9,971 research outputs found
The formation of IRIS diagnostics V. A quintessential model atom of C II and general formation properties of the C II lines at 133.5 nm
The 133.5 nm lines are important observables for the NASA/SMEX mission
Interface Region Imaging Spectrograph (IRIS). To make 3D non-LTE radiative
transfer computationally feasible it is crucial to have a model atom with as
few levels as possible while retaining the main physical processes. We here
develop such a model atom and we study the general formation properties of the
C II lines. We find that a nine-level model atom of C I-C III with the
transitions treated assuming complete frequency redistribution (CRD) suffices
to describe the 133.5 nm lines. 3D scattering effects are important for the
intensity in the core of the line. The lines are formed in the optically thick
regime. The core intensity is formed in layers where the temperature is about
10kK at the base of the transition region. The lines are 1.2-4 times wider than
the atomic absorption profile due to the formation in the optically thick
regime. The smaller opacity broadening happens for single peak intensity
profiles where the chromospheric temperature is low with a steep source
function increase into the transition region, the larger broadening happens
when there is a temperature increase from the photosphere to the low
chromosphere leading to a local source function maximum and a double peak
intensity profile with a central reversal. Assuming optically thin formation
with the standard coronal approximation leads to several errors: Neglecting
photoionization severly underestimates the amount of C II at temperatures below
16kK, erroneously shifts the formation from 10kK to 25kK and leads to too low
intensities.Comment: Accepted for publication by the Astrophysical Journa
Dihedral-Angle-Controlled Crossover from Static Hole Delocalization to Dynamic Hopping in Biaryl Cation Radicals
In cases of coherent charge-transfer mechanism in biaryl compounds the rates follow a squared cosine trend with varying dihedral angle. Herein we demonstrate using a series of biaryl cation radicals with varying dihedral angles that the hole stabilization shows two different regimes where the mechanism of the hole stabilization switches over from (static) delocalization over both aryl rings to (dynamic) hopping. The experimental data and DFT calculations of biaryls with different dihedral angles unequivocally support that a crossover from delocalization to hopping occurs at a unique dihedral angle where the electronic coupling (Hab) is one half of reorganization (λ), that is, Hab=λ/2. The implication of this finding in non-coherent charge-transfer rates is being investigated
Structural Characterization of Quaterphenyl Cation Radical:  X-ray Crystallographic Evidence of Quinoidal Charge Delocalization in Poly-\u3cem\u3ep\u3c/em\u3e-phenylene Cation Radicals
Quaterphenyl derivative (QP) containing tert-butyl solubilizing groups at the terminal positions yields a stable cation radical salt that was isolated, and its structure was established by X-ray crystallography. The crystal structure of neutral QP and its cation radical (QP+•SbCl6-) provides unequivocal evidence for the quinoidal stabilization of the cationic charge or polaron by smoothing out the torsional motion of the interconnected p-phenylene rings. Such an observation of stabilization of the cationic charge in a poly-p-phenylene (PPP) derivative forms the basis for the noted high conductivities in PPP oligomers in their doped state
Hexabenzo[4.4.4]propellane:  A Helical Molecular Platform for the Construction of Electroactive Materials
Helical hexabenzo[4.4.4]propellane (a relative of hexaphenylethane) and its derivatives are synthesized and their structures are established by X-ray crystallography. Isolation and X-ray crystallographic characterization of a robust trication-radical salt of hexamethoxypropellane derivative confirms that its framework is stable toward oxidative (aliphatic) C−C bond cleavage. It is also demonstrated that propellane can be easily brominated at the 4,4‘-positions of the biphenyl linkages for its usage as a molecular platform for the preparation of electroactive materials
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