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Classical-limit quantum dynamics is used to explain the origin of the quantum thresholds of chemical reactions from their classical dynamics when these are vibrationally nonadiabatic across the interaction region. This study is performed within the framework of an elementary model of chemical reaction that mimics the passage from the free rotation of the reagents to the bending vibration at the transition state to the free rotation of the products. [ABSTRACT FROM AUTHOR]

The 3D photodissociation dynamics of HCl and HF molecules adsorbed on ice is studied by quantum and classical simulations. The quantum calculations are carried out with the multiconfiguration time-dependent Hartree (MCTDH) approach. Dynamical observables like angular distributions in the momentum space of the H fragments, absorption cross sections are computed. The results are compared with our previous 2D studies. As expected, less encapsulation of the H atom between the ice surface and the halogen atom is obtained in the 3D study, resulting in less pronounced interference structures in the photoabsorption cross sections and in a decrease of the classical rainbow peaks observed in the 2D scheme. Although the amplitudes of the oscillations corresponding to quantum interferences in the asymptotic angular distribution of the H fragment are different between the 2D and 3D results, the qualitative pattern of the oscillations is similar in the 2D and 3D approaches. In addition, a good agreement is observed for the angular distribution between the classical and the quantum calculations. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of a HX (X=Cl,F) molecule adsorbed on a hexagonal ice surface at T=0 K is studied using time-dependent quantum wave packets and quasiclassical trajectories. The relevant potential energy surfaces are calculated using high-level ab initio methods. We present here two dimensional calculations for the dynamics of the hydrogen photofragment for both HCl and HF molecules. The purpose of this paper is to compare the photodissociation dynamics of the two molecules which are adsorbed on the ice surface with different equilibrium geometries. The total photodissociation cross section and the angular distribution are calculated. The comparison with classical trajectory calculations provides evidence for typical quantum effects and reveals rainbow structures. [ABSTRACT FROM AUTHOR]

Quasi-classical trajectory calculations for the Si(3P)+O2(X 3Σg-)→SiO(X 1Σ+)+O(1D) reaction have been carried out using the analytical ground 1A′ potential energy surface (PES) recently reported by Dayou and Spielfiedel [J. Chem. Phys. 119, 4237 (2003)]. The reaction has been studied for a wide range of collision energies (0.005–0.6 eV) with O2 in its ground rovibrational state. The barrierless PES leads to a decrease of the total reaction cross section with increasing collision energy. It has been brought to evidence that the reaction proceeds through different reaction mechanisms whose contributions to reactivity are highly dependent on the collision energy range. At low collision energy an abstraction mechanism occurs involving the collinear SiOO potential well. The associated short-lived intermediate complex leads to an inverted vibrational distribution peaked at v′=3 and low rotational excitation of SiO(v′,j′) with a preferentially backward scattering. At higher energies the reaction proceeds mainly through an insertion mechanism involving the bent and linear OSiO deep potential wells and associated long-lived intermediate complexes, giving rise to nearly statistical energy disposals into the product modes and a forward-backward symmetry of the differential cross section. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of the Ar2HBr van der Waals molecule is studied using the multiconfiguration time-dependent Hartree method. Standard Jacobian coordinates are used to describe the molecule. Two four-dimensional calculations are carried out where the rotation of the Ar2 molecule and, in addition, either the vibration of Ar2-Br or that of Ar2 are frozen. The time-evolution of the probability density in the different modes and the calculation of the dissociative flux show that the dissociating hydrogen atom preferentially moves out of the plane defined by Ar2 and Br. A comprehensive study of the cage effect in the process is presented. [ABSTRACT FROM AUTHOR]

We apply the multiconfiguration time-dependent Hartree method to the study of the photodissociation of the Ar-HBr van der Waals complex. The dynamics is studied in both Jacobi and valence coordinates. The evolution of the probability density in the different modes shows that the direct or nearly direct mechanism, where the hydrogen atom interacts only very weakly with the Ar and Br atoms, dominates the dissociation process. By projecting the quantum flux onto the vibrational levels of ArBr, we find at the maximum of the absorption profile, i.e., around E = 3 eV, that 65% of the dissociation gives rise to partial fragmentation into hot H fragments and bound ArBr molecules. [ABSTRACT FROM AUTHOR]

