Photoinduced absorption in disubstituted polyacetylenes: Comparison of theory with experiments

In a recently performed experiment Korovyanko et al [Phys. Rev. B 67, 035114 (2003)] have measured the photo-induced absorption (PA) spectrum of phenyl-disubstituted polyacetylenes (PDPA) from 1$B_{u}$ and 2$A_{g}$ excited states. In 1$B_{u}$ PA spectrum they identified two main features namely PA1 and PA2. While in the 2$A_{g}$ spectrum they identified only one feature called PA$_{g}$. In this paper we present a theoretical study of 1$B_{u}$ and 2$A_{g}$ PA spectra of oligo-PDPA's using correlated-electron Pariser-Parr-Pople (P-P-P) model and various configuration interaction (CI) methodologies. We compare the calculated spectra with the experiments, as well as with the calculated spectra of polyenes of the same conjugation lengths. Calculated spectra are in good agreement with the experiments. Based upon our calculations, we identify PA1 as the m$A_{g}$ state and PA$_{g}$ as the $nB_{u}$ state of the polymer. Regarding the PA2 feature, we present our speculations. Additionally, it is argued that the nature of excited states contributing to the $2A_{g}$-PA spectra of oligo-PDPA's is qualitatively different from those contributing to the spectra of polyenes.Comment: Revtex4, 14 pages, 8 figures (To appear in Phys. Rev. B, March 15 (2005) issue

Ab initio Wannier-function-based many-body approach to Born charge of crystalline insulators

In this paper we present an approach aimed at performing many-body calculations of Born-effective charges of crystalline insulators, by including the electron-correlation effects. The scheme is implemented entirely in the real space, using Wannier-functions as single-particle orbitals. Correlation effects are computed by including virtual excitations from the Hartree-Fock mean field, and the excitations are organized as per a Bethe-Goldstone-like many-body hierarchy. The results of our calculations suggest that the approach presented here is promising.Comment: 5 pages, to appear in Phys. Rev. B. (Rapid Comm., Dec 15, 2004

Ab initio Wannier-function-based correlated calculations of Born effective charges of crystalline Li2_{2}O and LiCl

In this paper we have used our recently developed ab initio Wannier-function-based methodology to perform extensive Hartree-Fock and correlated calculations on Li$_{2}$O and LiCl to compute their Born effective charges. Results thus obtained are in very good agreement with the experiments. In particular, for the case of Li$_{2}$O, we resolve a controversy originating in the experiment of Osaka and Shindo {[}Solid State Commun. 51 (1984) 421] who had predicted the effective charge of Li ions to be in the range 0.58--0.61, a value much smaller compared to its nominal value of unity, thereby, suggesting that the bonding in the material could be partially covalent. We demonstrate that effective charge computed by Osaka and Shindo is the Szigeti charge, and once the Born charge is computed, it is in excellent agreement with our computed value. Mulliken population analysis of Li$_{2}$O also confirms ionic nature of the bonding in the substance.Comment: 11 pages, 1 figure. To appear in Phys. Rev. B (Feb 2008

Energetics and electronic structure of phenyl-disubstituted polyacetylene: A first-principles study

Phenyl-disubstituted polyacetylene (PDPA) is an organic semiconductor which has been studied during the last years for its efficient photo-luminescence. In contrast, the molecular geometry, providing the basis for the electronic and optical properties, has been hardly investigated. In this paper, we apply a density-functional-theory based molecular-dynamics approach to reveal the molecular structure of PDPA in detail. We find that oligomers of this material are limited in length, being stable only up to eight repeat units, while the polymer is energetically unfavorable. These facts, which are in excellent agreement with experimental findings, are explained through a detailed analysis of the bond lengths. A consequence of the latter is the appearance of pronounced torsion angles of the phenyl rings with respect to the plane of the polyene backbone, ranging from $55^{\circ}$ up to $95^{\circ}$. We point out that such large torsion angles do not destroy the conjugation of the $\pi$ electrons from the backbone to the side phenyl rings, as is evident from the electronic charge density.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.

A large-scale correlated study of linear optical absorption and low-lying excited states of polyacenes: Pariser-Parr-Pople Hamiltonian

In this paper we present large-scale correlated calculations of linear optical absorption spectrum of oligo-acenes containing up to seven benzene rings. For the calculations we used the Pariser-Parr-Pople (P-P-P) Hamiltonian, along with the configuration interaction (CI) technique at various levels such as the full CI (FCI), the quadruple CI (QCI) and multi-reference singles-doubles CI (MRSDCI). The role of Coulomb parameters used in the P-P-P Hamiltonian was examined by considering standard Ohno parameters, as well as a screened set of parameters. A detailed analysis of the many-body character of the important excited states contributing to the linear absorption has also been performed. The results of our calculations have been compared extensively with the theoretical work of other authors, as well as with the experiments.Comment: 45 pages, 9 figure

Photoinduced absorption and nonlinear optical properties of disubstituted polyacetylenes: Theory

In this paper, we summarize results of our recent large-scale correlated calculations of nonlinear optical spectra and photoinduced absorption (PA) spectra of phenyl-disubstituted polyacetylenes (PDPA). Calculations were performed on oligomers of PDPA's using correlated-electron Pariser–Parr–Pople (P–P–P) model and configuration interaction (CI) methodology. Computed PA spectra are compared with the recent experiments of Korovyanko et al., and good agreement is obtained between the two.© Elsevie

A cadaveric study of aortic arch variations and its embryological basis

Background-Aortic arch variations are usually asymptomatic and are identified incidentally in radiological imaging and in autopsy finding.The aim of this study is to observe the incidence of variation in the branching pattern of arch of aorta in cadavers of Indian origin.Materials and Methods: A total of 45 cadavers were included in the study. Arch of aorta and its branches were dissected and noted. Origin of Brachiocephalic trunk,Left common carotid artery and left subclavian artery were noted.Results: Type 1-Normal three branching pattern was observed in 82.3%. Type II with twobranches was seen in 17.7% specimens.In this variant a common trunk giving rise to brachiocephalic trunk (BCT)and left common carotid artery(LCCA) and a separate origin of left subclavian artery to the left of common trunk was seen.A single heart with posterior and leftorigin of BCT was noted.Conclusion: Variation of aortic arch branching is ofradiological and surgical significance, particularly in the diagnosis and treatment of head,neck, and chest diseases.Awareness of these variations is particularly relevant for interventionalists who access these vessels during endovascular surgery