4,612 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
Probing autoionizing states of molecular oxygen with XUV transient absorption: Electronic symmetry dependent lineshapes and laser induced modification
The dynamics of autoionizing Rydberg states of oxygen are studied using
attosecond transient absorption technique, where extreme ultraviolet (XUV)
initiates molecular polarization and near infrared (NIR) pulse perturbs its
evolution. Transient absorption spectra show positive optical density (OD)
change in the case of and autoionizing states of oxygen
and negative OD change for states. Multiconfiguration
time-dependent Hartree-Fock (MCTDHF) calculation are used to simulate the
transient absorption spectra and their results agree with experimental
observations. The time evolution of superexcited states is probed in
electronically and vibrationally resolved fashion and we observe the dependence
of decay lifetimes on effective quantum number of the Rydberg series. We model
the effect of near-infrared (NIR) perturbation on molecular polarization and
find that the laser induced phase shift model agrees with the experimental and
MCTDHF results, while the laser induced attenuation model does not. We relate
the electron state symmetry dependent sign of the OD change to the Fano
parameters of the static absorption lineshapes.Comment: 15 pages, 8 figure
Resonant ion-pair formation in electron recombination with HF^+
The cross section for resonant ion-pair formation in the collision of
low-energy electrons with HF^+ is calculated by the solution of the
time-dependent Schrodinger equation with multiple coupled states using a wave
packet method. A diabatization procedure is proposed to obtain the electronic
couplings between quasidiabatic potentials of ^1Sigma^+ symmetry for HF. By
including these couplings between the neutral states, the cross section for
ion-pair formation increases with about two orders of magnitude compared with
the cross section for direct dissociation. Qualitative agreement with the
measured cross section is obtained. The oscillations in the calculated cross
section are analyzed. The cross section for ion-pair formation in electron
recombination with DF^+ is calculated to determine the effect of isotopic
substitution.Comment: 12 pages, 12 figure
Representation of a complex Green function on a real basis: I. General Theory
When the Hamiltonian of a system is represented by a finite matrix,
constructed from a discrete basis, the matrix representation of the resolvent
covers only one branch. We show how all branches can be specified by the phase
of a complex unit of time. This permits the Hamiltonian matrix to be
constructed on a real basis; the only duty of the basis is to span the
dynamical region of space, without regard for the particular asymptotic
boundary conditions that pertain to the problem of interest.Comment: about 40 pages with 5 eps-figure
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
Performance of the reconstruction algorithms of the FIRST experiment pixel sensors vertex detector
Hadrontherapy treatments use charged particles (e.g. protons and carbon ions) to treat tumors. During a therapeutic treatment with carbon ions, the beam undergoes nuclear fragmentation processes giving rise to significant yields of secondary charged particles. An accurate prediction of these production rates is necessary to estimate precisely the dose deposited into the tumours and the surrounding healthy tissues. Nowadays, a limited set of double differential carbon fragmentation cross-section is available. Experimental data are necessary to benchmark Monte Carlo simulations for their use in hadrontherapy. The purpose of the FIRST experiment is to study nuclear fragmentation processes of ions with kinetic energy in the range from 100 to 1000 MeV/u. Tracks are reconstructed using information from a pixel silicon detector based on the CMOS technology. The performances achieved using this device for hadrontherapy purpose are discussed. For each reconstruction step (clustering, tracking and vertexing), different methods are implemented. The algorithm performances and the accuracy on reconstructed observables are evaluated on the basis of simulated and experimental data
An efficient basis set representation for calculating electrons in molecules
The method of McCurdy, Baertschy, and Rescigno, J. Phys. B, 37, R137 (2004)
is generalized to obtain a straightforward, surprisingly accurate, and scalable
numerical representation for calculating the electronic wave functions of
molecules. It uses a basis set of product sinc functions arrayed on a Cartesian
grid, and yields 1 kcal/mol precision for valence transition energies with a
grid resolution of approximately 0.1 bohr. The Coulomb matrix elements are
replaced with matrix elements obtained from the kinetic energy operator. A
resolution-of-the-identity approximation renders the primitive one- and
two-electron matrix elements diagonal; in other words, the Coulomb operator is
local with respect to the grid indices. The calculation of contracted
two-electron matrix elements among orbitals requires only O(N log(N))
multiplication operations, not O(N^4), where N is the number of basis
functions; N = n^3 on cubic grids. The representation not only is numerically
expedient, but also produces energies and properties superior to those
calculated variationally. Absolute energies, absorption cross sections,
transition energies, and ionization potentials are reported for one- (He^+,
H_2^+ ), two- (H_2, He), ten- (CH_4) and 56-electron (C_8H_8) systems.Comment: Submitted to JC
Pseudospectral Calculation of the Wavefunction of Helium and the Negative Hydrogen Ion
We study the numerical solution of the non-relativistic Schr\"{o}dinger
equation for two-electron atoms in ground and excited S-states using
pseudospectral (PS) methods of calculation. The calculation achieves
convergence rates for the energy, Cauchy error in the wavefunction, and
variance in local energy that are exponentially fast for all practical
purposes. The method requires three separate subdomains to handle the
wavefunction's cusp-like behavior near the two-particle coalescences. The use
of three subdomains is essential to maintaining exponential convergence. A
comparison of several different treatments of the cusps and the semi-infinite
domain suggest that the simplest prescription is sufficient. For many purposes
it proves unnecessary to handle the logarithmic behavior near the
three-particle coalescence in a special way. The PS method has many virtues: no
explicit assumptions need be made about the asymptotic behavior of the
wavefunction near cusps or at large distances, the local energy is exactly
equal to the calculated global energy at all collocation points, local errors
go down everywhere with increasing resolution, the effective basis using
Chebyshev polynomials is complete and simple, and the method is easily
extensible to other bound states. This study serves as a proof-of-principle of
the method for more general two- and possibly three-electron applications.Comment: 23 pages, 20 figures, 2 tables, Final refereed version - Some
references added, some stylistic changes, added paragraph to matrix methods
section, added last sentence to abstract
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