24,184 research outputs found
Studies of the photoionization cross sections of CH_4
We present cross sections and asymmetry parameters for photoionization of the 1t_2 orbital of CH_4 using static‐exchange continuum orbitals of CH^+_4 to represent the photoelectron wave function. The calculations are done in the fixed‐nuclei approximation at a single internuclear geometry. To approximate the near‐threshold behavior of these cross sections, we assumed that the photoelectron spectrum is a composite of three electronic bands associated with the Jahn–Teller components of the distorted ion. The resulting cross sections reproduce the sharp rise seen at threshold in the experimental data and are in good agreement with experiment at higher energy. The agreement between the calculated and measured photoelectron asymmetry parameters is, however, less satisfactory
Epidemiologia das doenças das plantas úteis nos diferentes sistemas de plantio.
São discutidos a ocorrência de Microcyclus ulei e Thanatephorus cucumeris em seringueira; Phytophthora sp., Septobasidium pseudopediculatum e S. sacardinum em laranja; Crinipellis perniciosa em cupuaçu; Meloidogyne sp. em mamão; e de Xanthomonas campestris pv. manihotis em mandioca.bitstream/item/180863/1/Recuperacao-88-95.pd
Incidence of foliar diseases in mixed cultivation systems.
Incidência de doenças foliares em sistemas de policultivo: Mixed cropping systems can be an alternative to traditional cultivation methods in humid tropical areas such as Amazonia. The dispersion of species in the cultivated area will probably reduce the incidence of disease, because the non-host plants serve as a barrier to the spread of pathogens. Recultivation of abandoned area with the aid of mixed cropping systems has been studied. Since July 1993, the severity of infection of rubber trees has been assessed weekly and the incidence of disease in the other crops monitored monthly. Only rubber, orange, cassava and papaya have been affected. As the plants have not yet reached asufficient height to form pathogen barriers, the differences between the monocultures and the mixed cultivation systems are not significant.The occurrence of Microcyclus ulei and Thanatephorus cucumeris in rubber, Phytophthora citrophthora, Septobasidium pseudopediculatum and S. sacardium in orange. Meloidogyne sp. in papaya and Xanthomonas campestris p.v. manihotis in cassava are discussed
Motion of falling object
A simple setup was assembled to study the motion of an object while it falls.
The setup was used to determine the instantaneous velocity, terminal velocity
and acceleration due to gravity. Also, since the whole project was done within
$20 it can easily be popularized.Comment: 11 pages, 4 figur
Black Hole Formation with an Interacting Vacuum Energy Density
We discuss the gravitational collapse of a spherically symmetric massive core
of a star in which the fluid component is interacting with a growing vacuum
energy density. The influence of the variable vacuum in the collapsing core is
quantified by a phenomenological \beta-parameter as predicted by dimensional
arguments and the renormalization group approach. For all reasonable values of
this free parameter, we find that the vacuum energy density increases the
collapsing time but it cannot prevent the formation of a singular point.
However, the nature of the singularity depends on the values of \beta. In the
radiation case, a trapped surface is formed for \beta<1/2 whereas for
\beta>1/2, a naked singularity is developed. In general, the critical value is
\beta=1-2/3(1+\omega), where the \omega-parameter describes the equation of
state of the fluid component.Comment: 9 pages, 8 figure
Interacting dark energy in gravity
The field equations in gravity derived from the Palatini variational
principle and formulated in the Einstein conformal frame yield a cosmological
term which varies with time. Moreover, they break the conservation of the
energy--momentum tensor for matter, generating the interaction between matter
and dark energy. Unlike phenomenological models of interacting dark energy,
gravity derives such an interaction from a covariant Lagrangian which is
a function of a relativistically invariant quantity (the curvature scalar ).
We derive the expressions for the quantities describing this interaction in
terms of an arbitrary function , and examine how the simplest
phenomenological models of a variable cosmological constant are related to
gravity. Particularly, we show that for a flat,
homogeneous and isotropic, pressureless universe. For the Lagrangian of form
, which is the simplest way of introducing current cosmic acceleration
in gravity, the predicted matter--dark energy interaction rate changes
significantly in time, and its current value is relatively weak (on the order
of 1% of ), in agreement with astronomical observations.Comment: 8 pages; published versio
Clustering, Angular Size and Dark Energy
The influence of dark matter inhomogeneities on the angular size-redshift
test is investigated for a large class of flat cosmological models driven by
dark energy plus a cold dark matter component (XCDM model). The results are
presented in two steps. First, the mass inhomogeneities are modeled by a
generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is
characterized by a smoothness parameter and a power index ,
and, second, we provide a statistical analysis to angular size data for a large
sample of milliarcsecond compact radio sources. As a general result, we have
found that the parameter is totally unconstrained by this sample of
angular diameter data.Comment: 9 pages, 7 figures, accepted in Physical Review
Probing the two-scale-factor universality hypothesis by exact rotation symmetry-breaking mechanism
We probe the two-scale factor universality hypothesis by evaluating, firstly
explicitly and analytically at the one-loop order, the loop quantum corrections
to the amplitude ratios for O() scalar field theories with
rotation symmetry-breaking in three distinct and independent methods in which
the rotation symmetry-breaking mechanism is treated exactly. We show that the
rotation symmetry-breaking amplitude ratios turn out to be identical in the
three methods and equal to their respective rotation symmetry-breaking ones,
although the amplitudes themselves, in general, depend on the method employed
and on the rotation symmetry-breaking parameter. At the end, we show that all
these results can be generalized, through an inductive process based on a
general theorem emerging from the exact calculation, to any loop level and
physically interpreted based on symmetry ideas.Comment: 17 pages, 3 figure
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