On the photoemission spectra of CoO: Crystal-field analyses of the valence band structure

The valence band structure of CoO deduced from X-ray and ultraviolet photoemission spectroscopy is described in the framework of crystal-field theory. The energy of the transitions from the ground state of Co²⁺ to all states of Co³⁺ as well as the corresponding relative intensities are determined by taking, for the first time, strong-field configuration interaction into account.     

Semiempirical molecular orbital studies on p-quinodimethane. Electronic structure assignments based on total energies and comparisons with photoelectron and absorption spectra

The electronic structure of p-quinodimethane has been investigated using both CNDO/S and INDO molecular orbital approximations. It is found that the energetically favorable configuration is a “quinoid” construction leading to a spin-paired singlet ground state. Comparisons of the calculated excitation energies and orbital orderings with optical and photoemission measurements are consistent with this assignment. The “quinoid” configuration is found to be energetically unfavorable toward the formation of a low-lying triplet or “biradical”-like state. Charge density distributions, however, suggest a high ground state chemical reactivity.     

A thermal spin transition in [Co(bpy)3][LiCr(ox)3] (ox = C2O4(2-); bpy = 2,2'-bipyridine)

In the three-dimensional oxalate network structures [M(II)(bpy)3][M(I)-M(III)(ox)3] (ox= C2O4(2-); bpy = 2,2'-bipyridine) the negatively charged oxalate backbone provides perfect cavities for tris-bipyridyl complex cations. The size of the cavity can be adjusted by variation of the metal ions of the oxalate backbone. In [Co(bpy)3][NaCr(ox)3], the [Co(bpy)3]2 + complex is in its usual 4T1(t2g5e(g)2) high-spin ground state. Substituting Na+ by Li+ reduces the size of the cavity. The resulting chemical pressure destabilises the high-spin state of [Co(bpy)3]2+ to such an extent that the 2E(t2g6e(g)1) low-spin state becomes the actual ground state. As a result. [Co(bpy)3][LiCr(ox)3] becomes a spin-crossover system, as shown by temperature-dependent magnetic susceptibility measurements and single-crystal optical spectroscopy, as well as by an X-ray structure determination at 290 and 10 K.     

Transition metal phthalocyanines: Insight into the electronic structure from soft x-ray spectroscopy

Transition metal phthalocyanines (MPc's) are an interesting class of material, and their magnetic and electronic properties are determined by the orbital occupation of the transition metal 3d orbitals incorporated in the molecules center. Thus, the ground state configuration of the transition metal center is very important for a complete understanding of these materials. We present experimental data taken using x-ray absorption and x-ray photoemission spectroscopy together with a theoretical interpretation of MPc series with M=Zn, Cu, Ni, Co, Fe, and Mn. The combination of these methods allows us to narrow down possible dominating ground state configurations and shed a brighter light on the electronic structure of these complexes.     

Abnormal temperature dependence of photoemission intensity mediated by thermally driven reorientation of a monomolecular film

The photoemission intensities of organic molecular layers generally obey the Debye-Waller temperature dependence but not always. With the example of a monomolecular film formed from [1,1';4',1'-terphenyl]-4,4'-dimethanethiol, we show that pronounced deviations from Debye-Waller temperature behavior are possible and are likely caused by temperature-dependent changes in molecular orientation.     

The relative helix and hydrogen bond stability in the B domain of protein A as revealed by integrated tempering sampling molecular dynamics simulation

Molecular dynamics simulations using the integrated tempering sampling method were performed for the folding of wild-type B domain of protein A (BdpA). Starting from random and stretched structures, these simulations allow us to fold this protein into the native-like structure frequently, achieving very small backbone (1.7 A?) and all heavy-atom root-mean-square deviation (2.6 A?). Therefore, the method used here increases the efficiency of configuration sampling and thermodynamics characterization by molecular dynamics simulation. Although inconsistency exists between the calculation and experiments for the absolute stabilities, as a limitation of the force field parameters, the calculated order of helix stability (H3 > H2 > H1) is consistent with that determined by experiments for individual separate helices. The lowest free energy folding pathway of BdpA was found to start with a barrierless and non-cooperative structural collapse from the entirely extended (E) state, which leads to a physiologically unfolded (P) state consisting of multiple stable structures with few native inter-helical hydrophobic interactions formed. In the P state, only H3 is fully structured. The final formation of H1 (and to a lesser extent, H2) in the folded (F) state requires the packing of the inter-helical hydrophobic contacts. In addition, it was found that stabilities of backbone hydrogen bonds are significantly affected by their positions relative to the inter-helical hydrophobic core. As temperature increases, the stability of the hydrogen bonds exposed to the solvent tends to increase while that of the hydrogen bonds buried within the hydrophobic core decreases. Finally, we discuss implications of this study on the general folding mechanism of proteins.     

