Foudhil Bouakline

Bio: Foudhil Bouakline is an academic researcher from University of Potsdam. The author has contributed to research in topics: Identical particles & Vibrational energy relaxation. The author has an hindex of 6, co-authored 13 publications receiving 110 citations.

Seemingly Anomalous Angular Distributions in H + D2 Reactive Scattering

Abstract: When atoms and molecules collide, the energy embedded in the reaction products gets distributed among translations, vibrations, and rotations. Decades of meticulous experiments have mapped out the quantum mechanical rules underlying this distribution process, particularly in simple systems comprising just three light atoms. Now, Jankunas et al. (p. [1687][1]; see the Perspective by [ Yang et al. ][2]) describe a previously unappreciated wrinkle in the elementary reaction of an H atom with deuterium. Typically, products with low vibrational and rotational excitation tend to scatter backwards from the collision, whereas the spinning products scatter sideways. Above a certain vibrational threshold, however, spinning HD products were observed to scatter backwards. [1]: /lookup/volpage/336/1687 [2]: /lookup/doi/10.1126/science.1223680

Reduced and exact quantum dynamics of the vibrational relaxation of a molecular system interacting with a finite-dimensional bath.

Abstract: We investigate the vibrational relaxation of a Morse oscillator, nonlinearly coupled to a finite-dimensional bath of harmonic oscillators at zero temperature, using two different approaches: Reduced dynamics with the help of the Lindblad formalism of reduced density matrix theory in combination with Fermi's Golden Rule, and exact dynamics (within the chosen model) with the multiconfiguration time-dependent Hartree (MCTDH) method. Two different models have been constructed, the situation where the bath spectrum is exactly resonant with the anharmonic oscillator transition frequencies, and the case for which the subsystem is slightly off-resonant with the environment. At short times, reduced dynamics calculations describe the relaxation process qualitatively well but fail to reproduce recurrences observed with MCTDH for longer times. Lifetimes of all the vibrational levels of the Morse oscillator have been calculated, and both Lindblad and MCTDH results show the same dependence of the lifetimes on the initial vibrational state quantum number. A prediction, which should be generic for adsorbate systems is a striking, sharp increase of lifetimes of the subsystem vibrational levels close to the dissociation limit. This is contradictory with harmonic/linear extrapolation laws, which predict a monotonic decrease of the lifetime with initial vibrational quantum number.

Differential cross sections for H + D2 → HD(v′ = 2, j′ = 0,3,6,9) + D at center-of-mass collision energies of 1.25, 1.61, and 1.97 eV

Abstract: We have measured differential cross sections (DCSs) for the reaction H + D2 → HD(v′ = 2,j′ = 0,3,6,9) + D at center-of-mass collision energies Ecoll of 1.25, 1.61, and 1.97 eV using the photoloc technique. The DCSs show a strong dependence on the product rotational quantum number. For the HD(v′ = 2,j′ = 0) product, the DCS is bimodal but becomes oscillatory as the collision energy is increased. For the other product states, they are dominated by a single peak, which shifts from back to sideward scattering as j′ increases, and they are in general less sensitive to changes in the collision energy. The experimental results are compared to quantum mechanical calculations and show good, but not fully quantitative agreement.

Is it really possible to control aromaticity of benzene with light

Abstract: Recent theoretical investigations claim that tailored laser pulses may selectively steer benzene's aromatic ground state to localized non-aromatic excited states For instance, it has been shown that electronic wavepackets, involving the two lowest electronic eigenstates, exhibit subfemtosecond charge oscillation between equivalent Kekule resonance structures In this contribution, we show that such dynamical electron-localization in the molecule-fixed frame contravenes the principle of the indistinguishability of identical particles This breach stems from a total omission of the nuclear degrees of freedom, giving rise to nonsymmetric electronic wavepackets under nuclear permutations Enforcement of the latter leads to entanglement between the electronic and nuclear states To obey quantum statistics, the entangled molecular states should involve compensating nuclear-permutation symmetries This in turn engenders complete quenching of dynamical electron-localization in the molecule-fixed frame Indeed, for the (six-fold) equilibrium geometry of benzene, group-theoretic analysis reveals that any electronic wavepacket exhibits a (D6h) totally symmetric electronic density, at all times Thus, our results clearly show that the six carbon atoms, and the six C–C bonds, always have equal Mulliken charges, and equal bond orders, respectively However, electronic wavepackets may display dynamical localization of the electronic density in the space-fixed frame, whenever they involve both even and odd space-inversion (parity) or permutation-inversion symmetry Dynamical spatial-localization can be probed experimentally in the laboratory frame, but it should not be deemed equivalent to charge oscillation between benzene's identical electronic substructures, such as Kekule resonance structures

