50,008 research outputs found
Practical Synthesis of Unsymmetrical Tetraarylethylenes and Their Application for the Preparation of [TriphenylethyleneâSpacerâTriphenylethylene] Triads
We have demonstrated that reactions of diphenylmethyllithium with a variety of substituted benzophenones produces corresponding tertiary alcohols that are easily dehydrated, without any need for purification, to produce various unsymmetrical and symmetrical tetraarylethylenes in excellent yields. The simplicity of the method allows for the preparation of a variety of ethylenic derivatives in multigram (10â50 g) quantities with great ease. The methodology was successfully employed for the preparation of various triphenylethylene (TPE)-based triads (i.e., TPEâspacerâTPE) containing polyphenylene and fluoranyl-based spacers. The ready availability of various substituted tetraarylethylenes allowed us to shed light on the effect of substituents on the oxidation potentials (Eox) of various tetraarylethylenes. Moreover, the electronic coupling among the triphenylethylene moieties in various TPEâspacerâTPE triads was briefly probed by electrochemical and optical methods
Dispersive contributions to cross section ratio in forward regime
Two-photon exchange (TPE) contributions to elastic electron-proton scattering
in the forward regime are considered. The imaginary part of TPE amplitude in
these kinematics is related to the DIS nucleon structure functions. The real
part of the TPE amplitude is obtained from the imaginary part by means of
dispersion relations. We demonstrate that the dispersion integrals for the
relevant elastic -scattering amplitude converge and do not need
subtraction. This allows us to make clean prediction for the real part of the
TPE amplitude at forward angles. We furthermore compare and cross
sections which depends on the real part of TPE amplitude, and predict the
positron cross section to exceed the electron one by a few per cent, with the
difference ranging from 1.4% to 2.8% for electron energies in the range
from 3 to 45 GeV. We furthermore predict that the absolute value of this
asymmetry grows with energy, which makes it promising for experimental tests.Comment: 10 pages, 6 figure
Is there model-independent evidence of the two-photon-exchange effect in the electron-proton elastic scattering cross section?
We re-analyze the data of the elastic electron proton scattering to look for
model-independent evidence of the two-photon-exchange (TPE) effect. In contrast
to previous analyses, TPE effect is parametrized in forms which are free of
kinematical-singularity, in addition to being consistent with the constraint
derived from crossing symmetry and the charge conjugation. Moreover, we fix the
value of as determined from the data of the polarization transfer
experiment. We find that, at high values, the contribution
of the TPE effect to the slope of vs. is large and
comparable with that arising from . It also behaves quasi-linearly in
the region of current data, namely, in the range of .
Hence the fact that the current elastic cross section data shows little
nonlinearity with respect to can not be used to exclude the presence
of the TPE effect. More precise data at extreme angles will be crucial for a
model-independent extraction of the TPE effect.Comment: 11 pages, 4 figure
Ion pairing in model electrolytes: A study via three particle correlation functions
A novel integral equations approach is applied for studying ion pairing in
the restricted primitive model (RPM) electrolyte, i. e., the three point
extension (TPE) to the Ornstein-Zernike integral equations. In the TPE
approach, the three-particle correlation functions are obtained. The TPE results are compared to molecular
dynamics (MD) simulations and other theories. Good agreement between TPE and MD
is observed for a wide range of parameters, particularly where standard
integral equations theories fail, i. e., low salt concentration and high ionic
valence. Our results support the formation of ion pairs and aligned ion
complexes.Comment: 43 pages (including 18 EPS figs) - RevTeX 4 - J. Chem. Phys. (in
press
Review of two-photon exchange in electron scattering
We review the role of two-photon exchange (TPE) in electron-hadron
scattering, focusing in particular on hadronic frameworks suitable for
describing the low and moderate Q^2 region relevant to most experimental
studies. We discuss the effects of TPE on the extraction of nucleon form
factors and their role in the resolution of the proton electric to magnetic
form factor ratio puzzle. The implications of TPE on various other observables,
including neutron form factors, electroproduction of resonances and pions, and
nuclear form factors, are summarized. Measurements seeking to directly identify
TPE effects, such as through the angular dependence of polarization
measurements, nonlinear epsilon contributions to the cross sections, and via e+
p to e- p cross section ratios, are also outlined. In the weak sector, we
describe the role of TPE and gamma-Z interference in parity-violating electron
scattering, and assess their impact on the extraction of the strange form
factors of the nucleon and the weak charge of the proton.Comment: 73 pages, 40 figures, review article for Prog. Part. Nucl. Phys.
(dedicated to the memory of John A. Tjon
Two-Photon-Exchange Effects and Deformation
The two-photon-exchange (TPE) contribution in with
and small is calculated and its corrections to the ratios
of electromagnetic transition form factors and ,
are analysed. A simple hadronic model is used to estimate the TPE amplitude.
Two phenomenological models, MAID2007 and SAID, are used to approximate the
full cross sections which contain both the TPE and the
one-photon-exchange (OPE) contributions. The genuine the OPE amplitude is then
extracted from an integral equation by iteration. We find that the TPE
contribution is not sensitive to whether MAID or SAID is used as input in the
region with GeV.
It gives small correction to while for , the correction is
about -10\% at small and about at large for
GeV. The large correction from TPE at small must
be included in the analysis to get a reliable extraction of .Comment: Talk given at Conference:C16-07-2
- âŠ