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arXiv1996

On the absence of bound-state stabilization through short ultra-intense fields

We address the question of whether atomic bound states begin to stabilize in the short ultra-intense field limit. We provide a general theory of ionization probability and investigate its gauge invariance. For a wide range of potentials we find an upper and lower bound by non-perturbative methods, which clearly exclude the possibility that the ultra intense field might have a stabilizing effect on the atom. For short pulses we find almost complete ionization as the field strength increases.

quant-ph↗

arXiv1997

On the Influence of Pulse Shapes on Ionization Probability

We investigate analytical expressions for the upper and lower bounds for the ionization probability through ultra-intense shortly pulsed laser radiation. We take several different pulse shapes into account, including in particular those with a smooth adiabatic turn-on and turn-off. For all situations for which our bounds are applicable we do not find any evidence for bound-state stabilization.

quant-ph↗

arXiv1994

Exciton effects in soliton and bipolaron lattice states of doped electron-phonon Peierls systems

Exciton effects on soliton and bipolaron lattice states are investigated using an electron-lattice Peierls model with long-range Coulomb interactions. The Hartree-Fock (HF) approximation and the single-excitation configuration- interaction (single-CI) method are used to obtain optical absorption spectra. We discuss the following properties: (1) The attraction between the excited electron and the remaining hole makes the excitation energy smaller when the correlations are taken into account by the single-CI. The oscillator strengths of the lower excited states become relatively larger than in the HF calculations. (2) We look at variations of relative oscillator strengths of two or three kinds of excitons described by the single-CI. While the excess-electron concentration is small, the ratio of the oscillator strengths of the exciton with the lowest energy increases almost linearly. The oscillator strengths accumulate at this exciton as the concentration increases.

cond-mat↗

arXiv1996

Optical absorption spectra and geometric effects in higher fullerenes

The optical excitations in $\rug$ and higher fullerenes, including isomers of C$_{76}$, C$_{78}$, and C$_{84}$, are theoretically investigated. We use a tight binding model with long-range Coulomb interactions, treated by the Hartree-Fock and configuration-interaction methods. We find that the optical excitations in the energy region smaller than about 4 eV have most of their amplitudes at the pentagons. The oscillator strengths of projected absorption almost accord with those of the total absorption. When the projection is performed on each pentagon and pentagon dimers, the resultant spectrum in the low energy region is quite different from that of the total absorption. The spectral shapes of the total absorption are turned out to be determined mainly by the geometrical distributions of the pentagons in the fullerene structures.

cond-mat↗

arXiv1996

Ab initio simulations of liquid systems: Concentration dependence of the electric conductivity of NaSn alloys

Liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80% sodium) are studied using density functional calculations combined with molecular dynamics(Car-Parrinello method). The frequency-dependent electric conductivities for the systems are calculated by means of the Kubo-Greenwood formula. The extrapolated DC conductivities are in good agreement with the experimental data and reproduce the strong variation with the concentration. The maximum of conductivity is obtained, in agreement with experiment, near the equimolar composition. The strong variation of conductivity, ranging from almost semiconducting up to metallic behaviour, can be understood by an analysis of the densities-of-states.

mtrl-th↗

arXiv1995

A Lattice-Gas Model of Microemulsions

We develop a lattice gas model for the nonequilibrium dynamics of microemulsions. Our model is based on the immiscible lattice gas of Rothman and Keller, which we reformulate using a microscopic, particulate description so as to permit generalisation to more complicated interactions, and on the prescription of Chan and Liang for introducing such interparticle interactions into lattice gas dynamics. We present the results of simulations to demonstrate that our model exhibits the correct phenomenology, and we contrast it with both equilibrium lattice models of microemulsions, and to other lattice gas models.

comp-gas↗

arXiv1994

Stability of the Hydrogen and Hydrogen-like Molecules

We present a simple proof of the stability of the hydrogen molecule $(M^+M^+m^-m^-)$. It does not rely on the proton-to-electron mass ratio $M/m$ being very large, and actually holds for arbitrary values of $M/m$. Some asymmetric molecules of the type $(m_1^+m_2^+m_3^-m_4^-)$ are also stable. Possible applications to molecules containing antiparticles and to exotic hadrons in the quark model are briefly outlined. Revtex Version 3.0, 4 Figures available on request by Fax or Mail.

chem-ph↗

arXiv1994

Li$_5$ as a pseudorotating planar cluster

A pseudorotating state of Li$_5$ observed in a recent EPR experiment is investigated using the local density functional method. The calculated isotropic spin population indicates that a planar $C_{2v}$ structure is more consistent with the experimental result than a suggested trigonal bipyramid. A new pseudorotating state based on the planar structure is proposed.

chem-ph↗

arXiv1994

Quasi-classical path integral approach to supersymmetric quantum mechanics

{}From Feynman's path integral, we derive quasi-classical quantization rules in supersymmetric quantum mechanics (SUSY-QM). First, we derive a SUSY counterpart of Gutzwiller's formula, from which we obtain the quantization rule of Comtet, Bandrauk and Campbell when SUSY is good. When SUSY is broken, we arrive at a new quantization formula, which is found as good as and even sometime better than the WKB formula in evaluating energy spectra for certain one-dimensional bound state problems. The wave functions in the stationary phase approximation are also derived for SUSY and broken SUSY cases. Insofar as a broken SUSY case is concerned, there are strong indications that the new quasi-classical approximation formula always overestimates the energy eigenvalues while WKB always underestimates.