Does nuclear permutation symmetry allow dynamical localization in symmetric double-well achiral molecules?

Abstract: We discuss the effect of molecular symmetry on coherent tunneling in symmetric double-well potentials whose two molecular equilibrium configurations are interconverted by nuclear permutations This is illustrated with vibrational tunneling in ammonia molecules, electronic tunneling in the dihydrogen cation, and laser-induced rotational tunneling of homonuclear diatomics In this contribution, we reexamine the textbook picture of coherent tunneling in such potentials, which is depicted with a wavepacket shuttling back and forth between the two potential-wells We show that the common application of this picture to the aforementioned molecules contravenes the principle of the indistinguishability of identical particles This conflict originates from the sole consideration of the dynamics of the tunneling-mode, connecting the double-well energy minima, and complete omission of all the remaining molecular degrees of freedom This gives rise to double-well wavepackets that are nonsymmetric under nuclear permutations To obey quantum statistics, we show that the double-well eigenstates composing these wavepackets must be entangled with the wavefunctions that describe all the omitted molecular modes These wavefunctions have compensating and opposite nuclear permutation symmetry This in turn leads to complete quenching of interference effects behind localization in one potential-well or another Indeed, we demonstrate that the reduced density of probability of the symmetrized molecular wavefunction, where all the molecular coordinates but the tunneling-mode are integrated out, is symmetrically distributed over the two potential-wells, at all times This applies to any multilevel wavepacket of isotropic or fully aligned symmetric double-well achiral molecules However, in the case of coherent electronic or vibrational tunneling, fully aligned molecules may exhibit dynamical localization in the space-fixed frame, where the tunneling-mode density shuttles between the opposite directions of the alignment axis This dynamical spatial-localization results from linear combinations of molecular states that have opposite parity In summary, this study shows that dynamical localization of the tunneling-mode density on either of the two indistinguishable molecular equilibrium configurations of symmetric double-well achiral molecules is forbidden by quantum statistics, whereas its dynamical localization in the space-fixed frame is allowed by parity The subtle distinction between these two types of localization has far-reaching implications in the interpretation of many ultrafast molecular dynamics experiments

Synthesis of self-pillared zeolite nanosheets by repetitive branching.

Abstract: Hierarchical zeolites are a class of microporous catalysts and adsorbents that also contain mesopores, which allow for fast transport of bulky molecules and thereby enable improved performance in petrochemical and biomass processing. We used repetitive branching during one-step hydrothermal crystal growth to synthesize a new hierarchical zeolite made of orthogonally connected microporous nanosheets. The nanosheets are 2 nanometers thick and contain a network of 0.5-nanometer micropores. The house-of-cards arrangement of the nanosheets creates a permanent network of 2- to 7-nanometer mesopores, which, along with the high external surface area and reduced micropore diffusion length, account for higher reaction rates for bulky molecules relative to those of other mesoporous and conventional MFI zeolites.

Vibrational Control of Bimolecular Reactions with Methane by Mode, Bond, and Stereo Selectivity.

Abstract: Vibrational motions of a polyatomic molecule are multifold and can be as simple as stretches or bends or as complex as concerted motions of many atoms. Different modes of excitation often possess different capacities in driving a bimolecular chemical reaction, with distinct dynamic outcomes. Reactions with vibrationally excited methane and its isotopologs serve as a benchmark for advancing our fundamental understanding of polyatomic reaction dynamics. Here, some recent progress in this area is briefly reviewed. Particular emphasis is placed on the key concepts developed from those studies. The interconnections among mode and bond selectivity, Polanyi's rules, and newly introduced vibrational-induced steric phenomena are highlighted.

