1,721,154 research outputs found
Restoring detailed balance in the Landau-Teller probabilities for collision-induced vibrational transitions.
Full text linkMutual capture of dipolar molecules at low and very low energies. II. Numerical study.
No full textThe low-energy rate coefficients of capture of two identical dipolar polarizable rigid rotors in their lowest nonresonant (j(1) = 0 and j(2) = 0) and resonant (j(1) = 0, 1 and j(2) = 1, 0) states are calculated accurately within the close-coupling (CC) approach. The convergence of the quantum rate coefficients to their quantum-classical counterparts is studied. A comparison of the present accurate numerical with approximate analytical results (Nikitin, E. E.; Troe, J. J. Phys. Chem. A 2010, 114, 9762) indicates a good performance of the previous approach which was based on the interpolation between s-wave fly wheel quantal and all-wave classical adiabatic channel limits. The results obtained apply as well to the formation of transient molecular species in the encounter of two atoms at very low collision energy interacting via resonance dipole-dipole interaction.Deutsche Forschungsgemeindschaft [TR 69/17-1
70 years of Landau–Teller theory for collisional energy transfer. Semiclassical three-dimensional generalizations of the classical collinear model.
Full text linkThis article, in historical retrospective, describes the development of the celebrated Landau-Teller (LT) model of 1936 for vibrational-translational energy exchange in collisions of an atom with a diatomic molecule. We discuss semiclassical generalizations of the classical LT model and generalizations of the collinear LT model to account for the effects of rotation of the diatom on the vibrational relaxation rate. The former is based on the recovery of the Landau semiclassical exponent from the classical LT encounter time, and the latter on the definition of a 1-D driving mode within the manifold of the translational and rotational degrees of freedom of the colliding partners. The utility of generalized LT models is illustrated by three case studies that exemplify weak and strong effects of the rotation as well as the efficiencies of different driving modes in the vibrational relaxation of highly asymmetric diatoms
On the kinetic modeling of electron attachment to polyatomic molecules.
No full textThe description of electron attachment to neutral target molecules by kinetic modeling is compared with dynamical two-state and multi-state zero range potential treatments. It is shown that kinetic modeling, combining electron capture theory with empirical (experimentally fitted) electron–phonon coupling and inelastic scattering factors, approaches the results from the dynamical models provided that the coupling of the electronic wave function for attachment to the ground and excited vibrational states of the neutral is not too large. Strong coupling effects, e.g. resulting in large cusps of the cross sections, are not accounted for by the modeling with separated kinetic steps
Mutual capture of dipolar molecules at low and very low energies. I. Approximate analytical treatment.
No full textApproximate analytical expressions are derived for the low-energy rate coefficients of capture of two identical dipolar polarizable rigid rotors in their lowest nonresonant (j(1) = 0 and j(2) = 0) and resonant (j(1) = 0,1 and j(2) = 1,0) states. The considered range extends from the quantum, ultralow energy regime, characterized by s-wave capture, to the classical regime described within fly wheel and adiabatic channel approaches, respectively. This is illustrated by the table of contents graphic (available on the Web) that shows the scaled rate coefficients for the mutual capture of rotors in the resonant state versus the reduced wave vector between the Bethe zero-energy (left arrows) and classical high-energy (right arrow) limits for different ratios of the dipole dipole to dispersion interaction
Axially nonadiabatic channel treatment of low-energy capture in ion-rotating diatom collisions
No full textThe quantum version of an axially nonadiabatic channel (ANC) approximation, introduced in an earlier article for the calculation of complex-formation cross sections and rate constants in ion-diatom collisions [Maergoiz, A. I.; Nikitin, E. E.; Troe, J.; Ushakov, V. G. J. Chem. Phys. 2002, 117, 4201-4213] is tested against accurate quantum results. Cross sections and rate constants are determined for several representative systems with the participation of a diatom in the state j = 1, assuming various long-range potentials between the collision partners, such as anisotropic ion-induced dipole, second-order charge-permanent dipole, and first-order charge-quadrupole interaction. The ANC approximation well reproduces accurate quantum results in the perturbed rotor limit, while the standard quantum adiabatic channel (AC) approximation fails at low energy due to neglect of Coriolis coupling. However, the low-energy extrapolation of classical adiabatic channel results (ACCI) provides a reasonable approximation both to accurate and quantal ANC results down to collision energies when only few partial cross sections determine the total capture cross section. This unexpected feature of the ACCI approximation is due to two effects: (a) an artificial simulation of tunneling transmission and overbarrier reflection at centrifugal barriers by introducing a continuous distribution over total angular momenta and (b) a slight effective lowering of the centrifugal barriers compared to centrifugal barriers within the AC model. Low-temperature quantum rate constants are also presented
On the Bethe-Wigner-Shapiro limit of the rate coefficient for the capture of a rotating quadrupolar polarisable diatom by an ion.