The optical potential method initially proposed by Jolicard and Austin in the context of the stabilization method is reviewed here and used with the accurate and the efficient discrete variable representation method to obtain the energies and the widths (respectively, the real and the imaginary part of the resonance energies) of rovibrational predissociated states of diatomic molecules. In this method the resonances for an n coupled states problem are obtained by a direct diagonalization of the Hamiltonian matrix in the diabatic representation. This Hamiltonian matrix is directly evaluated in the discrete variable representation using the Fourier grid Hamiltonian method proposed by Marston and Balint-Kurti. In this approach, two optical potentials are tested and used here to impose the asymptotic behaviors of the boundary conditions which are compatible with the resonance states. The method is exemplified for the B 1Σ+–D’1 Σ+ Rydberg–valence predissociating interaction in the CO molecule. [ABSTRACT FROM AUTHOR]

The SiO molecule is the most widespread silicon-bearing molecule in the interstellar medium. Its largest abundances are found in regions of star formation, whereas its abundance is very low in quiescent cold dark clouds. The major source of SiO in the "shock" regions of the interstellar medium is the reaction Si(3P) + OH(X2∏) → SiO(X1∑+) + H(2S), however there is not experimental data available for it. Here we show and compare with quasi-classical trajectory method (QCT), the results of Mean Potencial Phase Space Theory (MPPST) calculations of integral cross section (ICS), thermal rate constant (k(T)) and ro-vibrational distributions, using the numerical approach (n-MPPST) and the analytical ones: the traditional framework (a1-MPPST) and a new feature suggested by our group (a2-MPPST). The ICS (and consequently the other observables) values depend strongly on the fitted potential in both reaction channels, so in order to develop more precise calculations we divided the fit process in two main regions depending on the collision energy. The obtained ICS values shows that the a2-MPPST has a greater agreement than a1-MPST and n-MPPST with QCT. We expect that subsequent refinements of MPPST like the showed on this work could lead to an alternative to more sophisticated and expensive calculations, and also be a valuable first approach to the triatomic colisions dynamics. [ABSTRACT FROM AUTHOR]

Although UV photon-induced CO ice desorption is clearly observed in many cold regions of the Universe as well as in the laboratory, the fundamental question of the mechanisms involved at the molecular scale remains debated. In particular, the exact nature of the involved energy transfers in the indirect desorption pathway highlighted in previous experiments is not explained. Using ab initio molecular dynamics simulations, we explore a new indirect desorption mechanism in which a highly vibrationally excited CO (v=40) within an aggregate of 50 CO molecules triggers the desorption of molecules at the surface. The desorption originates first from a mutual attraction between the excited molecule and the surrounding molecule(s), followed by a cascade of energy transfers, ultimately resulting in the desorption of vibrationally cold CO (∼95% in v=0). The theoretical vibrational distribution, along with the kinetic energy one, which peaks around 25 meV for CO with low rotational levels (v=0, J<7), is in excellent agreement with the results obtained from VUV laser induced desorption (157 nm) of CO (v=0, 1) probed using REMPI.

The dynamics of the Si( 3 P) + OH(X 2 Π) → SiO(X 1 Σ + , v ', j ') + H( 2 S) reaction is investigated by means of the quasi-classical trajectory method on the electronic ground state X 2 A' potential energy surface in the 10 -2 -1 eV collision energy range. Although the reaction involves the formation of a long-lived intermediate complex, a high probability for back-dissociation to the reactants is found because of inefficient intravibrational redistribution of energy among the complex modes. At low collision energies, the reactive events are governed by a dynamics with mixed direct/indirect features. As the collision energy increases, the intermediate complex lifetime increases and final state distributions are found to be in reasonable agreement with statistical predictions obtained using the mean potential phase space theory, thus highlighting the indirect character of the process. These rich and puzzling dynamical features are in line with what has been previously observed for the S( 3 P) + OH(X 2 Π) reaction.