Cold quantum-controlled rotationally inelastic scattering of HD with H 2 and D 2 reveals collisional partner reorientation

Abstract: Molecular interactions are best probed by scattering experiments. Interpretation of these studies has been limited by lack of control over the quantum states of the incoming collision partners. We report here the rotationally inelastic collisions of quantum-state prepared deuterium hydride (HD) with H2 and D2 using a method that provides an improved control over the input states. HD was coexpanded with its partner in a single supersonic beam, which reduced the collision temperature to 0–5 K, and thereby restricted the involved incoming partial waves to s and p. By preparing HD with its bond axis preferentially aligned parallel and perpendicular to the relative velocity of the colliding partners, we observed that the rotational relaxation of HD depends strongly on the initial bond-axis orientation. We developed a partial-wave analysis that conclusively demonstrates that the scattering mechanism involves the exchange of internal angular momentum between the colliding partners. The striking differences between H2/HD and D2/HD scattering suggest the presence of anisotropically sensitive resonances. Scattering of molecules at low temperature that are prepared in single quantum states illuminates the mechanism of rotationally inelastic collisions and reveals the reorientation of partner molecules. By correlating each outgoing partial wave with the incoming waves, partial-wave analysis of the scattering angular distribution determines the dominant short- and long-range anisotropies of the interaction potential.

Direct chemical dynamics simulations: coupling of classical and quasiclassical trajectories with electronic structure theory

Abstract: In classical and quasiclassical trajectory chemical dynamics simulations, the atomistic dynamics of collisions, chemical reactions, and energy transfer are studied by solving the classical equations of motion. These equations require the potential energy and its gradient for the chemical system under study, and they may be obtained directly from an electronic structure theory. This article reviews such direct dynamics simulations. The accuracy of classical chemical dynamics is considered, with simulations highlighted for the F− + CH3OOH reaction and of energy transfer in collisions of CO2 with a perfluorinated self-assembled monolayer (F-SAM) surface. Procedures for interfacing chemical dynamics and electronic structure theory computer codes are discussed. A Hessian-based predictor–corrector algorithm and high-accuracy Hessian updating algorithm, for enhancing the efficiency of direct dynamics simulations, are described. In these simulations, an ensemble of trajectories is calculated which represents the experimental and chemical system under study. Algorithms are described for selecting the appropriate initial conditions for bimolecular and unimolecular reactions, gas-surface collisions, and initializing trajectories at transition states and conical intersections. Illustrative direct dynamics simulations are presented for the Cl− + CH3I SN2 reaction, unimolecular decomposition of the epoxy resin constituent CH3NHCHCHCH3 versus temperature, collisions and reactions of N-protonated diglycine with a F-SAM surface that has a reactive head group, and the product energy partitioning for the post-transition state dynamics of C2H5F HF + C2H4 dissociation. © 2012 John Wiley & Sons, Ltd.

The multi-configurational time-dependent Hartree approach revisited

Abstract: The multi-configurational time-dependent Hartree (MCTDH) approach facilitates accurate high-dimensional quantum dynamics simulations. In the approach, the wavefunction is expanded in a direct product of self-adapting time-dependent single-particle functions (SPFs). The equations of motion for the expansion coefficients and the SPFs are obtained via the Dirac-Frenkel variational principle. While this derivation yields well-defined differential equations for the motion of occupied SPFs, singularities in the working equations resulting from unoccupied SPFs have to be removed by a regularization procedure. Here, an alternative derivation of the MCTDH equations of motion is presented. It employs an analysis of the time-dependence of the single-particle density matrices up to second order. While the analysis of the first order terms yields the known equations of motion for the occupied SPFs, the analysis of the second order terms provides new equations which allow one to identify optimal choices for the unoccupied SPFs. The effect of the optimal choice of the unoccupied SPFs on the structure of the MCTDH equations of motion and their regularization is discussed. Generalized equations applicable in the multi-layer MCTDH framework are presented. Finally, the effects resulting from the initial choice of the unoccupied SPFs are illustrated by a simple numerical example.