UPS spectra of H2O,CH3OH and C5H9OH adsorbed onto Cu(111)/Na and Na(cp)

The present communication presents ultraviolet photoemission spectra (UPS) of three different “alcohols”; water (H₂O), methanol (CH₃OH), and cyclopentanol (C₅H₉OH), chemisorbed onto a Cu(111) surface partially covered by sodium atoms as well as onto closely packed sodium films, a free electron adsorbent. Whereas all three alcohols ROH bind reversibly and associatively to Cu(111) they react with adsorbed sodium atoms to metal bound alcoxides RO. The chemisorption bond, characterized by the interaction between O 2pπ orbitals and metal atoms as an electron donor, the alcoxide being the acceptor, is similar for all groups R. The O 2pπ orbitals shift to higher UPS binding energies with increasing electron density, i.e. decreasing r_s/a_o of the sodium overlayer. Only for HONa, the sterically smallest group R, does the alcoxide growth continue in three dimensions. Although, possibly failing to reproduce the electron density profile of a free electron surface, Hartree-Fock-Slater cluster calculations of small models ROH and RONa₃ enable correlations to be made between UPS intensity peaks and one electron orbitals.     

Band alignment and charge transport characteristics of TPP/ZnO hybrid for photovoltaic cell potential

Growth of ZnO nanocrystals has been carried out by a two-electrode electrochemical cell arrangement. The nanocrystals serve as substrates onto which tetraphenylporphyrin (TPP) is evaporated under integrated UHV conditions for the development of TPP/ZnO hybrid specimen. The quality of the hybrid specimen is investigated by XPS measurement. Direct interaction between the valence electrons of the adsorbed TPP molecules and ZnO electronic levels is revealed by Ultraviolet Photoemission Spectroscopy (UPS). Absorption spectroscopic studies are used to examine the ground state electronic structure and polarization dependence (excitation to π* and σ* vacant orbitals) of the hybrid specimen. The studies further show efficient charge transfer from donor (TPP) to proximate acceptor (ZnO). Thus, porphyrin molecules act as an active layer for generation of electron and as an enhancement for subsequent sudden transfer of the charge to the ZnO at a timescale faster than that of core hole.     

Calculations of ionization and excitation energies for SiH3 X (X = F,Cl, Br and I) using the discrete variational Xα method

The electronic structure of silyl halide SiH₃ X molecules are investigated using the discrete variational Xα method based on the Hartree—Fock—Slater model. Theoretical ionization and excitation energies are in very good agreement with the experimental results of UPS and UV spectra. The effects of Si 3d orbitals are found to be significant on the bondings and orbital energies.     

Structural and electronic properties of linear and angular polycyclic aromatic hydrocarbons

The structural and electronic properties of linear and angular polycyclic aromatic hydrocarbons (PAHs) have been studied by ab initio calculations. We show that the HOMO–LUMO gaps of linear and angular PAHs are inversely proportional to their width and length and angular PAHs always exhibit larger HOMO–LUMO gaps than their linear isomers, indicating the relative higher kinetic stability for angular PAHs. The frontier orbitals of HOMO and LUMO are localized at the zigzag edges in linear PAHs, while they are distributed uniformly over the molecules in angular PAHs. These results offer key insight for understanding the size and topology depending effects, which is of great use for experimental syntheses of PAHs with specific electronic properties.     

First principles study of the electron transport through cis-polyacetylene based molecular wires

The electron transport properties of cis-polyacetylene and cis-polyacetylene based molecular wires (oligo(cyclopentadiene), oligo(pyrrole), and oligo(furan)) have been studied theoretically using a combination of density-functional theory and non-equilibrium Green′s functions method. The results demonstrate that the introduction of bridging group X (X=CH₂, NH, and O) in cis-polyacetylene has a profound effect on the electron transport behavior of the molecules. The conductance of the four molecular wires decreases in the order of polyacetylene>oligo(cyclopentadiene)>oligo(furan)>oligo(pyrrole). In particular, the conductances of oligo(furan) and oligo(pyrrole) are much lower than those of polyacetylene and oligo(cyclopentadiene). The mechanism of this difference of electron transport properties of these four molecular systems is analyzed in terms of their geometric structures, electronic structures, transmission spectra, and spatial distribution of frontier orbitals. It is found that the energy levels of frontier molecular orbitals and the evolution of spatial distribution of frontier molecular orbitals with the applied bias are the essential reason for generating this difference of electron transport behaviors of the four molecular systems.     

