Coulomb correlations and two-photon spectra of conjugated polymers
Abstract Electron—electron correlations in conjugated polymers are shown to produce weak two-photon absorption (TPA) below the one-photon gap E g and intense TPA above E g . TPA intensities vary strongly with the alternation δ of transfer integrals t (1 ± δ) and with U/t in Hubbard models. Pariser—Parr—Pople (PPP) models reconcile TPA spectra of conjugated molecules and polymers.
We give a brief introduction to the study of the algebraic structures -- and their geometrical interpretations -- which arise in the BRST construction of a conformal string background. Starting from the chiral algebra $\cA$ of a string background, we consider a number of elementary but universal operations on the chiral algebra. From these operations we deduce a certain fundamental odd Poisson structure, known as a Gerstenhaber algebra, on the BRST cohomology of $\cA$. For the 2D string background, the correponding G-algebra can be partially described in term of a geometrical G-algebra of the affine plane $\bC^2$. This paper will appear in the proceedings of {\it Strings 95}.