Inverse photoemission from metal surfaces

Ultraviolet inverse photoemission spectroscopy (IPES) is a technique for exploring unoccupied electronic states, particularly in the energy range between the Fermi level and vacuum level, a range inaccessible in ordinary photoemission. Theories of inverse photoemission and its special instrumentation requirements are outlined. IPES measurements on clean metal surfaces have revealed an abundance of new Shockley surface states and the Rydberg series of image states converging on the vacuum level. Empty surface states of d-like character have also been seen. The systematics of the occurrence of surface states (including image states) associated with s,p bulk band gaps are well described by a simple adaptation of multiple-reflection theory. This model is propounded and its implications discussed with regard to effective masses, surface corrugation, and determination of the surface barrier potential. IPES measurements on adsorbate-covered metal surfaces have revealed antibonding levels (O on Ni surfaces being the prime example) and valuable information on empty molecular levels (the 2π state in adsorbed CO and NO being the prime example). A straightforward Blyholder interpretation is modified by considerations of electronic relaxation and screening in the emission process. We compare and contrast the role of these effects in photoemission and inverse photoemission. Polarization selection rules and molecular shape resonances are also discussed.     

Photoelectron spectroscopy of silver clusters

A comparison of x-ray photoemission from Ag clusters deposited on polygraphite and highly oriented pyrolitic graphite shows the influence of the support both on the valence band and on the core 3d level of the metal. Positive shifts have been obtained with respect to the bulk for the Fermi edge and the 3d peaks depending on the number of silver atoms deposited on the substrates. When the deposition is very small (cluster regime) the positive shifts of the binding energies are quite different for different substrates and cannot have a common origin. In contrast with recent work, we show that several effects contribute to these shifts: initial state effects like charge redistribution as well as final state effects like the hole-electron interaction.     

Correlation contributions to the radical cation states of linear even polyenes: An open-shell RHF-CNDO/S(CI) analysis of the optical and photoelectron spectra

Configuration interaction wave functions are calculated for the low-lying radical cation states of trans-butadiene, hexatriene, and octatetraene within the open-shell RHF -CNDO /S (CI ) approach. The consequences of various one-electron contributions to the interpretation of existing photoemission and radical cation optical spectra are emphasized. Electron correlation is shown to be essential to achieve adequate energy and intensity profiles assuming photoelectron or optical excitation. The excitation energies and transition amplitudes (optical and photoemission) are also found to be sensitive to the molecular geometry. The present results are consistent with previous interpretations that photoionization measurements probe the neutralmolecule alternating single-double bond-length structure, whereas optical excitation samples an ion-state–state–induced “relaxed” reference configuration having a weakened bond-length alternation. Calculated trends in the spectroscopic properties are extrapolated to extended members of the even-polyene series.     

X‐Ray Photoemission Studies of the Ternary Intermetallic Compounds Li2MGa and LiMGa2 (M = Rh,Pd, Ir,Pt)

X‐ray photoelectron and x‐ray excited Auger spectra were measured for the intermetallic compounds LiMGa₂ and Li₂MGa (M = Rh, Pd, Ir, Pt). The valence band spectra exhibit characteristic differences in the location of the M d‐band between group 9 elements (Rh, Ir) and group 10 elements (Pd, Pt) on one side and between LiMGa₂ and Li₂MGa on the other. The experimentally observed differences are in excellent agreement with results from band structure calculations. The combination of binding energy shifts with Auger kinetic energy shifts allowed a separation of initial and final state contributions. Core hole screening is very efficient in accordance with the metallic character of the investigated phases. The magnitude of the screening correlates with the theoretically predicted composition of the density of states at the Fermi level. Application of Wertheim's electrostatic model allowed to estimate the charge distribution for LiRhGa₂ and Li₂RhGa. The sign of the charges agrees with expectations that result from the Extended Zintl Concept. The results show, how dangerous it is to draw conclusions on the chemistry of such systems from photoemission data alone.     

