Interpretation of a low-lying excited state of the reaction center of Rb.sphaeroides as a double triplet

The recently observed transient absorption of the lowest excited state of the special pair P* at 2710 cm-1 is assigned as a singlet which arises from the coupling of the two lowest triplets from the two dimer halves. INDO calculations are used to predict its intensity. The analogy of the coupling mechanism to the trip-doublet spectrum from P+ is shown and the influence of the double triplet on the Stark effect of P* is investigated.Comment: 10 pages Latex, 1 figure, to be published in Chemical Physics Letters 199

Effective Field Theory of the Single-Nucleon Sector

We address the issue of a consistent power counting scheme in manifestly Lorentz-invariant baryon chiral perturbation theory. We discuss the inclusion of vector mesons in the calculation of the nucleon electromagnetic form factors. We comment on the chiral expansion of the nucleon mass to order O(q**6).Comment: 4 pages, talk at "Chiral Symmetry in Hadron and Nuclear Physics" (Chiral 07), Osaka, Japan, 13-16 Nov. 200

Correlated spinless fermions on the honeycomb lattice revisited

We investigate the quantum many-body instabilities of the extended Hubbard model for spinless fermions on the honeycomb lattice with repulsive nearest-neighbor and 2nd nearest-neighbor density-density interactions. Recent exact diagonalization and infinite density matrix renormalization group results suggest that a putative topological Mott insulator phase driven by the 2nd nearest-neighbor repulsion is suppressed, while other numerically exact approaches support the topological Mott insulator scenario. In the present work, we employ the functional renormalization group (fRG) for correlated fermionic systems. Our fRG results hint at a strong suppression of the scattering processes stabilizing the topological Mott insulator. From analyzing the effects of fermionic fluctuations, we obtain a phase diagram which is the result of the competition of various charge ordering instabilities.Comment: 9 pages, 8 figure

Compton scattering from a pion and off-shell effects

We discuss the most general form of the electromagnetic vertex of an off-shell pion. The framework of chiral perturbation theory is used to illustrate the representation dependence of the electromagnetic three-point Green's function. For that purpose we discuss the concept of field transformations which, in comparison with the standard Gasser and Leutwyler Lagrangian, generate equivalent Lagrangians containing additional terms at order p^4 proportional to the lowest-order equation of motion. We consider Compton scattering from a pion to show that calculations involving different off-shell effects in the s- and u-channel pole diagrams may nevertheless lead to the same on-shell Compton scattering amplitude. This is a result of the equivalence theorem which states that two equivalent Lagrangians predict the same S-matrix elements even though they may generate a different off-shell behaviour of Green's functions. We conclude that off-shell effects are not only model dependent but also representation dependent.Comment: 10 pages, RevTex, no figures, invited talk at the conference on "Perspectives in Nuclear Physics at Intermediate Energies", Trieste, Italy, 8-12 May 199

Compton Scattering and Generalized Polarizabilities

In recent years, real and virtual Compton scattering off the nucleon have attracted considerable interest from both the experimental and theoretical sides. Real Compton scattering gives access to the so-called electromagnetic polarizabilities containing the structure information beyond the global properties of the nucleon such as its charge, mass, and magnetic moment. These polarizabilities have an intuitive interpretation in terms of induced dipole moments and thus characterize the response of the constituents of the nucleon to a soft external stimulus. The virtual Compton scattering reaction $e^-p\to e^-p\gamma$ allows one to map out the {\em local} response to external fields and can be described in terms of generalized electromagnetic polarizabilities. A simple classical interpretation in terms of the induced electric and magnetic polarization densities is proposed. We will discuss experimental results for the polarizabilities of the proton and compare them with theoretical predictions.Comment: 5 pages, 3 figures, invited lead talk at the 19th European Few-Body Conference, Groningen, The Netherlands, August 23-27, 200

Soft X-ray Emission and Resonant Inelastic Scattering Study of Polycyclic Hydrocarbons

This thesis comprises a detailed study of the electronic properties of the polycyclic hydrocarbonstetracene, perylene and coronene. The at room temperature grown organiclayers consisted of a up to several hundred nanometer thickness, and have been studiedwith the experimental techniques soft x-ray emission spectroscopy (SXES) and NEXAFS-spectroscopyby means of synchrotron radiation. In particular, resonant inelastic scattering(RIXS) was employed in order to obtain possible band structure or MO-symmetry informationof the studied systems. The studied materials consist of large organic molecules,evaporated under high vacuum an silicon wafers. The goal was to figure out, whether insidethese organic layers the hydrocarbon molecules polymerize and form a band structureor if the hydrocarbon molecules do not interact with each other and retain their originalmolecular like symmetry and behavior. The discrete peaks in the hydrocarbon NEXAFS-spectracould be assigned by comparison with the literature to chemical shifted C1s $\rightarrow \pi^{*}$transitions, caused by different chemical surrounded C-atoms. Hence, due to excitationan a certain $\pi$*-resonance only Cls-electrons of equal C-atoms can be excited, which leadsto a site selective excitation. The fluorescent decay of the created hole, which is as welllocalized an these former excited C-atoms was recorded energy dispersed in the SXE-spectrometer. This method is usually referred to as SXE. Therefore, the local MO-densitywas recorded. $\textit{Hartree-Fock}$ based ground state MO-calculations have been performed foreach molecule, in order to simulate the SXE-spectra. Good agreement between experimentand simulation was shown, under the strict retainment of symmetry selection rulesalong the transitions. This result states, that the different hydrocarbon molecules conservetheir symmetry and structure in an up to several hundred nanometer thick organic layer.Only in the region of the valence band maximum ($\pi$-states) weck indications for a banddispersion could be observed