757 research outputs found
Nonadiabatic quantum transition-state theory in the golden-rule limit. I. Theory and application to model systems
We propose a new quantum transition-state theory for calculating Fermi's
golden-rule rates in complex multidimensional systems. This method is able to
account for the nuclear quantum effects of delocalization, zero-point energy
and tunnelling in an electron-transfer reaction. It is related to instanton
theory but can be computed by path-integral sampling and is thus applicable to
treat molecular reactions in solution. A constraint functional based on energy
conservation is introduced which ensures that the dominant paths contributing
to the reaction rate are sampled. We prove that the theory gives exact results
for a system of crossed linear potentials and also the correct classical limit
for any system. In numerical tests, the new method is also seen to be accurate
for anharmonic systems, and even gives good predictions for rates in the Marcus
inverted regime.Comment: 18 pages and 6 figure
The nature and role of the gold-krypton interactions in small neutral gold clusters
© 2015 American Chemical Society. We investigate the nature and role of krypton embedding in small neutral gold clusters. For some of these clusters, we observe a particular site-dependent character of the Kr binding that does not completely follow the criterion of binding at low-coordinated sites, widely accepted for interaction of a noble gas with closed-shell metal systems such as metal surfaces. We aim at understanding the effect of low dimensionality and open-shell electronic structure of the odd-numbered clusters on the noble gas-metal cluster interaction. First, we investigate the role of attractive and repulsive forces, and the frontier molecular orbitals. Second, we investigate the Au-Kr interaction in terms of reactivity and bonding character. We use a reactivity index derived from Fukui formalism, and criteria provided by the electron localization function (ELF), in order to classify the type of bonding. We carry out this study on the minimum energy structures of neutral gold clusters, as obtained using pseudo potential plane-wave density functional theory (DFT). A model is proposed that includes the effect of attractive electrostatic, van der Waals and repulsive forces, together with effects originating from orbital overlap. This satisfactorily explains minimum configurations of the noble gas-gold cluster systems, the site preference of the noble gas atoms, and changes in electronic properties
Physics
Ion cyclotron resonance study of energy dependence of ion-molecule reaction in gaseous hydrogen, and fluorine 19 isotopic NMR chemical shifts due to chlorine 35 and chlorine 37 isotope
Far-from-equilibrium noise heating and laser cooling dynamics in radio-frequency Paul traps
We study the stochastic dynamics of a particle in a periodically driven
potential. For atomic ions trapped in radio-frequency Paul traps, noise heating
and laser cooling typically act slowly in comparison with the unperturbed
motion. These stochastic processes can be accounted for in terms of a
probability distribution defined over the action variables, which would
otherwise be conserved within the regular regions of the Hamiltonian phase
space. We present a semiclassical theory of low-saturation laser cooling
applicable from the limit of low-amplitude motion to large-amplitude motion,
accounting fully for the time-dependent and anharmonic trap. We employ our
approach to a detailed study of the stochastic dynamics of a single ion,
drawing general conclusions regarding the nonequilibrium dynamics of
laser-cooled trapped ions. We predict a regime of anharmonic motion in which
laser cooling becomes diffusive (i.e., it is equally likely to cool the ion as
it is to heat it), and can also turn into effective heating. This implies that
a high-energy ion could be easily lost from the trap despite being laser
cooled; however, we find that this loss can be counteracted using a laser
detuning much larger than Doppler detuning.Comment: 23 pages, 7 figure
Multi-Instantons and Exact Results I: Conjectures, WKB Expansions, and Instanton Interactions
We consider specific quantum mechanical model problems for which perturbation
theory fails to explain physical properties like the eigenvalue spectrum even
qualitatively, even if the asymptotic perturbation series is augmented by
resummation prescriptions to "cure" the divergence in large orders of
perturbation theory. Generalizations of perturbation theory are necessary which
include instanton configurations, characterized by nonanalytic factors
exp(-a/g) where a is a constant and g is the coupling. In the case of
one-dimensional quantum mechanical potentials with two or more degenerate
minima, the energy levels may be represented as an infinite sum of terms each
of which involves a certain power of a nonanalytic factor and represents itself
an infinite divergent series. We attempt to provide a unified representation of
related derivations previously found scattered in the literature. For the
considered quantum mechanical problems, we discuss the derivation of the
instanton contributions from a semi-classical calculation of the corresponding
partition function in the path integral formalism. We also explain the relation
with the corresponding WKB expansion of the solutions of the Schroedinger
equation, or alternatively of the Fredholm determinant det(H-E) (and some
explicit calculations that verify this correspondence). We finally recall how
these conjectures naturally emerge from a leading-order summation of
multi-instanton contributions to the path integral representation of the
partition function. The same strategy could result in new conjectures for
problems where our present understanding is more limited.Comment: 66 pages, LaTeX; refs. to part II preprint update
Josephson dynamics for coupled polariton modes under the atom-field interaction in the cavity
We consider a new approach to the problem of Bose-Einstein condensation (BEC)
of polaritons for atom-field interaction under the strong coupling regime in
the cavity. We investigate the dynamics of two macroscopically populated
polariton modes corresponding to the upper and lower branch energy states
coupled via Kerr-like nonlinearity of atomic medium. We found out the
dispersion relations for new type of collective excitations in the system under
consideration. Various temporal regimes like linear (nonlinear) Josephson
transition and/or Rabi oscillations, macroscopic quantum self-trapping (MQST)
dynamics for population imbalance of polariton modes are predicted. We also
examine the switching properties for time-averaged population imbalance
depending on initial conditions, effective nonlinear parameter of atomic medium
and kinetic energy of low-branch polaritons.Comment: 10 pages, 6 postscript figures, uses svjour.cl
Surface-enhanced pair transfer in quadrupole states of neutron-rich Sn isotopes
We investigate the neutron pair transfer modes associated with the low-lying
quadrupole states in neutron-rich Sn isotopes by means of the quasiparticle
random phase approximation based on the Skyrme-Hartree-Fock-Bogoliubov mean
field model. The transition strength of the quadrupole pair-addition mode
feeding the state is enhanced in the Sn isotopes with . The
transition density of the pair-addition mode has a large spatial extension in
the exterior of nucleus, reaching far to fm. The quadrupole
pair-addition mode reflects sensitively a possible increase of the effective
pairing interaction strength in the surface and exterior regions of
neutron-rich nuclei.Comment: 14 page
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