Quasiparticle spectra and excitons of organic molecules deposited on substrates: G0W0-BSE approach applied to benzene on graphene and metallic substrates

We present an alternative methodology for calculating the quasi-particle energy, energy loss, and optical spectra of a molecule deposited on graphene or a metallic substrate. To test the accuracy of the method it is first applied to the isolated benzene (C6H6) molecule. The quasiparticle energy levels and especially the energies of the benzene excitons (triplet, singlet, optically active and inactive) are in very good agreement with available experimental results. It is shown that the vicinity of the various substrates (pristine/doped graphene or (jellium) metal surface) reduces the quasiparticle HOMO-LUMO gap by an amount that slightly depends on the substrate type. This is consistent with the simple image theory predictions. It is even shown that the substrate does not change the energy of the excitons in the isolated molecule. We prove (in terms of simple image theory) that energies of the excitons are indeed influenced by two mechanisms which cancel each other. We demonstrate that the benzene singlet optically active (E1u) exciton couples to real electronic excitations in the substrate. This causes it substantial decay, such as {\Gamma} = 174 meV for pristine graphene and {\Gamma} = 362 meV for metal surfaces as the substrate. However, we find that doping graphene does not influence the E1u exciton decay rate.Comment: 16 pages, 14 figure

Synthesis of Some Quaternary Ammonium Salts Derived from Esters of Muconic Acid

Quaternary ammonium salts of di-(B-dimethylammonium ethyl) -muconate (I) and di-(B-dimethylammonium isopropyl)-muconate (II) , were prepared as possible neuromuscular blocking agents

TDDFT study of time-dependent and static screening in graphene

Time-dependent density functional theory (TDDFT) within the random phase approximation (RPA) is used to obtain the time evolution of the induced potential produce by the sudden formation of a C 1s core hole inside a graphene monolayer, and to show how the system reaches the equilibrium potential. The characteristic oscillations in the time-dependent screening potential are related to the excitations of π and σ+π plasmons as well as the low energy 2D plasmons in doped graphene. The equilibrium RPA screened potential is compared with the DFT effective potential, yielding good qualitative agreement. The self energy of a point charge near a graphene monolayer is shown to demonstrate an image potential type behavior, Ze/(z−z0), down to very short distances (4 a.u.) above the graphene layer. Both results are found to agree near quantitatively with the DFT ground state energy shift of a Li+ ion placed near a graphene monolayer

Dynamical effects and conductance asymmetry in metal-insulator-metal systems with different electrodes

We investigated the electron tunneling process in a planar system consisting of two semi-infinite metal electrodes of different dielectric properties separated by a narrow vacuum gap. This system can be viewed as a model of a metal-insulator-metal structure, or even as a very rough model of scanning tunneling microscopy. We used a local limit of a self-consistent dynamic calculation to evaluate effective tunneling barriers, and discussed the influence of different screening properties of the metallic electrodes on these barriers as well as on the conductance. We found that the barriers are significantly additionally lowered due to the difference in the charge fluctuation frequencies of the electrodes, and that the conductance minimum in such systems is shifted to a nonzero value. We compared our results with some measurements of I-V characteristics and work functions and found that our calculations contribute to the understanding of the observed conductance asymmetry

Synthetic Studies in the Chloramphenicol Series. III. Synthesis of threo-DL-Chloramphenicol from DL-Serine Ethyl Ether

A synthesis of threo-DL-chloramphenicol (I) from a-phthalimido-~-ethoxy-DL-propiophenone (II) is described. The crude ketone II obtained from a-phthalimido-~-ethoxy-DL-propionyl chloride via Friedel-Crafts reaction was reduced with aluminium isopropoxide to give a yield of 17.2 % of the corresponding carbinol III. This carbinol gave in a series of reactions threo -DL-chloramphenicol in an overall yield of 2.4 %

TDDFT study of time-dependent and static screening in graphene

Time-dependent density functional theory (TDDFT) within the random phase approximation (RPA) is used to obtain the time evolution of the induced potential produce by the sudden formation of a C 1s core hole inside a graphene monolayer, and to show how the system reaches the equilibrium potential. The characteristic oscillations in the time-dependent screening potential are related to the excitations of π and σ + π plasmons as well as the low energy 2D plasmons in doped graphene. The equilibrium RPA screened potential is compared with the DFT effective potential, yielding good qualitative agreement. The self energy of a point charge near a graphene monolayer is shown to demonstrate an image potential type behavior, Ze/(z − z0), down to very short distances (4 a.u.) above the graphene layer. Both results are found to agree near quantitatively with the DFT ground state energy shift of a Li+ ion placed near a graphene monolayer.D.J.M. acknowledges financial support from the Spanish “Juan de la Cierva” program (JCI-2010-08156), MICINN (FIS2010-21282-C02-01), “Grupos Consolidados UPV/EHU del Gobierno Vasco” (IT-319-07), and ACI-Promociona (ACI2009-1036).Peer Reviewe

On rationality of nonnegative matrix factorization

Nonnegative matrix factorization (NMF) is the problem of decomposing a given nonnegative n × m matrix M into a product of a nonnegative n × d matrix W and a nonnegative d × m matrix H. NMF has a wide variety of applications, including bioinformatics, chemometrics, communication complexity, machine learning, polyhedral combinatorics, among many others. A longstanding open question, posed by Cohen and Rothblum in 1993, is whether every rational matrix M has an NMF with minimal d whose factors W and H are also rational. We answer this question negatively, by exhibiting a matrix M for which W and H require irrational entries. As an application of this result, we show that state minimization of labeled Markov chains can require the introduction of irrational transition probabilities. We complement these irrationality results with an NP- complete version of NMF for which rational numbers suffice