T. T. Nguyen-Dang

Ultrafast molecular imaging by laser-induced electron diffraction

We address the feasibility of imaging geometric and orbital structures 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 CO{sub 2} molecule excited by a near-infrared few-cycle laser pulse. The molecular geometry (bond lengths) is determined within 3% of accuracy from a diffraction pattern which also reflects the nodal properties of the initial molecular orbital. Robustness of the structure determination is discussed with respect to vibrational and rotational motions with a complete interpretation of the laser-induced mechanisms.

Nonvariational time-dependent multiconfiguration self-consistent field equations for electronic dynamics in laser-driven molecules

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, tools, and facilities of existing, well-developed quantum chemical MCSCF codes. The theory uses, in particular, a unitary representation of time-dependent configuration mixings and orbital transformations. Within a short-time, or adiabatic approximation, the TDMCSCF scheme amounts to a second-order split-operator algorithm involving generically the two noncommuting one-electron and two-electron parts of the time-dependent electronic Hamiltonian. We implement the scheme to calculate the laser-induced dynamics of the two-electron H2 molecule described within a minimal basis, and show how electron correlation is affected by the interaction of the molecule with a strong laser field.

Laser induced electron diffraction: a tool for molecular orbital imaging

eld polarization, the position and relative heights of the associated fringes can be related to the molecular geometrical and orbital structure, using a simple inversion algorithm which takes into account the symmetry of the initial molecular orbital from which the ionized electron is produced. We show that it is possible to extract inter-atomic distances in the molecule from an averaged photon-electron signal with an accuracy of a few percents.

Time‐resolved laser control of vibrational excitations in molecules

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 formulation of the inverse problem associated with the time‐resolved control (tracking) of vibrational excitations. The control equation to be solved is quadratic in the area, when the object of the time‐resolved control is the total vibrational energy, and linear when the object to be controlled is an average elongation (position tracking), or the average energy of a remotely coupled mode. This yields a control algorithm which requires no iteration and is easy to implement. Numerical tests of the algorithm are performed on the energy and position trackings in simple one‐dimensional model systems. An excellent analytical, approximate description of the laser‐drive...

Dynamical quenching of laser-induced dissociations of heteronuclear diatomic molecules in intense infrared fields

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 synchronization of the molecular motions with the oscillations of the laser electric field [see F. Châteauneuf, T. Nguyen-Dang, N. Ouellet, and O. Atabek, J. Chem. Phys. 108, 3974 (1998)]. To this end, the wave packet dynamics of the HD+ and, to a lesser extent, the HCl+ molecular ions are considered in an intense IR laser field of variable frequency. Variations in the absolute phase of the laser electric field, a form of variations in the initial conditions, reveal new signatures of the DDQ effect due to the presence of nonzero permanent dipole moments in these molecules. The added permanent dipole/field interaction terms induce a discrimination between parallel an...

Dynamical quenching of field-induced dissociation of H2+ in intense infrared lasers

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 conditions of the laser field are considered. A new stabilization mechanism, called dynamical dissociation quenching, is found operative in the IR spectral range. In a time-resolved picture, this effect is shown to arise when a proper synchronization between the molecular motions and the laser field oscillations is ensured. In the Floquet, dressed molecule picture, the effect is related to interferences between the Floquet resonances that are excited initially by the nonadiabatic, sudden preparation of the ion. The Floquet analysis of the wave packets in this low frequency regime reveals important intersystem couplings between Floquet blocks, reflecting the highly...

Adiabatic time evolution of atoms and molecules in intense radiation fields

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 Schrodinger equation under a general unitary transformation U(t). The system will follow an adiabatic time evolution, if the transformed Hamiltonian, K(t)=U° HU−iℏU° U, is divisible into an effective Hamiltonian h(t), defining adiabatic quasistationary states, and an interaction term Ω(t), whose effect on the adiabatic states exactly cancels the nonadiabatic couplings arising from the adiabatic states’ parametric dependence on the time. This decoupling condition, which ensures adiabaticity in the system’s dynamics, can be expressed in a state independent manner, and governs the choice of the unitary operator U(t), as well as the construction of the effective Hamiltonian h(t). Using a restricted class of unitary transformations, the formalism is applied to the time evolution of an atomic or molecular system in interaction with a spatially uniform electromagnetic field, and gives an adiabatic approximation of higher order to the solutions of the semiclassical Schrodinger equation for this system. The adiabatic approximation so obtained exhibits two properties that make it suitable for the studies of intense field molecular dynamics: It is valid for any temporal profile of the field, and improves further as the field intensity increases, as reflected in the weakening of the associated residual nonadiabatic couplings with increasing field strength.

Molecular dichotomy within an intense high-frequency laser field

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 considered and the validity of the α0−2/3 scaling law of the Floquet eigenvalues, with respect to the field intensity parameter α0 of the HFFT, is discussed. To test the molecular dichotomy effect, numerical calculations are performed using a quantum chemical package (Gaussian 94), modified appropriately to incorporate the cycle-averaged displacements of the nuclear–electron Coulomb potential as found in the HFFT hamiltonian. Results of calculations on the two-electron H2 molecule are presented with an emphasis placed on the character of the total and orbital charge distributions and on trends to be observed in the electronic correlation at high intensities.

Nonperturbative wave packet dynamics of the photodissociation of H2+ in ultrashort laser pulses

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 coupled‐channel wave functions in momentum space. These structures appear in the channels’ momentum wave functions at P≠0, as the v=0 ground vibrational state is promoted to the dissociative channel then accelerated. The variations of these structures reflect the interplay between local laser‐induced transitions and the accelerating–decelerating action of intrinsic molecular forces. The wave packet dynamics are studied for rectangular and Gaussian pulses of varying durations and peak intensities. In addition, two forms of channel couplings were considered corresponding to two different choices of the gauge: the electric‐field (EF) gauge, in which the matter–field i...

Direct numerical integration of coupled equations with non-adiabatic interactions

Abstract 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.