Intramolecular screening of crystal fields and the X-ray-photoemission determination of charge transfer in TTF-TCNQ

A recent controversy concerns the XPS data for TTF-TCNQ, which shows two binding energies for the N?1s level. We show that XPS chemical shifts due to charge flow on TCNQ, in response to the crystal potential, can cancel the Madelung splitting of the N?1s levels. Then we decompose the N?1s spectrum into its two components, which have widths characteristic of N?1s in TCNQ⁰ and TCNQ^?, and whose intensities suggest charge transfer of ～ 0.56–0.67.     

Optical properties of nanodiamond layers

Thin ultradisperse diamond (UDD) layers deposited from a water suspension are studied by optical and x-ray photoelectron spectroscopy (XPS). The effective band gap determined by the 10⁴-cm^?1 criterion for ozone-cleaned UDD is 3.5 eV. The broad structureless photoluminescence band (380–520 nm) is associated with radiative recombination through a system of continuously distributed energy levels in the band gap of diamond nanoclusters. The optical absorption of the material at 250–1000 nm originates from absorption on the disordered nanocluster surface containing threefold-coordinated carbon. The surface of UDD clusters subjected to acid cleaning contains nitrogen-oxygen complexes adsorbed in the form of NO ₃ ^? nitrate ions. Annealing in a hydrogen atmosphere results in desorption of the nitrate ions from the cluster surface. The evolution of the oxygen (O1s) and nitrogen (N1s) lines in the XPS spectra under annealing of a UDD layer is studied comprehensively.     

Isotope shift of182Hg and an update of nuclear moments and charge radii in the isotope range181Hg-206Hg

The technique of collinear fast-beam laser spectroscopy has been used to measure the isotope shifts of the even-even isotopes of Hg (Z=80) in the mass range 182≤A≤198 at the on-line mass separator ISOLDE at CERN. The atomic transition studied (6s 6p ³ P ₂- 6s7s ³ S ₁,λ=546.1 nm) starts from a metastable state, which is populated in a quasi resonant charge transfer process. The resulting changes in nuclear mean square charge radii show clearly that¹⁸²Hg follows the trend of the heavier, even, weakly oblate isotopes. Correspondingly the huge odd-even shape staggering in the light Hg isotopes continues and the nuclear shape staggering and shape coexistence persists down to the last isotope investigated,¹⁸¹Hg. An update of isotope shift and hyperfine structure data for^181–206Hg is given, with a revised evaluation of the differences in nuclear mean square charge radii and of spectroscopic quadrupole moments.     

On the core electron binding energy of carbon and the effective charge of the carbon atom

From Mulliken population data based on ab initio calculations of a series of substituted aromatic compounds and from well-calibrated XPS data (core electron binding energies, E_b) an empirical relation between E_b(C1s) and the charge on the carbon atoms, q_c, has been found: E_b(C1s = 6.42q_c + 4.52q²_c + 285.8 (eV). It is also found that XPS data for other carbon-containing species, notably the cyanide ion, can be described by this relation.     

A simple model for the quantitative evaluation of the XPS line shape asymmetries in nearly free electron and noble metals

We show that the Born approximation for the scattering by a linearly screened simple model “hole” potential, derived by pseudopotential data, is sufficient to provide an accurate evaluation of the XPS line shape asymmetry parameter α. We get a simple analytical close form expression for α as a function of r_s, which emphasizes the role of electronic screening and evidentiates that α is increasing with increasing r_s.     

X-ray photoelectron spectroscopy of merocyanine dyes: Part IV. Correlation of O1s binding energy with atomic charge calculated by Smyth's method. Application to the merocyanine 540 dye

Smyth's equation for calculating the partial ionic character of bonds yields better results than Pauling's equation when the electronegativity difference between the bonded atoms exceeds unity. This equation is used to establish the following binding energy/partial charge correlation for the O1s line:O1s binding energy (eV) = 534.55 + 3.29 q. This relation is used to evaluate the conjugation of benzoxazolinic and carbonyl oxygen atoms in the MC 540 dye from the measured XPS binding energies. The partial charge on the carbonyl oxygen atoms is close to −1 and the weight of the ⁻O-C⩽ form can be evaluated to ca. 80%.     

Comparative study of the core level photoemission of the ZrB2 and ZrB12

X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were used to investigate the binding energies and valence band for ZrB₂ and ZrB₁₂. The Zr 3d and B 1s core levels were identified. The Zr 3d core level shows a spin–orbit split 3d_5/2 and 3d_3/2 while that for B 1s core level exhibited a single symmetric peak, these being typical of zirconium and boride signals. Comparing the Zr 3d and B 1s core levels with metallic Zr, B₂O₃ and ZrO₂ reference materials only a negative chemical shift for Zr 3d associated to ZrB₂ was observed, which suggests that the charge transfer model based on the concept of electronegativity was not applicable to explain the superconductivity in the ZrB₁₂ sample. The measured valence band using UPS is consistent with the band-structure calculations indicating a higher density of states (DOS) at E_F for ZrB₁₂ respect to ZrB₂. Finally, we found that the weak mixed B-p and Zr-d states for ZrB₁₂ is crucial for the superconductivity due to the state population increased the DOS at the E_F.     