Electronic structure of metal clusters

Photoemission spectra of valence electrons in metal clusters, together with threshold ionization potential measurements, provide a coherent picture of the development of the electronic structure from the isolated atom to the large metallic cluster. An insulator-metal transition occurs at an intermediate cluster size, which serves to define the boundary between small and large clusters. Although the outer electrons may be delocalized over the entire cluster, a small cluster remains insulating until the density of states near the Fermi level exceeds 1/kT. In large clusters, with increasing cluster size, the band structure approaches that of the bulk metal. However, the bands remain significantly narrowed even in a 1000-atom cluster, giving an indication of the importance of long-range order. The core-electron binding-energy shifts of supported metal clusters depend on changes in the band structure in the initial state, as well as on various final-state effects, including changes in core hole screening and the coulomb energy of the final-state charge. For cluster supported on amorphous carbon, this macroscopic coulomb shift is often dominant, as evidenced by the parallel shifts of the core-electron binding energy and the Fermi edge. Auger data confirm that final-state effects dominate in cluster of Sn and some other metals. Surface atom core-level shifts provide a valuable guide to the contributions of initial-state changes in band structure to cluster core-electron binding energy shifts, especially for Au and Pt. The available data indicate that the shift observed in supported, metallic clusters arise largely from the charge left on the cluster by photoemission. As the metal-insulator transition is approached from above, metallic screening is suppressed and the shift is determined by the local environment.     

Unidirectional Pt silicide nanowires grown on vicinal Si(100)

We investigated the structure and electronic properties of unidirectional Pt(2)Si nanowires (NWs) grown on a Si(100)-2 degrees off surface. We found that Pt(2)Si NWs were formed along the step edges of the Si(100)-2 degrees off surface with c(4x6) reconstructions that occurred on the terraces of Si(100) using scanning tunneling microscopy and the structure of formed NWs was found to be Pt(2)Si by core-level photoemission spectroscopy. Moreover, we confirmed that the electronic band structures of the NWs along the NW direction are different from those perpendicular to the NWs and the surface state induced by the Pt(2)Si NWs was observed with a small density of state using the angle-resolved photoemission spectra.     

Isolated monohydrates of a model peptide chain: effect of a first water molecule on the secondary structure of a capped phenylalanine

The formation of monohydrates of capped phenylalanine model peptides, CH(3)-CO-Phe-NH(2) and CH(3)-CO-Phe-NH-CH(3), in a supersonic expansion has been investigated using laser spectroscopy and quantum chemistry methods. Conformational distributions of the monohydrates have been revealed by IR/UV double-resonance spectroscopy and their structures assigned by comparison with DFT-D calculations. A careful analysis of the final hydrate distribution together with a detailed theoretical investigation of the potential energy surface of the monohydrates demonstrates that solvation occurs from the conformational distribution of the isolated peptide monomers. The distribution of the monohydrates appears to be strongly dependent on both the initial monomer conformation (extended or folded backbone) and the solvation site initially occupied by the water molecule. The solvation processes taking place during the cooling can be categorized as follows: (a) solvation without significant structural changes of the peptide, (b) solvation inducing significant distortions of the backbone but retaining the secondary structure, and (c) solvation triggering backbone isomerizations, leading to a modification of the peptide secondary structure. It is observed that solvation by a single water molecule can fold a β-strand into a γ-turn structure (type c) or induce a significant opening of a γ-turn characterized by an elongated C(7) hydrogen bond (type b). These structural changes can be considered as a first step toward the polyproline II condensed-phase structure, illustrating the role played by the very first water molecule in the solvation process.     

