is shown that the adiabatic approximation is appropriate in all cases, except for the case of 1:1 Fermi resonance between th...

It is shown that coupled equations for multiphoton processes can be written with either momentum transition moments or dipole transition moments. In perturbation (low field intensity) theories, nonadiabatic corrections are necessary to make the adiabatic momentum electronic transition probabilities nonzero and equivalent to electronic dipole transi...

Electrons, nuclei, and photons are treated simultaneous quantum mechanically for molecular multiphoton processes. Coupled equations are derived for these processes in weak and intense electromagnetic fields. Different gauges are shown to give rise to different adiabatic separations for all three constituents. The Bloch Nordsieck transformation of Q...

The Bloch–Nordsieck transformation of quantum electrodynamics is applied to the molecular Hamiltonian in order to include explicitly radiative corrections in molecular dynamics. General coupled equations are obtained for all field strengths. In the adiabatic limit, we obtain dressed potential surfaces which undergo radiative distortions relative to...

This paper illustrates how the topological theory of molecular structure can be used to provide an answer as to whether or not homoaromatic conjugation is present in a given system. The study hinges upon the particular properties of a cyclopropyl ring. As a consequence of the proximity of the bond and ring critical points in this system, the ring p...

The topological properties of a molecular charge distribution serve to define a theory of structure and structural stability.

This chapter deals with the quantum mechanical definition of the average properties of an atom. It is demonstrated that the topological property that defines the atom determines the definition of its average properties. It reviews only the basic topological properties of a charge distribution in this chapter. Their role in the definition of molecul...

An algorithm for calculating atomic properties, based on the topological theory of molecular structure, is described and applied. For any given molecular system atoms are rigorously defined in terms of the topological properties of the system's charge distribution rho (x) in three-dimensional space. The essential feature of this distribution is tha...

The relation between the structural stability of a molecular system as determined by the topological properties of its charge distribution and the energetic stability of the same system as determined by the properties of its potential energy hypersurface is studied. In general, it is found that one may associate a given molecular structure with an...

This thesis is concerned with the rigorous definitions of the two central concepts embodied in the notion of molecular structure, namely, the concepts of atoms and bonds. The basis for the present approach is provided by the topological properties of the charge distribution in a given molecular system. The essential observation is that the only loc...

In this paper we review and exemplify a new and rigorous approach to the problem of molecular structure and its morphogenesis: the theory of quantum topology. The basis for this approach is provided by the topology of the total charge density in a given molecular system. The essential observation is that the only local maxima of a ground state dist...

This paper illustrates how the concepts of atoms and bonds may be given definite expression in terms of the topological properties of the charge density, (r), and how, as a consequence of these identifications, one is led to a definition of structure and to a phenomenological analysis of structural stability. This approach finds its natural express...

Schwinger’s quantum action principle is used to obtain a quantum mechanical description of a subspace and its properties. The subspaces considered are those regions (Ω) of real space as defined by a property of the system’s charge distribution ρ (r), namely, that they be bounded by a surface S (r) through which the flux in ∇ρ (r) is zero at every p...

This work is a continuation and extension of the delineation of the properties of a quantum subspace—a region of the real space of a molecular system bounded by a surface through which the flux in the gradient of the (observable) charge density is zero. Such subspaces are of interest as they constitute a basis for theoretical definitions of chemica...