CuSi6团簇几何结构和性质的密度泛函理论研究

利用密度泛函理论(DFT)的B3LYP方法,采用全电子基组6-311+G(d)研究了CuSi6团簇的几何构型和电子结构性质,计算表明CuSi6团簇存在多个能量相近的稳定异构体,且结构中存在多个Cu-Si键,多个低能异构体共存解释了实验中观察到的CuSi6团簇较强的现象。对于CuSi6团簇,计算得到的三个最稳定异构体的垂直电离能,电子亲和能和HOMO-LUMO能隙均相对较大,也表明这三个异构体较为稳定。     

The UPS of some compounds containing the heteroatoms phosphorus,nitrogen, and oxygen

The “lone-pair” regions of the He(I) UPS of 25 compounds containing phosphorus, nitrogen, and oxygen as heteroatoms are presented and discussed. The semiempirical MNDO SCF MO method has been employed to compute the orbital energies of many of the molecules. Although this theoretical treatment seems to predict correct geometries and conformations for the molecules, it does not always reproduce the most logical orbital-ordering sequence. In particular, the phosphorus “lone-pair” orbitals tend to be computed too stable. The controversial interpretation of the UPS of tris-dimethylaminophosphine is reviewed.     

中高能电子和N_2（~1∑_g~＋）、CO（~1∑~＋）弹性散射全截面的计算

此文采用由原子轨道线性组合构成的自洽场分子轨道（ＳＣＦ－ＬＣＡＯ－ＭＯ），用一级玻恩近似（ＦＢＡ）计算了能量为１００－５０００ｅＶ的电子与弹性散射的微分截面和全截面。由于采用了Ｇａｕｓｓ函数的线性组合拟合Ｓｌａｔｅｒ函数的方法（ＳＴＯ－ＫＧ），得到了弹性散射微分截面的解析表达式，使计算大大简化。计算得到的电子和Ｎ２，ＣＯ散射的总截面和实验结果进行了比较，当入射电子的能量大于１ｋｅＶ时，理论值和实验值之间符合得很好。     

Ab Initio Simulation of Charge Transfer at the Semiconductor Quantum Dot/TiO2 Interface in Quantum Dot‐Sensitized Solar Cells

Quantum dot‐sensitized solar cells (QDSSCs) have emerged as a promising solar architecture for next‐generation solar cells. The QDSSCs exhibit a remarkably fast electron transfer from the quantum dot (QD) donor to the TiO₂ acceptor with size quantization properties of QDs that allows for the modulation of band energies to control photoresponse and photoconversion efficiency of solar cells. To understand the mechanisms that underpin this rapid charge transfer, the electronic properties of CdSe and PbSe QDs with different sizes on the TiO₂ substrate are simulated using a rigorous ab initio density functional method. This method capitalizes on localized orbital basis set, which is computationally less intensive. Quite intriguingly, a remarkable set of electron bridging states between QDs and TiO₂ occurring via the strong bonding between the conduction bands of QDs and TiO₂ is revealed. Such bridging states account for the fast adiabatic charge transfer from the QD donor to the TiO₂ acceptor, and may be a general feature for strongly coupled donor/acceptor systems. All the QDs/TiO₂ systems exhibit type II band alignments, with conduction band offsets that increase with the decrease in QD size. This facilitates the charge transfer from QDs donors to TiO₂ acceptors and explains the dependence of the increased charge transfer rate with the decreased QD size.     

