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  • 1
    Electronic Resource
    Electronic Resource
    Molecule–corrugated surface collisions: Converged close coupling wave packet and quasiclassical trajectory calculations for N2 scattering from corrugated lattices (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 704-720 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The close coupling wave packet (CCWP) and quasiclassical trajectory methods are used to study rotationally inelastic scattering of N2 from static, corrugated surfaces. The collision energy in these calculations ranges from 10 to 100 meV; 18 711 quantum states are included in the highest energy calculations to ensure convergence. The scattered molecules are analyzed with respect to the polarization of the final angular momentum vector and the amount of energy transferred into rotational motion and translational motion parallel to the surface. Comparisons of quantum and quasiclassical results show that quantum effects are important even with the relatively large mass of N2 and the high scattering energies used and can be seen even after summing over many final quantum states. A test of a factorization relation derived from the coordinate-representation sudden (CRS) approximation gives qualitative agreement with the exact quantum results.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465743
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  • 2
    Electronic Resource
    Electronic Resource
    Treatment of zero-point motions in cluster dynamics: Semiclassical time-dependent self-consistent-field simulation of (Ne)N (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 8637-8643 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A semiclassical time-dependent self-consistent-field (TDSCF) method is developed for dealing with the difficulties caused by the nonpreservation of zero-point energy in classical molecular dynamics simulation. The method is applied to a collinear model of a (Ne)12 cluster at very low temperatures. Classically, this system dissociates rapidly due to its zero-point energy. We show that the system remains stable when treated by the new method. The normal mode dynamics of the anharmonic cluster are calculated and discussed. Interesting results are obtained on the lifetimes of single-mode states and energies due to the coupling between the modes. Some of the single-mode states have subpicosecond lifetimes, while others are stable for at least 60 ps. The results illustrate the power of semiclassical TDSCF as a tool for studying the vibrational dynamics of anharmonic cluster at very low temperatures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465587
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  • 3
    Electronic Resource
    Electronic Resource
    Quantum dynamics simulations of nonadiabatic processes in many-atom systems: Photoexcited Ba(Ar)10 and Ba(Ar)20 clusters (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 5803-5814 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantum simulations are reported for the dynamics following the photoexcitation Ba(1S)→Ba(1P) in Ba(Ar)10 and Ba(Ar)20 clusters. The evolution in time is studied in a framework that treats quantum-mechanically all the coupled degrees of freedom. The focus is on the role of nonadiabatic transitions between the three adiabatic surfaces corresponding to the P states of the Ba atom. The time scales of electronic relaxation and of electronic depolarization (orbital reorientation) are computed, and the competition between adiabatic and nonadiabatic effects is assessed. The calculations are carried out by a new scheme that extends the recent classically based separable potential method. Semiclassical surface-hopping simulations are used to produce effective single-mode potentials on which nuclear "orbitals'' are then generated. The full wave packet is constructed from the electronic states involved, and from these nuclear wave functions. Among the main results we find that nonadiabatic transitions become appreciable around 1 ps after photoexcitation, and they are stronger in the smaller cluster. Comparing Tully's semiclassical method with the quantum simulations, good qualitative agreement is found. Quantitatively, the semiclassical predictions for the electronic states branching rations deviate from the quantum results roughly by a factor of 2 after 1 ps. In the smaller cluster direct dissociation of the Ba atom dominates over energy redistribution within the cluster, the opposite being true for the large system. This example demonstrates the feasibility of quantum simulations of nonadiabatic processes in large systems with the new method. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471312
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  • 4
    Electronic Resource
    Electronic Resource
    Quantum dynamics of many-atom systems by the classically based separable potential (CSP) method: Calculations for I−(Ar)12 in full dimensionality (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8855-8864 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A recently developed method for time-dependent quantum simulations of large systems on short time scales is applied to the dynamics following electron photodetachment from the clusters I−(Ar)2 and I−(Ar)12. The problem is treated in full dimensionality, incorporating all vibrational degrees of freedom, by the classically based separable potential (CSP) approach. This is essentially an approximate time-dependent quantization of classical dynamics: Classical molecular dynamics is used to generate effective, single mode separable time-dependent potentials for each degree of freedom. The quantum dynamics is then propagated separately for each mode, using the effective potentials that implicitly include effects such as energy transfer between the modes. In the current application of the CSP method we calculate properties relevant for the interpretation of spectroscopies, such as correlation functions of wave packets, as well as time-dependent atom–atom distribution functions, pertinent to future diffraction experiments using ultrafast pulses. The insight obtained from the quantum dynamics of these clusters is discussed. In particular, light is thrown on the differences in the dynamics associated with the system landing on the three different electronic surfaces of the neutral I(2P)⋅(Ar)n system. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468939