X-ray photoelectron spectra and structure of iron polynuclear complexes

Iron trimethylacetate complexes with different ligands have been investigated by X-ray photoelectron spectroscopy (XPS). The Fe 2p, Fe 3p, C 1s, O 1s, and N 1s spectra obtained at successive replacement of the Fe-O coordination bonds with Fe-N bonds have been analyzed. A satellite component is found in the spectra of iron trimethylacetate complexes, which is indicative of the high-spin state of Fe^II and Fe^III iron atoms. The XPS data made it possible to determine the degree of covalence of the metal-ligand bonding and reveal two nonequivalent states for iron atoms.     

Charge-transfer shake-up satellites accompanying core ionization in organic donor/acceptor molecules: bis(4-dimethylaminophenyl)squaraine and its derivatives

Solid state XPS spectra of bis(4-dimethylaminophenyl)squaraine and a number of derivatives are presented. The complex multipeak N1s and O1s satellite spectra are analyzed in terms of intra and inter-molecular contributions and related to corresponding electronic transitions of the neutral species. It is concluded that a distinctive low-energy O1s satellite characterizes the intermolecular excited state charge transfer between the donor and acceptor fragments on adjacent overlapping sites. A CNDO/S(S + DES CI) equivalent-core computation on O1s ionization of the parent monomer supports this interpretation. Spectral comparisons indicate that chemical substitution to yield intramolecular hydrogen bonding may provide a more favorable pathway for O1s core-hole screening, and thus inhibit intermolecular charge-transfer. The broad asymmetric C1s signals are shown to reflect a wide spread of chemically shifted carbon sites characteristic of a highly charged polarized monomer unit. The consequences of adsorbed O₂/H₂O on the spectral properties and electronic conductivity are discussed.     

XPS evidence for band bending at semiconducting oxide surfaces

The magnitude of the surface potential, directly influenced by defect properties and donor or acceptor chemisorption, has been studied using XPS for p-type nickel oxide surfaces. Above 200 W, the KE shifts are independent of X-ray intensity. A simple explanation of the dependence of the surface potential (V_ch) on band bending (V_s) is proposed with preliminary experimental evidence allowing derivation of values of V_s.     

Depth distribution of the fluorine concentration during radiative carbonization of PVDF

The XPS integral intensity of the F1s line and its satellite is measured during the long-term radiative carbonization of PVDF (polyvinylidene fluoride). A model is proposed that describes the effect of the fluorine depth distribution on the shape and intensity of the F1s spectra. A comparison of the experimental data with the model calculations provides estimates for the concentration inhomogeneity during the radiative carbonization of PVDF, for the photoelectron escape depth, and for the probability of a single energy loss by a photoelectron in its motion towards the surface. A technique determining the fluorine concentration is presented. It is based on the occurrence of chemical shifts of the C1s line towards larger bond energies for the carbon atoms chemically bonded to one or two fluorine atoms.     

Surface charging effects on valence band spectra in X-ray photoemission: Crystalline and Amorphous As2S3

In X-ray photoemission (XPS) studies on insulators, strong electric fields associated with surface charging can perturb the observed spectra. We find that the standard technique of flooding with thermal electrons to neutralize the net charge does not eliminate this effect for valence band spectra, but that the use of thin or (for photoconductors) illuminated samples does solve the problem. These conclusions are demonstrated by experiments revealing new structure in, and real differences between, the XPS valence band spectra of crystalline and amorphous As₂S₃.     

Gas phase and solid state X-ray photoelectron spectra of the substituted acetylenes (SF5)n C2 (CF3)2−n (n = 0, 1, 2): A comparison of the pentafluorosulfur and trifluoromethyl groups

The core level X-ray photoelectron spectra (XPS) of CF₃CCCF₃, CF₃CCSF₅ and SF₅CCSF₅ have been measured in the solid state. Gas phase spectra of CF₃CCCF₃ and CF₃CCSF₅ have also been obtained. The XPS data, interpreted with the point charge potential model and semiempirical MNDO (minimum neglect of differential overlap) molecular orbital calculations, indicate that the electron withdrawing effect of the ?CF₃ group is greater than that of the ?SF₅ group. Results further suggest that sulfur 3d orbitals do not play a detectable role in the bonding or charge distribution in these molecules. Carbon 1s linewidths of ?CF₃ carbon atoms are found to be much narrower than those arising from the acetylenic carbon atoms. The narrower lines correlate with the much higher binding energy of the ?CF₃ carbon atoms. Large shifts (nearly 1 eV) in heteroatom core level binding energy differences (for example, F 1s — C 1s) between the gas phase and solid state data are observed. These shifts are attributed to solid state effects (Madelung potential, intermolecular bonding interactions, and/or extramolecular relaxation contributions). From these comparisons it is clear that solid state effects are not uniform in their influence on the photoionized sites in these molecules.     