cond-mat↗

arXiv1998

Two-Color Coherent Photodissociation of Nitrogen Oxide in Intense Laser Fields

A simple one-dimensional semi-classical model with a Morse potential is used to investigate the possibility of two-color infrared multi-photon dissociation of vibrationally excited nitrogen oxide. The amplitude ratio effects and adiabatic effects are investigated. Some initial states are found to have thresholds smaller than expected from single-mode considerations and multiple thresholds exist for initial states up to 32. PACS: 42.50.Hz

chem-ph↗

arXiv1995

Classical and quantum Malus' law

The classical and the quantum Malus' Laws for light and spin are discussed. It is shown that for spin-1/2, the quantum Malus' Law is equivalent in form to the classical Malus' Law provided that the statistical average involves a quasi-distribution function that can become negative. A generalization of Malus' Law for arbitrary spin-s is obtained in the form of a Feynman path-integral representation for the Malus amplitude. The classical limit of the Malus amplitude for large s is discussed.

quant-ph↗

arXiv1994

Radiative association and inverse predissociation of oxygen atoms

The formation of $\mbox{O}_2$ by radiative association and by inverse predissociation of ground state oxygen atoms is studied using quantum-mechanical methods. Cross sections, emission spectra, and rate coefficients are presented and compared with prior experimental and theoretical results. At temperatures below 1000~K radiative association occurs by approach along the $1\,{}^3Π_u$ state of $\mbox{O}_2$ and above 1000~K inverse predissociation through the $\mbox{B}\,{}^3Σ_u^-$ state is the dominant mechanism. This conclusion is supported by a quantitative comparison between the calculations and data obtained from hot oxygen plasma spectroscopy.

chem-ph↗

arXiv1995

Spectrum Generating Algebra for X$_{3}$ Molecules

A new spectrum generating algebra for a unified description of rotations and vibrations in polyatomic molecules is introduced. An application to nonlinear X$_3$ molecules shows that this model (i) incorporates exactly the relevant point group, (ii) provides a complete classification of oblate top states, and (iii) treats properly both degenerate and nondegenerate vibrations.

chem-ph↗

arXiv1996

Boson-realization model for the vibrational spectra of tetrahedral molecules

An algebraic model of Boson-realization is proposed to study the vibrational spectra of a tetrahedral molecule, where ten sets of boson creation and annihilation operators are used to construct the Hamiltonian with $T_{d}$ symmetry. There are two schemes in our model. The first scheme provides an eight-parameter fit to the published experimental vibrational eigenvalues of methane with a root-mean-square deviation 11.61 $cm^{-1}$. The second scheme, where the bending oscillators are assumed to be harmonic and the interactions between the bending vibrations are neglected, provided a five-parameter fit with a root-mean-square deviation 12.42 $cm^{-1}$

chem-ph↗

arXiv1996

From Heisenberg matrix mechanics to EBK quantization: theory and first applications

Despite the seminal connection between classical multiply-periodic motion and Heisenberg matrix mechanics and the massive amount of work done on the associated problem of semiclassical (EBK) quantization of bound states, we show that there are, nevertheless, a number of previously unexploited aspects of this relationship that bear on the quantum-classical correspondence. In particular, we emphasize a quantum variational principle that implies the classical variational principle for invariant tori. We also expose the more indirect connection between commutation relations and quantization of action variables. With the help of several standard models with one or two degrees of freedom, we then illustrate how the methods of Heisenberg matrix mechanics described in this paper may be used to obtain quantum solutions with a modest increase in effort compared to semiclassical calculations. We also describe and apply a method for obtaining leading quantum corrections to EBK results. Finally, we suggest several new or modified applications of EBK quantization.

chem-ph↗

arXiv1996

Variational calculations of dispersion coefficients for interactions between H, He, and Li atoms

The dispersion coefficients $C_6$, $C_8$, and $C_{10}$ for the interactions between H, He, and Li are calculated using variational wave functions in Hylleraas basis sets with multiple exponential scale factors. With these highly correlated wave functions, significant improvements are made upon previous calculations and our results provide definitive values for these coefficients.

atom-ph↗

arXiv1996

Separation of the Exchange-Correlation Potential into Exchange plus Correlation: an Optimized Effective Potential Approach

Most approximate exchange-correlation functionals used within density functional theory are constructed as the sum of two distinct contributions for exchange and correlation. Separating the exchange component from the entire functional is useful since, for exchange, exact relations exist under uniform density scaling and spin scaling. In the past, accurate exchange-correlation potentials have been generated from essentially exact densities constructed using information from either quantum chemistry or quantum Monte Carlo calculations but they have not been correctly decomposed into their separate exchange and correlation components, except for two-electron systems. exchange and correlation components (except for two-electron systems). Using a recently proposed method, equivalent to the solution of an optimized effective potential problem with the corresponding orbitals replaced by the exact Kohn-Sham orbitals, we obtain the separation according to the density functional theory definition. We compare the results for the Ne and Be atoms with those obtained by the previously used approximate separation scheme.

cond-mat↗

arXiv1996

$ΔI=4$ and $ΔI=8$ bifurcations in rotational bands of diatomic molecules

It is shown that the recently observed $ΔI=4$ bifurcation seen in superdeformed nuclear bands is also occurring in rotational bands of diatomic molecules. In addition, signs of a $ΔI=8$ bifurcation, of the same order of magnitude as the $ΔI=4$ one, are observed both in superdeformed nuclear bands and rotational bands of diatomic molecules.

chem-ph↗