50 research outputs found
From angle-action to Cartesian coordinates: A key transformation for molecular dynamics
The transformation from angle-action variables to Cartesian coordinates is a
crucial step of the (semi) classical description of bimolecular collisions and
photo-fragmentations. The basic reason is that dynamical conditions
corresponding to experiments are ideally generated in angle-action variables
whereas the classical equations of motion are ideally solved in Cartesian
coordinates by standard numerical approaches. To our knowledge, the previous
transformation is available in the literature only for triatomic systems. The
goal of the present work is to derive it for polyatomic ones.Comment: 10 pages, 11 figures, submitted to J. Chem. Phy
Dynamical Reaction Pathways in Eley-Rideal Recombination of Nitrogen from W(100)
The scattering of atomic nitrogen over a N-pre-adsorbed W(100) surface is theoretically described in the case of normal incidence off a single adsorbate. Dynamical reaction mechanisms, in particular Eley-Rideal (ER) abstraction, are scrutinized in the 0.1-3.0 eV collision energy range and the influence of temperature on reactivity is considered between 300 and 1500 K. Dynamics simulations suggest that, though non-activated reaction pathways exist, the abstraction process exhibits a significant collision energy threshold (0.5 eV). Such a feature, which has not been reported so far in the literature, is the consequence of a repulsive interaction between the impinging and the pre-adsorbed nitrogens along with a strong attraction towards the tungsten atoms. Above threshold, the cross section for ER reaction is found one order of magnitude lower than the one for hot-atoms formation. The abstraction process involves the collision of the impinging atom with the surface prior to reaction but temperature effects, when modeled via a generalized Langevin oscillator model, do not affect significantly reactivity
Surface Temperature Effects on the Dynamics of Nâ Eley-Rideal Recombination on W(100)
Quasiclassical trajectories simulations are performed to study the influence of surface temperature on the dynamics of a N atom colliding a N-preadsorbed W(100) surface under normal incidence. A generalized Langevin surface oscillator scheme is used to allow energy transfer between the nitrogen atoms and the surface. The influence of the surface temperature on the N2 formed molecules via Eley-Rideal recombination is analyzed at T = 300, 800, and 1500 K. Ro-vibrational distributions of the N2 molecules are only slightly affected by the presence of the thermal bath whereas kinetic energy is rather strongly decreased when going from a static surface model to a moving surface one. In terms of reactivity, the moving surface model leads to an increase of atomic trapping cross section yielding to an increase of the so-called hot atoms population and a decrease of the direct Eley-Rideal cross section. The energy exchange between the surface and the nitrogen atoms is semi-quantitatively interpreted by a simple binary collision model
Exact, BornâOppenheimer, and quantum-chemistry-like calculations in helium clusters doped with light molecules: The He2N2(X) system
9 pages, 2 figures, 4 tables.-- PACS nrs.: 34.20.-b; 31.50.-x; 31.15.A-; 33.15.Mt; 33.20.Vq; 36.40.-c.Helium clusters doped with diatomic molecules, He(N)âBC, have been recently studied by means of a quantum-chemistry-like approach. The model treats He atoms as âelectronsâ and dopants as ânucleiâ in standard electronic structure calculations. Due to the large mass difference between He atoms and electrons, and to the replacement of Coulomb interactions by intermolecular potentials, it is worth assessing up to what extent are the approximations involved in this model, i.e., decoupling of the BC rotation from the He-atom orbital angular momenta and BornâOppenheimer separation of the BC stretch versus the He motions, accurate enough. These issues have been previously tackled elsewhere for the 4He2âBr2(X) system, which contains a heavy dopant [Roncero et al., Int. J. Quantum Chem. 107, 2756 (2007)]. Here, we consider a similar cluster but with a much lighter dopant such as N2(X). Although the model does not provide the correct energy levels for the cluster, positions and intensities of the main detectable lines of the vibrotational Raman spectrum at low temperature are accurately reproduced.This work has been partially supported by the DGICYT Spanish Grant Nos. FIS2007-62006 and CTQ2004-02415/BQU. M.P.de L.-C. acknowledges the support of a MEC-CSIC
Spanish Grant No. 2007501004. The calculations presented here were performed at Centro de CĂĄlculo of IMAFF (CSIC).Peer reviewe
Study of the H+O2 reaction by means of quantum mechanical and statistical approaches: The dynamics on two different potential energy surfaces
The possible existence of a complex-forming pathway for the H+O2 reaction has been investigated by means of both quantum mechanical and statistical techniques. Reaction probabilities, integral cross sections, and differential cross sections have been obtained with a statistical quantum method and the mean potential phase space theory. The statistical predictions are compared to exact results calculated by means of time dependent wave packet methods and a previously reported time independent exact quantum mechanical approach using the double many-body expansion (DMBE IV) potential energy surface (PES) [Pastrana et al., J. Phys. Chem. 94, 8073 (1990)] and the recently developed surface (denoted XXZLG) by Xu et al. [J. Chem. Phys. 122, 244305 (2005)]. The statistical approaches are found to reproduce only some of the exact total reaction probabilities for low total angular momenta obtained with the DMBE IV PES and some of the cross sections calculated at energy values close to the reaction threshold for the XXZLG surface. Serious discrepancies with the exact integral cross sections at higher energy put into question the possible statistical nature of the title reaction. However, at a collision energy of 1.6 eV, statistical rotationally resolved cross sections managed to reproduce the experimental cross sections for the H+O2(v=0,j=1)-->OH(v[prime]=1,j[prime])+O process reasonably well. ©2008 American Institute of Physic
Toward a Single-Valued DMBE Potential Energy Surface for CHNO(3A). 1. Diatomic Fragments
As a first step toward the construction of a single-valued double many-body expansion potential energy surface for CHNO(3A), we have carried out CASSCF and CASPT2 calculations of six diatomic curves, involving a total of nine electronic states. The ab initio curves have been represented analytically using the extended HartreeâFock approximate correlation energy model. In all cases, the semiempirical curves have been found to agree well with the available spectroscopic RKR data
On the key factors of angular correlations in complex-forming elementary reactions
In the mid-seventies, Case and Herschbach argued that
for
complex-forming three-atom reactions governed by long-range forces and
performed in supersonic molecular beam experiments, vectorial properties
are determined by a single parameter Î' = â©L'/(L' + j')âȘ, L' and j' being
respectively the moduli of the orbital and rotational angular momenta of the
products. A simple mathematical relation between vectorial properties and
Î' was then proposed. However, Î' must be determined
beforehand by
phase space theory calculations. Besides, we have recently shown that scalar
properties are mainly controled by two factors Ï'1 and Ï'2
respectively
called angular excitation and diatomic inertial contribution. We show here
that these factors control also vectorial properties. Moreover, the way they
control them is summarized in a set of four figures. The advantage of our
method is that Ï'1 and Ï'2 are related to the mechanical
parameters of the
reaction by very simple formulas, contrary to Î'. Last by not
least, our
parameters appear to be mostly independent, so that vectorial properties
cannot be said to strictly depend on Î'. Nevertheless, it turns out
that the
rule proposed by Case and Herschbach is reasonable in many realistic
situations