Synthesis and Characterizations of Regioregular Poly(3‐alkylthiophene) with Alter1483ting Dodecyl/1H,1H,2H,2H‐Perfluorooctyl Side Chains

A regioregular poly[4′‐dodecyl‐3‐(1H,1H,2H,2H‐perfluorooctyl)‐2,2′‐bithiophene] (P3DDFT) with alternating alkyl and semifluoroalkyl side chains were synthesized. Short ethylene spacer between perfluorohexyl part and thiophene did not largely affect the absorption and emission properties of the polythiophene backbone in comparison with poly(3‐dodecylthiophene) (P3DDT). P3DDFT showed a larger onset of the oxidation potential (+0.17 V) observed by cyclic voltamogram due to the electron withdrawing effect of the fluoroalkyl part. Thermal analysis and X‐ray diffraction patterns indicated that P3DDFT in the solid state forms a semicrystalline lamellar structure that is similar to that of P3DDT. Ultraviolet photoemission spectroscopy was also used to investigate their electron structure in the films. Comparison of hole mobilities in the films suggested that P3DDFT could have a less ordered packing structure compared to P3DDT both in the bulk and at the dielectric interface.

     

Glass transition broadening via nanofiller-contiguous polymer network in aromatic thermosetting copolyester nanocomposites

The glass transition is a genuine imprint of temperature-dependent structural relaxation dynamics of backbone chains in amorphous polymers, which can also reflect features of chemical transformations induced in macromolecular architectures. Optimization of thermophysical properties of polymer nanocomposites beyond the state of the art is contingent on strong interfacial bonding between nanofiller particles and host polymer matrix chains that accordingly modifies glass transition characteristics. Contemporary polymer nanocomposite configurations have demonstrated only marginal glass transition temperature shifts utilizing conventional polymer matrix and functionalized nanofiller combinations. We present nanofiller-contiguous polymer network with aromatic thermosetting copolyester nanocomposites in which carbon nanofillers covalently conjugate with cure advancing crosslinked backbone chains through functional end-groups of constituent precursor oligomers upon an in situ polymerization reaction. Via thoroughly transformed backbone chain configuration, the polymer nanocomposites demonstrate unprecedented glass transition peak broadening by about 100 °C along with significant temperature upshift of around 80 °C. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1595–1603     

Coverage-dependent adsorption geometry of octithiophene on Au(111)

The adsorption behavior of α-octithiophene (8T) on the Au(111) surface as a function of 8T coverage has been studied with low-temperature scanning tunneling microscopy, high resolution electron energy loss spectroscopy as well as with angle-resolved two-photon photoemission and ultraviolet photoemission spectroscopy. In the sub-monolayer regime 8T adopts a flat-lying adsorption geometry. Upon reaching the monolayer coverage the orientation of 8T molecules changes towards a tilted configuration, with the long molecular axis parallel to the surface plane, facilitating attractive intermolecular π-π-interactions. The photoemission intensity from the highest occupied molecular orbitals (HOMO and HOMO - 1) possesses a strong dependence on the adsorption geometry due to the direction of the involved transition dipole moment for the respective photoemission process. The change in molecular orientation as a function of coverage in the first molecular layer mirrors the delicate balance between intermolecular and molecule/substrate interactions. Fine tuning of these interactions opens up the possibility to control the molecular structure and accordingly the desirable functionality.     

Transmembrane Signaling Mediated by Water in Bovine Rhodopsin

Abstract— Unhydrated air-dried films of rhodopsin from bovine rod outer segment membranes do not produce its active state, metarhodopsin II. In order to reveal requirements for its formation, we studied changes in H-bonding of water, peptide carbonyl and carboxylic acid in the photochemical reactions by means of difference Fourier transform infrared spectroscopy, under both hydrated and unhydrated conditions. A water molecule near Glull3, which undergoes H-bonding change in bathorhodopsin, remained in the unhydrated film, but with a weaker H-bonding state than in the hydrated film. The other water molecules, which shift in lumirhodopsin and metarhodopsin I as well as in bathorhodopsin of the hydrated film, were not observed in the unhydrated film. Effects of the dehydration were detected in all the C=O stretching vibrations of the peptide backbone and of Asp83 in the formation of bathorhodopsin. The C=O stretching band of Asp83 of lumirhodopsin and metarhodopsin I is intensified in the unhydrated film. We propose that structural changes at the intradiscal site in the interaction between the Schiff base and Glull3 affect water molecules, the peptide backbone, Asp83 and Glul22 in helices B and C through consecutive photochemical processes to metarhodopsin II.