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  • 5
    Electronic Resource
    Electronic Resource
    Electronic excitation dynamics of Li(H2)2: Dissociation mechanisms, lifetimes, and the validity of a hybrid quantum/classical approach (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 4056-4062 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissociation dynamics of the cluster Li(H2)2, following the 2s→2p excitation of the Li atom, is studied in the framework of a collinear model. The process was investigated by exact quantum wave packet calculations, and the results were used to test a hybrid quantum/classical method, in which the highly quantum mechanical initial state of the cluster is described by a wave function, and the latter is used to sample initial positions and momenta for a classical treatment of the excited state dynamics. We found that the dynamics was dominated by two predissociation processes, Li*(H2)2→Li*–H2+H2 and Li*(H2)2→Li*+(H2)2, with the latter process having a higher yield. A relatively long dissociation lifetime, ∼10 ps, was found for the excited cluster. The slow vibrational predissociation rate was interpreted as due to the very low density of state involved. The hybrid quantum/classical approach was found to give product vibrational energy and velocity distributions in good accord with the distribution from exact calculation. However, the lifetimes from the hybrid approach were found to be much shorter than those from the exact quantum calculations. The hybrid approach is thus applicable even to photoexcitation of quantum clusters for studying certain selected properties. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468533
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  • 6
    Electronic Resource
    Electronic Resource
    A quantitative approximation for the quantum dynamics of hydrogen transfer: Transition state dynamics and decay in ClHCl− (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 1975-1987 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general method for studying transition state spectroscopy and dynamics in hydrogen atom transfer reactions is presented. This approach is based on the time-dependent self-consistent field (TDSCF) approximation and is applied to a study of the ClHCl− photodetachment experiments of Metz et al. [Metz et al., J. Chem. Phys. 88, 1463 (1988)]. Comparison of results of exact time-dependent and TDSCF calculations are made for collinear and three-dimensional (J=0) approximations for the quantum dynamics. When ClHCl is constrained to be collinear, the TDSCF calculation overcorrelates the motions in the H atom displacement and ClCl extension coordinates. This results in relatively poor agreement with the exact result for many properties of the wave function. In contrast, when the system is propagated in the three vibrational coordinates of the system, the transition state dynamics are effectively over much more rapidly. Consequently, the TDSCF approximation yields results of very good quantitative accuracy over the time required for most of the wave function to decay off of the transition state. Comparison is also made between the wave function that results from the exact propagation and from TDSCF when the wave function in the ClCl stretch coordinate is approximated by a Gaussian wave packet. Here the magnitude of the overlap between the two TDSCF wave functions in the H atom coordinates, for quantum and semiclassical propagations of the wave function in the ClCl distance coordinate, is greater than 0.98 over the time of the propagations. These TDSCF calculations are repeated for a wave function that is approximated by a product of a two-dimensional wave function in the hydrogen atom coordinates and a one-dimensional wave function in the ClCl extension coordinate and even better quantitative agreement with the exact propagation is achieved. The success of this method for studying ClHCl gives us confidence that TDSCF will provide a general powerful tool for studies of hydrogen and proton transfer reactions in large systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467707
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  • 7
    Electronic Resource
    Electronic Resource