The electronic structure of SrTiO3 and some simple related oxides (MgO,Al2O3, SrO,TiO2)

The valence band density of states (VBDOS) of the insulating oxides SrTiO₃, TiO₂, SrO, MgO and Al₂O₃ obtained by X-ray photoelectron spectroscopy (XPS), are reported. Qualitatively, the VBDOS of these oxides are similar to one another. The XPS results are compared with results from soft X-ray emission spectroscopy (XES), ultraviolet photoemission spectroscopy (UPS), and theoretical calculations. There are some differences (in particular for TiO₂) between the XES and XPS results, which are probably due to matrix element effects enhancing different features of the VBDOS in the two techniques. The XPS results definitively establish the position of the O 2s level, which had been erroneously assigned in previous low-energy UPS measurements. Cluster-type calculations are demonstrated to give a reasonable representation of the VBDOS for the oxides.     

Some comments on the electronic properties of the surface space charge layer of copper phthalocyanine thin films

A simple analysis, using a theory of the surface space charge layer of semiconductors, of the published values of the work function φ and surface ionization energy Φ_s of copper phthalocyanine (CuPc) thin films was performed. Using a well known position of the Fermi level E_F within the band gap E_g the values of its absolute band bending eV_s and surface electron affinity X_s were determined. A small negative value of the absolute band bending eV_s = −0.17 ∓ 0.15 eV has been interpreted by the existence of the filled electronic surface states localized in the band gap below the Fermi level E_F. Such states were predicted theoretically for thin films and the crystalline surface of CuPc, and attributed to surface lattice defects of a high concentration.     

Nitrogen 1s electron binding energy assignment in carbon nitride thin films with different structures

Carbon nitride thin films deposited by dc unbalanced magnetron sputtering have been analyzed by high-resolution X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS data show that N 1s binding states depend on substrate temperature (T_s). By comparison with the Raman spectra, N 1s binding states are assigned in which nitrogen atoms are mainly bound to sp² and sp³ carbon atoms at T_s = 100°C, whereas at T_s = 500°C nitrogen atoms are mainly bonded to sp², sp³ and sp¹ carbon atoms.     

Electronic properties of the space charge layer of the copper phthalocyanine thin films

Photoemission Yield Spectroscopy (PYS) has been used in studies of the electronic properties of the space charge layer of UHV annealed copper phthalocyanine (CuPc) thin films. The work functionφ, ionization energyΦ, absolute band bendingeV _s and surface electron affinityX _s is determined. Moreover, two filled electronic state bands localized in the band gap are observed. Their origin is briefly discussed.     

Correlation of inner shell binding energies with shifts of charge transfer bands

Inner shell binding energies were determined for the elements in the crystalline compounds A^IIIX^VO₄ where A  B, Al, Ga, Fe and X  P, As with fourfold coordination of A, and in the corresponding dihydrates of Al having a sixfold coordination of Al. The O 1s binding energies for the dihydrates could be resolved into two bands arising from XO₄ -groups and from the water molecules respectively. The O 1s binding energies from the XO₄ groups decrease by about 0.6 eV in going from the anhydrous compounds to their dihydrates whereas the Al 2p binding energies remain constant within the limits of error. These results correlate surprisingly well with shifts of the first charge transfer band of trivalent iron in these compounds. The binding energy differences between phosphates and arsenates are also in good agreement with concurrent shifts of this charge transfer band. The consequences of these results for the concept of optical electronegativities are discussed.     

XPS study of the valence band spectrum of amorphous Fe-B alloys

XPS spectra of Fe 3s and Fe 3p levels and Fe 3d valence band of amorphous iron-boron alloys were measured as a function of boron concentration over the range from 12 to 25 at% B. It was found that a characteristic hump appeared at lower energy region in Fe 3d band spectrum of every amorphous Fe-B alloy. The origin of the hump was discussed in terms of the formation of a new bonding state between Fe and B atoms in amorphous Fe-B alloys.     

Inner shell excitation,valence excitation and core ionization in NF3 studied by electron energy-loss and X-ray photoelectron spectroscopies

Electron energy-loss Spectroscopy (EELS) at impact energies of 2.5–3 keV has been used to obtain the electron excitation spectra for the N 1s (K-shell), F 1s (K-shell) and valence shell regions of NF₃. The inner shell spectra were recorded using small angle scattering (?1° ) while the valence shell spectrum was obtained at zero degree scattering angle. The inner shell excitation spectra show a strongly enhanced 1s→ δ^* type transition and continuum features which are typical for molecules with highly electronegative ligands. One of the peaks in an earlier published photoabsorption study of the N 1s region has been shown to be due to a N₂ impurity. The valence shell electron energy-loss spectrum shows a number of transitions which are considered to be mainly due to valence-valence type transitions, with also some evidence of Rydberg structure.The X-ray photoelectron spectra (XPS) of the N 1s and F 1s electrons along with their associated satellite structures have also been recorded using Al Kα (1486.58 eV) radiation. The vertical ionization potentials for the N 1s and F 1s electrons were found to be 414.36 (10) eV and 693.24 (10) eV, respectively. Both spectra exhibit a rich and different satellite structure. These “shake-up” features in the satellite XPS spectra are compared with continuum features of the inner shell electron energy-loss spectra and also with the valence shell spectrum.