No full textThe low-energy (Bethe–Wigner–Shapiro) behaviour of the rate coefficient for capture of a polarisable quadrupolar rotationally excited diatom by an ion is calculated. The multichannel character of capture shows itself in the dependence of the effective attractive potential on the intrinsic angular momentum of the diatom and in a nonadiabatic diagonal correction to the potential. The predicted energy dependence of the rate coefficients is compared with previous numerically accurate results for the capture of H2 molecules by H2+ ions
Dynamics of ion-molecule complex formation at very low energies and temperatures
No full textThe dynamics of complex formation in collisions of rotationally excited diatomic molecules with ions at very low collision energies and translational temperatures is discussed. Under these conditions, the locking of the intrinsic angular momentum of the diatomic molecule to the collision axis occurs in the region of centrifugal barriers, and the motion of collision partners across these barriers bears quantum features (tunneling transmission and overbarrier reflection). The capture in this energy range is described by coupled radial wave equations that are solved numerically. In two limits, the respective rate coefficients are expressed analytically: for low collision energies (adiabatic channel approximation with classical relative motion of partners) and at zero collision energy (Bethe limit, s-wave capture). By comparison with accurate numerical quantum results, it is shown that these two limits are satisfactorily bridged by the so-called axially-nonadiabatic channel model that considers uncoupled relative motion of the colliding partners across effective potentials; the latter are generated by adiabatic channel potentials and Coriolis interaction in the perturbed rotor basis. The limits of applicability of the standard adiabatic channel model are discussed. The general approach is illustrated by the capture of H-2 and HCl in collisions with ions
Electron capture by finite-size polarizable molecules and clusters.
Full text linkThe effects of finite molecular target size are investigated for partial-wave selected capture of electrons by isotropically polarizable molecules and clusters within a generalized Vogt-Wannier model. It is shown how expressions for partial-wave selected capture probabilities of zero-size targets from Dashevskaya et al. (Phys. Chem. Chem. Phys., 2009, 11, 9364) can be modified to account for finite target sizes of the molecules and clusters. The transition from quantum to classical, from single- to multiple- and all-wave, behaviour of capture probabilities, cross sections, and rate constants is illustrated.EOARD [FA 8655-09-1-3001
Determination of adhesion energies by means of hydrogen loading
No full textDetailed knowledge of the adhesion of metal films to polymer or metal substrates is required in daily application as well as in fundamental research. However, the determination of the relevant adhesion energies is difficult. So-called scotch tape tests or scratching tests only give approximate values. We present a new technique to quantitatively determine the adhesion energy between two materials by hydrogen loading. Hydrogen loading results in in-plane stress between the film and its substrate which results, by crossing a critical stress, in local film detachment and buckling. The critical stress depends on the material as well as on the film thickness. It can be used as a measure for the adhesion energy between the film and the substrate. A simple model will be presented to calculate the adhesion energy from critical stress data. Adhesion energies y = 0.2 and = 0.1 J/m(2) are obtained for the Pd/polycarbonate and the Cr-oxide/polycarbonate interface, respectively. (c) 2005 Elsevier B.V. All rights reserved
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