    A perturbed-mean-field approach to the decay rates of excited vibrational states in extended systems: An application to I2(Ne)n (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 4355-4366 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A method for calculating decay rates of vibrational modes in large polyatomic systems is proposed and tested. The high frequency excited vibration is treated quantum mechanically, and the soft modes are described classically. The initial state is described by the hybrid quantum/classical self-consistent-field (SCF) approximation. The formalism is based on a golden-rule expression. The driving potential is the difference between the full Hamiltonian and the mean field Hamiltonian (SCF) causing the decay of the initial state to final mixed quantum/classical SCF states. These states are calculated using an extension of the usual static mean-field techniques to systems with mixed quantum and classical degrees of freedom. The formalism for obtaining the mean-field states and calculating the decay rates is presented, and the method is applied to a diatomic molecule treated quantum mechanically, embedded in a 1D model for a rare gas cluster treated classically. The dependence of the eigenenergies of the quantum and the decay rates with temperature is studied. The influence on the system size is also presented and compared with the prediction of the isolated binary collision model. The effect of a change in the linear density of the cluster on the eigenenergies of the vibrational mode is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466318
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  • 8
    Electronic Resource
    Electronic Resource
    Quantum dynamics of large polyatomic systems using a classically based separable potential method (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 6046-6056 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method for approximate solution of the time-dependent vibrational Schrödinger equation, applicable to extended molecular systems, is presented. The new method is essentially an approximate time-dependent quantization of classical dynamics. A molecular dynamics simulation is used to obtain a separable, effective time-dependent potential for each mode, that implicitly includes also the effects of all the other modes on this degree of freedom. A time-dependent wave packet is then propagated separately for each mode, using the corresponding effective potential. The new approximation is valid for short time scale processes only, but it is easily applicable to large realistic systems. Test calculations against exact quantum and time-dependent self-consistent field (TDSCF) results are carried out for two examples; photodissociation of HI in the collinear Xe...HI cluster, and electron photodetachment from the collinear Ar...I−...Ar cluster. For illustration, the new scheme is also applied to photodetachment from large linear clusters Arn...I−...Arn (n=2–8) and the results are discussed. For the test systems, the results of the new method are virtually identical to those following from the computationally much more demanding TDSCF approach, and they are in excellent agreement with the exact results. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469339
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  • 9
    Electronic Resource
    Electronic Resource
    Quantum Monte Carlo studies of small B(H2)n clusters (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2577-2591 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure and stability of clusters of a boron atom with one to eight H2 molecules is investigated. For the simplest BH2 clusters, the lowest ab initio adiabatic potentials for o-H2 and p-H2 interacting with a boron atom are used. For the larger clusters (n=2–8), the p-H2 is treated as a sphere, and the total potential is taken to be the sum of pairwise additive B–H2 and H2–H2 interactions which include, in the former case, an anisotropy due to the orientation of the unpaired B 2p electron. This electronic interaction is considerably more attractive when H2 approaches the B atom in a plane perpendicular to the orientation of the 2p orbital. The local and global minima on these potential surfaces were located and diffusion quantum Monte Carlo simulations were used to determine the energies and properties of the ground state wave functions for these B–(H2)n clusters. For the B(H2) cluster, a comparison is made with the results of variational calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468432
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  • 10
    Electronic Resource
    Electronic Resource
    Solvation effects on association reactions in microclusters: Classical trajectory study of H+Cl(Ar)n (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 343-355 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The role of solvent effects in association reactions is studied in atom-cluster collisions. Classical trajectory studies of the systems H+Cl(Ar)n (n=1,12) are used to investigate the influence of size, structure, and internal energy of the "microsolvation'' on the H+Cl association reaction. The following effects of solvating the chlorine in an Arn cluster are found. (1) In the H+ClAr system there is a large "third body'' effect. The single solvent atom stabilizes the newly formed HCl molecule by removing some of its excess energy. The cross section found at low energies is a substantial fraction of the gas-kinetic cross section. The molecule is produced in highly excited vibrational-rotational states. (2) Some production of long-lived HCl...Ar complexes, with lifetimes of 1 ps and larger, is found for the H+ClAr collisions. Weak coupling stemming from the geometry of the cluster is the cause for long life times. These resonance states decay into HCl+Ar. (3) At low collision energy (E=10 kJ/mol) for H+Cl(Ar)12, the H+Cl association shows a sharp threshold effect with cluster temperature. For temperatures T≥45 K the cluster is liquidlike, and the reaction probability is high. For T≤40 K the cluster is solidlike, and there is no reactivity. This suggests the potential use of reactions as a signature for the meltinglike transition in clusters. (4) At high collision energies (E=100 kJ/mol) H atoms can penetrate also the solidlike Cl(Ar)12 cluster. At this energy, the solid–liquid phase change is found not to increase the reaction probability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468141
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