Quantum reflection of rare gas atoms and molecules from a grating

Quantum reflection is a universal property of atoms and molecules when scattered from surfaces in ultracold collisions. Recent experimental work has documented the quantum reflection and diffraction of He atoms, dimers, trimers and Neon atoms when reflected from a grating. Conditions for the observation of emerging beam resonances have been discussed and measured. In this paper, we provide a theoretical simulation of the quantum reflection in these cases from a grating. We confirm, as expected the universal dependence on the incident de Broglie wavelength only of the threshold angles for the observation of emerging beam resonances. However, the angular dependence of the reflection efficiencies, that is the ratio of scattered intensity into specific diffraction channels relative to the total intensity is found to be dependent on the specifics of the incident particle. The dependence of the reflection efficiency on the identity of the particle is intimately related to the fact that the incident energy dependence of quantum reflection depends on the details of the particle surface interaction.Comment: 18 pages, 5 figures, 2 table

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 N₂ 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

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

A theoretical simulation of the resonant Raman spectroscopy of the H2O⋯Cl2 and H2O⋯Br2 halogen-bonded complexes

The resonant Raman spectra of the H2O⋯Cl2 and H2O⋯Br2 halogen-bonded complexes have been studied in the framework of a 2-dimensional model previously used in the simulation of their UV-visible absorption spectra using time-dependent techniques. In addition to the vibrational progression along the dihalogen mode, a progression is observed along the intermolecular mode and its combination with the intramolecular one. The relative intensity of the inter to intramolecular vibrational progressions is about 15% for H2O⋯Cl2 and 33% for H2O⋯Br2. These results make resonant Raman spectra a potential tool for detecting the presence of halogen bonded complexes in condensed phase media such as clathrates and ice.Fil: Franklin Mergarejo, Ricardo. Université Paris Sud; Francia. Centre National de la Recherche Scientifique; Francia. InSTEC; Cuba. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Rubayo Soneira, Jesús. InSTEC; CubaFil: Halberstadt, Nadine. Université Paris Sud; Francia. Centre National de la Recherche Scientifique; FranciaFil: Janda, Kenneth C.. University of California at Irvine; Estados UnidosFil: Apkarian, V. Ara. University of California at Irvine; Estados Unido

Quasi-classical trajectories study of Ne2Br2(B) vibrational predissociation: Kinetics and product distributions

The vibrational predissociation of the Ne2Br2(B) van der Waals complex has been investigated using the quasi-classical trajectory method (QCT), in the range of vibrational levels v' = 16-23. Extensive comparison is made with the most recent experimental observations [Pio et al., J. Chem. Phys. 133, 014305 (2010)], molecular dynamics with quantum transitions (MDQT) simulations [Miguel et al., Faraday Discuss. 118, 257 (2001)], and preliminary results from 24-dimensional Cartesian coupled coherent state (CCCS) calculations. A sequential mechanism is found to accurately describe the theoretical dynamical evolution of intermediate and final product populations, and both QCT and CCCS provide very good estimates for the dissociation lifetimes. The capabilities of QCT in the description of the fragmentation kinetics is analyzed in detail by using reduced-dimensionality models of the complexes and concepts from phase-space transport theory. The problem of fast decoupling of the different coherent states in CCCS simulations, resulting from the high dimensionality of phase space, is tackled using a re-expansion scheme. QCT ro-vibrational product state distributions are reported. Due to the weakness of the vdW couplings and the low density of vibrational states, QCT predicts a larger than observed propensity for \Delta v' = -1 and -2 channels for the respective dissociation of the first and second Ne atoms.Comment: 16 pages, 6 figures, 4 tables. Accepted for publication in J. Chem. Phy

Ultrafast structural dynamics in electronically excited solid neon. II. Molecular-dynamics simulations of the electronic bubble formation

Mol.-dynamics simulations of structural relaxation in electronically excited NO-doped solid neon are presented. The NO impurity is excited to its lowest A (3ss) Rydberg state, inducing a rearrangement of the surrounding medium in the form of a bubble, due to repulsion between the Rydberg electron and the closed-shell surrounding species. The simulations were carried out using the thermal harmonic quantum correction in order to account for quantum effects. The first shell response is characterized by an initial expansion and an oscillatory response, which point to coherent dynamics and confirm the exptl. obsd. slower dynamics than in solid argon. The medium response is characterized by a collective oscillatory behavior of the shells around the impurity. We investigated the role of quantum effects, nature of the NO-Ne ground- and excited-state potentials and of the Ne-Ne potential on the dynamics. It appears that the expansion stage is mainly detd. by the fact that the repulsive interaction between the excited impurity and the Ne lattice reaches out beyond the first shell, so that the first three shells are simultaneously pushed outward. Combined with the short-range character of the Ne-Ne interaction, this sets in motion a larger mass than in solid argon. The collective response is mainly due to the short-range Ne-Ne interaction and the tight nature of the Ne lattice. Quantum effects play a negligible role in the overall dynamics, at least to the level of approxn. of our simulations. [on SciFinder (R)

The medium response to an impulsive redistribution of charge in solid argon: molecular dynamics simulations and normal mode analysis

Excitation of the A(3ss) Rydberg state of NO leads to an extensive rearrangement of the environment, which was studied by classical mol. dynamics simulations and normal mode anal., using pair potentials from the literature. The medium response is independent of the details at long range of the excited state NO A-Ar potential, stressing the fact that it is mainly driven by the short range repulsive forces between the Rydberg electron and the matrix atoms. The authors establish the inertial character of the 1st shell response in the initial 100-150 fs after excitation, as the next shells are silent over this time scale. The expansion of the 1st shell at early times, induces the propagation of a supersonic wave along the (011) axis of the crystal, which define 12 linear chains of atoms with the impurity. The early time response is followed by vibrational coherences with a complex behavior. The normal modes anal. of the crystal shell by shell shows good agreement with the power spectra of the MD trajectories. It allows one to identify the most significant modes in the medium response. Overall, the dynamics of the system may be regarded as that of a NOAr12 supermol., embedded in an Ar lattice and undergoing vibrational energy redistribution. [on SciFinder (R)