402 research outputs found
Dissociative electron attachment to the H2O molecule. I. Complex-valued potential-energy surfaces for the 2B1, 2A1, and 2B2 metastable states of the water anion
We present the results of calculations defining global, three-dimensional
representations of the complex-valued potential-energy surfaces of the doublet
B1, doublet A1, and doublet B2 metastable states of the water anion that
underlie the physical process of dissociative electron attachment to water. The
real part of the resonance energies is obtained from configuration-interaction
calculations performed in a restricted Hilbert space, while the imaginary part
of the energies (the widths) is derived from complex Kohn scattering
calculations. A diabatization is performed on the 2A1 and 2B2 surfaces, due to
the presence of a conical intersection between them. We discuss the
implications that the shapes of the constructed potential-energy surfaces will
have upon the nuclear dynamics of dissociative electron attachment to H2O.
This work originally appeared as Phys Rev A 75, 012710 (2007). Typesetting
errors in the published version have been corrected here.Comment: Corrected version of PRA 75, 012710 (2007
Dissociative electron attachment to the H2O molecule. II. Nuclear dynamics on coupled electronic surfaces within the local complex potential model
We report the results of a first-principles study of dissociative electron
attachment to H2O. The cross sections are obtained from nuclear dynamics
calculations carried out in full dimensionality within the local complex
potential model by using the multi-configuration time-dependent Hartree method.
The calculations employ our previously obtained global, complex-valued,
potential-energy surfaces for the three (doublet B1, doublet A1, and doublet
B2) electronic Feshbach resonances involved in this process. These three
metastable states of H2O- undergo several degeneracies, and we incorporate both
the Renner-Teller coupling between the B1 and A1 states as well as the conical
intersection between the A1 and B2 states into our treatment. The nuclear
dynamics are inherently multidimensional and involve branching between
different final product arrangements as well as extensive excitation of the
diatomic fragment. Our results successfully mirror the qualitative features of
the major fragment channels observed, but are less successful in reproducing
the available results for some of the minor channels. We comment on the
applicability of the local complex potential model to such a complicated
resonant system.Comment: Corrected version of Phys Rev A 75, 012711 (2007
Multiconfiguration Time-Dependent Hartree-Fock Treatment of Electronic and Nuclear Dynamics in Diatomic Molecules
The multiconfiguration time-dependent Hartree-Fock (MCTDHF) method is
formulated for treating the coupled electronic and nuclear dynamics of diatomic
molecules without the Born- Oppenheimer approximation. The method treats the
full dimensionality of the electronic motion, uses no model interactions, and
is in principle capable of an exact nonrelativistic description of diatomics in
electromagnetic fields. An expansion of the wave function in terms of
configurations of orbitals whose dependence on internuclear distance is only
that provided by the underlying prolate spheroidal coordinate system is
demonstrated to provide the key simplifications of the working equations that
allow their practical solution. Photoionization cross sections are also
computed from the MCTDHF wave function in calculations using short pulses.Comment: Submitted to Phys Rev
Direct extraction of intense-field-induced polarization in the continuum on the attosecond time scale from transient absorption
A procedure is suggested for using transient absorption spectroscopy above the ionization threshold to measure the polarization of the continuum induced by an intense optical pulse. In this way transient absorption measurement can be used to probe subfemtosecond intense field dynamics in atoms and molecules. The method is based on an approximation to the dependence of these spectra on time delay between an attosecond XUV probe pulse and an intense pump pulse that is tested over a wide range of intensities and time delays by all-electrons-active calculations using the multiconfiguration time-dependent Hartree-Fock method in the case of neon
Temperature-pressure scaling for air-fluidized grains on approaches to Point J
We present experiments on a monolayer of air-fluidized beads in which a
jamming transition is approached by increasing pressure, increasing packing
fraction, and decreasing kinetic energy. This is accomplished, along with a
noninvasive measurement of pressure, by tilting the system and examining
behavior vs depth. We construct an equation of state and analyze relaxation
time vs effective temperature. By making time and effective temperature
dimensionless using factors of pressure, bead size, and bead mass, we obtain a
good collapse of the data but to a functional form that differs from that of
thermal hard-sphere systems. The relaxation time appears to diverge only as the
effective temperature to pressure ratio goes to zero
Nuclear Anapole Moments
Nuclear anapole moments are parity-odd, time-reversal-even E1 moments of the
electromagnetic current operator. Although the existence of this moment was
recognized theoretically soon after the discovery of parity nonconservation
(PNC), its experimental isolation was achieved only recently, when a new level
of precision was reached in a measurement of the hyperfine dependence of atomic
PNC in 133Cs. An important anapole moment bound in 205Tl also exists. In this
paper, we present the details of the first calculation of these anapole moments
in the framework commonly used in other studies of hadronic PNC, a meson
exchange potential that includes long-range pion exchange and enough degrees of
freedom to describe the five independent amplitudes induced by
short-range interactions. The resulting contributions of pi-, rho-, and
omega-exchange to the single-nucleon anapole moment, to parity admixtures in
the nuclear ground state, and to PNC exchange currents are evaluated, using
configuration-mixed shell-model wave functions. The experimental anapole moment
constraints on the PNC meson-nucleon coupling constants are derived and
compared with those from other tests of the hadronic weak interaction. While
the bounds obtained from the anapole moment results are consistent with the
broad ``reasonable ranges'' defined by theory, they are not in good agreement
with the constraints from the other experiments. We explore possible
explanations for the discrepancy and comment on the potential importance of new
experiments.Comment: 53 pages; 10 figures; revtex; submitted to Phys Rev
Atomic Electric Dipole Moments: The Schiff Theorem and Its Corrections
Searches for the permanent electric dipole moments (EDMs) of diamagnetic
atoms provide powerful probes of CP-violating hadronic and semileptonic
interactions. The theoretical interpretation of such experiments, however,
requires careful implementation of a well-known theorem by Schiff that implies
a vanishing net EDM for an atom built entirely from point-like, nonrelativistic
constituents that interact only electrostatically. Any experimental observation
of a nonzero atomic EDM would result from corrections to the point-like,
nonrelativistic, electrostatic assumption. We reformulate Schiff's theorem at
the operator level and delineate the electronic and nuclear operators whose
atomic matrix elements generate corrections to "Schiff screening". We obtain a
form for the operator responsible for the leading correction associated with
finite nuclear size -- the so-called "Schiff moment" operator -- and observe
that it differs from the corresponding operator used in previous Schiff moment
computations. We show that the more general Schiff moment operator reduces to
the previously employed operator only under certain approximations that are not
generally justified. We also identify other corrections to Schiff screening
that may not be included properly in previous theoretical treatments. We
discuss practical considerations for obtaining a complete computation of
corrections to Schiff screening in atomic EDM calculations.Comment: 31 pages, 2 figures, typeset by REVTe
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