Lowest Π–Π* electronic transitions in linear and two-dimensional polycyclic aromatic hydrocarbons: enhanced electron density edge effect

Polycyclic aromatic hydrocarbons (PAHs) form an important class of molecules as they are ubiquitous, pollute air and cause severe health problems. Lowest vertical π–π* singlet–singlet or triplet–triplet excitation energies and corresponding oscillator strengths were studied for several linear and two-dimensional PAHs employing time-dependent density functional theory. Excited-state electron density, molecular electrostatic potential (MEP) and spin density distributions in the PAHs, along with ground-state chemical hardness, were also studied. It has been found that, generally, excitation energies and oscillator strengths decrease with increase in PAH size, and excitation energies and chemical hardness are strongly linearly correlated. Enhanced electron density edge effect, which was found to occur in the ground states of the molecules, continues to hold in their excited states also. A strong similarity between the ground and π–π* excited-state MEP maps suggests that σ electrons are the main contributors to the enhanced electron density at the edges. Due to their strong electronic absorption transitions in the visible and infrared regions, the PAHs can be used for harnessing solar energy efficiently.     

Do Dyson Orbitals resemble canonical Hartree–Fock orbitals?

ABSTRACT

Dyson orbitals are overlaps between states with N and N±1 electrons and provide conceptual links between transition probabilities of electron detachment or attachment, density matrices, total energies and general principles of chemical bonding. Canonical, Hartree–Fock orbitals are compared with Dyson orbitals obtained with electron–propagator calculations that retain all elements of the self–energy matrix, wherein all orbital–relaxation and electron–correlation corrections to Koopmans results reside. For valence ionization energies and electron affinities of representative closed–shell molecules, canonical, Hartree–Fock orbitals usually are excellent approximations to Dyson orbitals, although there are some notable cases where the resemblance is not as strong. Numerical relationships between pole strengths and the Koopmans contributions to Dyson orbitals also are inferred from the data.     

Noncanonical localized orbitals and chemisorption

A localization scheme is applied to the Hückel N-atom linear chain in the limit that the chain becomes semi-infinite. It is shown that eigenvalues which arise from the localization procedure serve to separate a set of orbitals localized near the surface from a set of bulk orbitals. The effect of localization on the binding energy of an adatom is also reported where the adatom is assumed to be identical to the bulk atoms. Approximate binding energies are computed by taking combinations of the adatom orbital plus localized orbitals. Agreement to within 10% with the exact energy is found for the case of localization to two end atoms.     

Investigation of photoprocesses in complex molecular systems (macroheterocycles)

Spectral properties and photophysical processes in two macroheterocycles consisting of two pairs of chemically bound fluorophors 1-(3,4-dicyanophenyl)-benzotriazole and 9,10-bis (4-aminophenyl)-anthracene (MI) and 1-(3,4- dicyanophenyl)-benzotriazole and rhodamine (MII) are examined by quantum-chemical and experimental methods. It is demonstrated that macroheterocycles are not bifluorophor molecules in classical understanding by virtue of their unique structure. All processes of electronic excitation energy deactivation in such systems proceed as in a single molecule, and electronic states in them are common with different relative contributions of molecular orbitals belonging to donor and acceptor fragments. The presence of identical fragments results in splitting of electronic states with the forbidden lower state from which the fluorescence is improbable.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 13–18, October, 2004.     

Relationships between geometrical and electronic structures and optical properties of 1,8-naphthosultam substituents and derivatives: TDDFT study

Geometrical and electronic structure and optical properties of several substituents and derivatives of 1,8-naphthosultam are studied by quantum-chemical DFT and TDDFT. It is found that the substituents –NO₂, –CF₃, and –N(CH₃)₂ affect the geometrical and electronic structure the most. It is shown that the Stokes shift is greatest (≈260 nm) for compounds with the strong donor substituent –N(CH₃)₂, while strong acceptor substituents provide a quite small Stokes shift. The dependence of the Stokes shift on the difference in energies of the frontier orbitals of the ground and excited states of molecules is found. Of the 1,8-naphthosultam substituents considered, compounds with –N(CH₃)₂ substituent, which emit in the biological window region, can be advised for use in optical bioimaging. The results can be used as a basis for the development and creation of new functional materials and biologically active compounds.     

Electron pairs,localized orbitals and electron correlation

Presuming zeroth-order electronic wavefunctions generated from localized SCF or FORS molecular orbitals, the correlation energy is expressed as a bilinear form in terms of the pair populations of these orbitals and the projections of a correlation operator onto these orbitals. The latter are determined by fitting the correlation energies of large sets of organic molecules, which are reproduced with a mean absolute deviation of 1–3 kcal mol^?1. The resulting formula provides a ‘back-of-the-envelope’ method for estimating correlation energies and furnishes an analysis of these energies in terms of physical concepts and chemical structure. It Predicts the correlation energy of diamond (per carbon atom) to within 6 kcal mol^?1.     