The dynamics of the Si( 3 P) + OH(X 2 Π) → SiO(X 1 Σ + ) + H( 2 S) reaction is investigated by means of the time-dependent wave packet (TDWP) approach using an ab initio potential energy surface recently developed by Dayou et al. ( J. Chem. Phys. 2013 , 139 , 204305 ) for the ground X 2 A' electronic state. Total reaction probabilities have been calculated for the first 15 rotational states j = 0-14 of OH(v=0,j) at a total angular momentum J = 0 up to a collision energy of 1 eV. Integral cross sections and state-selected rate constants for the temperature range 10-500 K were obtained within the J-shifting approximation. The reaction probabilities display highly oscillatory structures indicating the contribution of long-lived quasibound states supported by the deep SiOH/HSiO wells. The cross sections behave with collision energies as expected for a barrierless reaction and are slightly sensitive to the initial rotational excitation of OH. The thermal rate constants show a marked temperature dependence below 200 K with a maximum value around 15 K. The TDWP results globally agree with the results of earlier quasi-classical trajectory (QCT) calculations carried out by Rivero-Santamaria et al. ( Chem. Phys. Lett. 2014 , 610-611 , 335 - 340 ) with the same potential energy surface. In particular, the thermal rate constants display a similar temperature dependence, with TDWP values smaller than the QCT ones over the whole temperature range.

Marine aerosols represent the most important aerosol fraction in the Earth atmosphere. Field studies have revealed that fatty acids form an organic film at the surface of sea salt particles, altering the properties of the aerosol. By means of classical molecular dynamics simulation, the surface organization of palmitic acid (PA) on a salt surface, NaCl, has been investigated at two different temperatures, 235 and 300 K, and with relative humidity varying from 0 to 40%. Calculations show that water promotes the formation of well-ordered close-packed PA islands. As a result, some area of the salt may be covered by water only or by PA molecules supported by water. Depending on the relative humidity, the hydrophilic/hydrophobic character of the sea salt surface varies. This heterogeneous coating gives rise locally to very different surface properties and hence may affect the transfer of gas phase species to the salt and their reactivity.

The chlorine/water interface is of crucial importance in the context of atmospheric chemistry. Modeling the structure and dynamics at this interface requires an accurate description of the interaction potential energy surfaces. We propose here an analytical intermolecular potential that reproduces the interaction between the Cl2 molecule and a water molecule. Our functional form is fitted to a set of high level ab initio data using the coupled-cluster single double (triple)/aug-cc-p-VTZ level of electronic structure theory for the Cl2 - H2O complex. The potential fitted to reproduce the three minima structures of 1:1 complex is validated by the comparison of ab initio results of Cl2 interacting with an increasing number of water molecules. Finally, the model potential is used to study the physisorption of Cl2 on a perfectly ordered hexagonal ice slab. The calculated adsorption energy, in the range 0.27 eV, shows a good agreement with previous experimental results.

We report the first global potential energy surface (PES) for the X(2)A' ground electronic state of the Si((3)P) + OH(X(2)Π) → SiO(X(1)Σg(+)) + H((2)S) reaction. The PES is based on a large number of ab initio energies obtained from multireference configuration interaction calculations plus Davidson correction (MRCI+Q) using basis sets of quadruple zeta quality. Corrections were applied to the ab initio energies in the reactant channel allowing a proper description of long-range interactions between Si((3)P) and OH(X(2)Π). An analytical representation of the global PES has been developed by means of the reproducing kernel Hilbert space method. The reaction is found barrierless. Two minima, corresponding to the SiOH and HSiO isomers, and six saddle points, among which the isomerization transition state, have been characterized on the PES. The vibrational spectra of the SiOH/HSiO radicals have been computed from second-order perturbation theory and quantum dynamics methods. The structural, energetic, and spectroscopic properties of the two isomers are in good agreement with experimental data and previous high quality calculations.

Quasi-classical trajectory calculations for the Si(3P)+O2(X 3Sigmag-)-->SiO(X 1Sigma+)+O(1D) reaction have been carried out using the analytical ground 1A' potential energy surface (PES) recently reported by Dayou and Spielfiedel [J. Chem. Phys. 119, 4237 (2003)]. The reaction has been studied for a wide range of collision energies (0.005-0.6 eV) with O2 in its ground rovibrational state. The barrierless PES leads to a decrease of the total reaction cross section with increasing collision energy. It has been brought to evidence that the reaction proceeds through different reaction mechanisms whose contributions to reactivity are highly dependent on the collision energy range. At low collision energy an abstraction mechanism occurs involving the collinear SiOO potential well. The associated short-lived intermediate complex leads to an inverted vibrational distribution peaked at v'=3 and low rotational excitation of SiO(v',j') with a preferentially backward scattering. At higher energies the reaction proceeds mainly through an insertion mechanism involving the bent and linear OSiO deep potential wells and associated long-lived intermediate complexes, giving rise to nearly statistical energy disposals into the product modes and a forward-backward symmetry of the differential cross section.