4,260 research outputs found
ABIOTIC DEGRADATION OF IODOSULFURON-METHYL-ESTER IN AQUEOUS SOLUTION
The abiotic degradation of iodosulfuron-methyl-ester was investigated under both alkaline and acidic
pH conditions in the dark, and results showed it to be a rather stable molecule in neutral or slightly
alkaline environments. Photochemical reactions were studied using a high-pressure mercury arc lamp,
and results showed that direct phototransformation is possible under normal environmental conditions
(ì > 290 nm). High-performance liquid chromatography (HPLC-UV and HPLC-MS) analyses were
used to identify the degradates and to study the kinetics of photodecomposition and hydrolysis. Five
main products of iodosulfuron-methyl-ester degradation were tentatively identified, and one of them
(4-methoxy-6-methyl-1,3,5-triazin-2-amine) was confirmed using an authentic standard. Among the
phototransformation mechanisms, photosubstitution of the iodide atom by a hydroxyl group,
photodissociation of the N-S bond, and photoassisted hydrolysis were observed. The quantum
efficiencies (multiwavelength quantum yield) of the photodegradation under different conditions were
determined, and values of 0.054 ( 0.02 (pH 9.6), 0.08 ( 0.02 (pH 7), and 0.044 ( 0.008 (pH 5.3)
were obtained
Development of a Classical Force Field for the Oxidised Si Surface: Application to Hydrophilic Wafer Bonding
We have developed a classical two- and three-body interaction potential to
simulate the hydroxylated, natively oxidised Si surface in contact with water
solutions, based on the combination and extension of the Stillinger-Weber
potential and of a potential originally developed to simulate SiO2 polymorphs.
The potential parameters are chosen to reproduce the structure, charge
distribution, tensile surface stress and interactions with single water
molecules of a natively oxidised Si surface model previously obtained by means
of accurate density functional theory simulations. We have applied the
potential to the case of hydrophilic silicon wafer bonding at room temperature,
revealing maximum room temperature work of adhesion values for natively
oxidised and amorphous silica surfaces of 97 mJ/m2 and 90mJ/m2, respectively,
at a water adsorption coverage of approximately 1 monolayer. The difference
arises from the stronger interaction of the natively oxidised surface with
liquid water, resulting in a higher heat of immersion (203 mJ/m2 vs. 166
mJ/m2), and may be explained in terms of the more pronounced water structuring
close to the surface in alternating layers of larger and smaller density with
respect to the liquid bulk. The computed force-displacement bonding curves may
be a useful input for cohesive zone models where both the topographic details
of the surfaces and the dependence of the attractive force on the initial
surface separation and wetting can be taken into account
Scalar mesons in radiative \phi\to K^0\bar{K}^0\gamma decay
We study the radiative decay within a
phenomenological framework by considering the contributions of the
and scalar resonances. We calculate the branching ratio by employing the coupling constants
and as determined by different experimental groups.Comment: 10 pages, 3 figure
Stress Development and Impurity Segregation during Oxidation of the Si(100) Surface
We have studied the segregation of P and B impurities during oxidation of the
Si(100) surface by means of combined static and dynamical first-principles
simulations based on density functional theory. In the bare surface, dopants
segregate to chemically stable surface sites or to locally compressed
subsurface sites. Surface oxidation is accompanied by development of tensile
surface stress up to 2.9 N/m at a coverage of 1.5 monolayers of oxygen and by
formation of oxidised Si species with charges increasing approximately linearly
with the number of neighbouring oxygen atoms. Substitutional P and B defects
are energetically unstable within the native oxide layer, and are
preferentially located at or beneath the Si/SiOx interface. Consistently,
first-principles molecular dynamics simulations of native oxide formation on
doped surfaces reveal that dopants avoid the formation of P-O and B-O bonds,
suggesting a surface oxidation mechanism whereby impurities remain trapped at
the Si/SiOx interface. This seems to preclude a direct influence of impurities
on the surface electrostatics and, hence, on the interactions with an external
environment
Two-photon decays of hadronic molecules
In many calculations of the two--photon decay of hadronic molecules, the
decay matrix element is estimated using the wave function at the origin
prescription, in analogy to the two-photon decay of parapositronium. We
question the applicability of this procedure to the two-photon decay of
hadronic molecules for it introduces an uncontrolled model dependence into the
calculation. As an alternative approach, we propose an explicit evaluation of
the hadron loop. For shallow bound states, this can be done as an expansion in
powers of the range of the molecule binding force. In the leading order one
gets the well-known point-like limit answer. We estimate, in a self-consistent
and gauge invariant way, the leading range corrections for the two-photon decay
width of weakly bound hadronic molecules emerging from kaon loops. We find them
to be small. The role of possible short-ranged operators and of the width of
the scalars remains to be investigated.Comment: LaTeX2e, 26 pages, new figure and additional appendix added, version
to appear in Phys.Rev.
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