A Theory of Electronic Energy Transfer in Complex Molecular Systems

The Förster theory for energy transfer is critically reviewed in the context of the present-day theory of nonradiative transitions, and the fundamental tenets of the Förster theory are shown to be erroneous. A new theory of electronic energy transfer is constructed taking into account the electronic transition theory. The intermolecular interaction between donor and acceptor molecules is assumed to perturb electron states of isolated molecules before the donor-molecule excitation. A distinguishing characteristic of the intermolecular interaction is spatial delocalization of the wave functions of electron states of the interacting molecules. It is this fact that has made it possible to realize ordinary photophysical processes between electron states of various molecules in a bimolecular system. In the experiments under study, the result of the intermolecular nonradiative photoprocess is given as evidence of electronic energy transfer from the donor molecule to the acceptor molecule.     

Design of perylene-diimides-based small-molecules semiconductors for organic solar cells

A series of perylene-diimide-based small molecules have been designed to explore their optical, electronic and charge transport properties as organic solar cell materials. The frontier molecular orbitals analysis has turned out that the vertical electronic transitions of absorption are characterised as intramolecular charge transfer between perylene diimide moieties and substituent aromatic groups. Our results suggest that the optical and electronic properties and reorganisation energies are affected by the introduction of different aromatic groups to these molecules. The calculation results showed that the designed molecules own the large longest wavelength of absorption spectrum, the oscillator strength and absorption region values. On the basis of the investigated results, the designed molecules could be used as solar cell material with intense broad absorption spectra. Furthermore, they are expected to be the promising candidates for hole and/or electron transport materials.     

Identification of excited singlet states of chlorophenol isomers

The UV spectra of optical absorption of para-, meta-, and ortho-chlorophenol are recorded in the gas phase. The bands of UV spectra are assigned to the electronic transitions of molecules to definite excited singlet states on the basis of calculations by the TDDFT B3LYP/6-311++G(d, p) method. In each case the electron configuration making the predominant contribution to the particular singlet state is determined. The energies of singlet electronic transitions are shown to depend on the energy spacing between the molecular orbitals involved in these transitions.     

Quantum mechanics: cleaning up metaphysical baggage

The dynamic electronic structure of atoms and molecules can be directly observed by means of the (e, 2e) reaction, which measures the distribution of energies and momenta of two electrons in coincidence after a knockout reaction initiated by an electron beam of known momentum incident on a molecular gas target. The molecular state for each event is identified by the electron separation energy. The recoil momentum for each event is known from the difference of measured initial and final momenta. It has been verified that values of this momentum are equal under suitable conditions to the momentum of the electron in the target immediately before knockout. Thus the spherically-averaged electron momentum distribution for each molecular orbital is measured. This is directly related to molecular orbitals calculated by the methods of quantum chemistry. Properties obtained by this method for different types of molecules are discussed.     

Enhanced charge transfer in a monolayer of the organic charge transfer complex TTF-TNAP on Au(111)

Electronic doping is a key concept for tuning the properties of organic materials. In bulk structures, the charge transfer between donor and acceptor is mainly given by the respective ionization potential and electron affinity. In contrast, monolayers of charge transfer complexes in contact with a metal are affected by an intriguing interplay of hybridization and screening at the metallic interface, determining the resulting charge state. Using scanning tunneling microscopy and spectroscopy, we characterize the electronic properties of the organic acceptor molecule 11,11,12,12-tetracyanonaptho-2,6-quinodimethane (TNAP) adsorbed on a Au(111) surface. The ordered islands remain in a weakly physisorbed state with no charge transfer interaction with the substrate. When the electron donor tetrathiafulvalene (TTF) is added, ordered arrays of alternating TNAP and TTF rows are assembled. In these structures, we find the lowest unoccupied molecular orbital (LUMO) of the free TNAP molecule shifted well below the Fermi level of the substrate. The TNAP is thus charged with more than one electron.