139 research outputs found
Photocatalytic oxidation mechanism of alkanes in contact with titanium dioxide
Isobutane was photooxidized on titanium dioxide between -16 and +180 C in tertiary butanol and acetone. The formation of tertiary butanol preceded the formation of acetone. Above 20 C the latter compound became clearly predominant. The reaction kinetics obeyed a steady state model of oxygen chemisorption with the involvement of isobutane in the physisorbed phase
Variation in the thermionic work function of semiconductor powders exposed to electromagnetic radiation
The study of the variation of thermoelectronic work function potential of TiO2 in the presence of isobutane shows that this gas is not adsorbed on this solid, in either the presence or the absence of ultraviolet radiation. These results, as well as those obtained in a previous work, lead to the mechanism of the photo-oxidation of isobutane at room temperature, in which excited atomic oxygen is the active species
Change in the thermionic work function of semiconductor powders exposed to electromagnetic radiation
The variations of the thermoelectronic work function of titanium dioxide, submitted to an ultraviolet or visible and infrared radiation, in the presence of oxygen, are studied by the vibrating condenser method. It is shown that during the ultraviolet irradiation, a desorption of a first species of oxygen simultaneously occurs with the adsorption of a second species of oxygen and that this phenomenon is found for any structure of TiO2 (anatase or rutile) any texture, oxygen pressure, radiation intensity, and nature of introduced dopes
Information theory of open fragmenting systems
An information theory description of finite systems explicitly evolving in time is presented. We impose a MaxEnt variational principle on the Shannon entropy at a given time while the constraints are set at a former time. The resulting density matrix contains explicit time odd components in the form of collective flows. As a specific application we consider the dynamics of the expansion in connection with heavy ion experiments. Lattice gas and classical molecular dynamics simulations are shown. © 2007 American Institute of Physics.Fil:Ison, M.J. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Dorso, C.O. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina
Exact pairing correlations in one-dimensionally trapped fermions with stochastic mean-field wave-functions
Accepted for publication in Physical Review Letters.The canonical thermodynamic properties of a one-dimensional system of interacting spin-1/2 fermions with an attractive zero-range pseudo-potential are investigated within an exact approach. The density operator is evaluated as the statistical average of dyadics formed from a stochastic mean-field propagation of independent Slater determinants. For an harmonically trapped Fermi gas and for fermions confined in a 1D-like torus, we observe the transition to a quasi-BCS state with Cooper-like momentum correlations and an algebraic long-range order. For few trapped fermions in a rotating torus, a dominant superfluid component with quantized circulation can be isolated
Sign-free stochastic mean-field approach to strongly correlated phases of ultracold fermions
We propose a new projector quantum Monte-Carlo method to investigate the
ground state of ultracold fermionic atoms modeled by a lattice Hamiltonian with
on-site interaction. The many-body state is reconstructed from Slater
determinants that randomly evolve in imaginary-time according to a stochastic
mean-field motion. The dynamics prohibits the crossing of the exact nodal
surface and no sign problem occurs in the Monte-Carlo estimate of observables.
The method is applied to calculate ground-state energies and correlation
functions of the repulsive two-dimensional Hubbard model. Numerical results for
the unitary Fermi gas validate simulations with nodal constraints.Comment: Accepted for publication in New Journal of Physic
Generalized Gibbs ensembles for time dependent processes
An information theory description of finite systems explicitly evolving in
time is presented for classical as well as quantum mechanics. We impose a
variational principle on the Shannon entropy at a given time while the
constraints are set at a former time. The resulting density matrix deviates
from the Boltzmann kernel and contains explicit time odd components which can
be interpreted as collective flows. Applications include quantum brownian
motion, linear response theory, out of equilibrium situations for which the
relevant information is collected within different time scales before entropy
saturation, and the dynamics of the expansion
Non-Linear Vibrations in Nuclei
We have perfomed Time Dependant Hartree-Fock (TDHF) calculations on the non
linear response of nuclei. We have shown that quadrupole (and dipole) motion
produces monopole (and quadrupole) oscillations in all atomic nuclei. We have
shown that these findings can be interpreted as a large coupling between one
and two phonon states leading to large anharmonicities.Comment: 4 pages, 3 figure
Local Density Approximation for proton-neutron pairing correlations. I. Formalism
In the present study we generalize the self-consistent
Hartree-Fock-Bogoliubov (HFB) theory formulated in the coordinate space to the
case which incorporates an arbitrary mixing between protons and neutrons in the
particle-hole (p-h) and particle-particle (p-p or pairing) channels. We define
the HFB density matrices, discuss their spin-isospin structure, and construct
the most general energy density functional that is quadratic in local
densities. The consequences of the local gauge invariance are discussed and the
particular case of the Skyrme energy density functional is studied. By varying
the total energy with respect to the density matrices the self-consistent
one-body HFB Hamiltonian is obtained and the structure of the resulting mean
fields is shown. The consequences of the time-reversal symmetry, charge
invariance, and proton-neutron symmetry are summarized. The complete list of
expressions required to calculate total energy is presented.Comment: 22 RevTeX page
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