Oxidation of a potassium monolayer on Ru(001) studied with photoemission,NEXAFS and vibrational EELS

The interaction of oxygen with a monolayer of potassium on a Ru(001) surface has been investigated with photoemission and NEXAFS. O 1s core-level data exhibit for low exposures of oxygen a single peak at 531.8 eV. This indicates together with the earlier observation of KO bond formation by EELS that a single K_xO_y species with equivalent oxygen atoms is formed. NEXAFS data indicate a partially filled O π* orbital and a strongly shifted σ-resonance. Photoemission, vibrational and Auger data suggest an ionic species which is close to potassium Superoxide with the OO bond oriented parallel to the surface.     

Ionization and excitation of some atomic targets and metal oxides by electron impact

We have calculated total inelastic and total ionization cross-sections for collisions of electrons on atomic targets oxygen (O), aluminium (Al) and copper (Cu) and metal oxides AlO and Al₂O, at impact energies from near excitation threshold to 2000 eV. A complex (optical) energy-dependent interaction potential is used to derive total inelastic cross-sections resulting from ionization as well as excitation processes. The inelastic cross-sections are bifurcated into discrete and continuum contributions and total ionization cross-sections have been deduced therefrom. Our calculation also provides information, hitherto sparse, on the excitation processes in the atomic targets O, Al, Cu and metal oxides AlO, Al₂O. Adequate comparisons are made with other theoretical and experimental data.     

Core ionization potentials in dimethyl ether and methyl amine

The ionization potentials for core electrons in dimethyl ether and methyl amine have been measured. For carbon 1s electrons the values are 292.25(4) eV for the ether and 291.60(4) eV for the amine. The nitrogen 1s ionization potential is 405.15(4) eV; that for oxygen is 538.59(4) eV. The two ionization potentials for the ether are very close to those that have been reported for the related compound methanol. Those for methyl amine reflect the fact that nitrogen is more electro-negative than carbon. The carbon 1s ionization potential in methyl amine together with the equivalent cores approximation gives an estimate of 169 kcal/mole for the heat of formation of N₂H₅⁺. The corresponding proton affinity for hydrazine is 221 kcal/mole, very close to that of ammonia, 214 kcal/mole. The large shifts of nitrogen and oxygen core ionization potentials relative to N₂ and O₂ may be due to unusually low relaxation energies for the diatomic molecules rather than to high negative charges on the atoms in the respective compounds. Since different methods for estimating relaxation energies give results differing by as much as 6 eV, it is not possible to say which is the more important effect.     

Electronic structure of amino acids and ureas

The outer electronic levels of glycine, alanine, glycine ethyl ester, urea, and thiourea have been investigated by means of photoelectron spectroscopy using He I (584 Å) and He II (304 Å) radiation and CNDO/2 molecular orbital calculations. In the amino acids the molecular orbital (MO) ordering ha been found to be [σ core and carbonyl π > a″(O) a′(O) > a″(N)] with a first ionization potential of ～8.8 eV. Glycine and alanine are foun to exist as the undissociated amino carboxylic acids rather than zwitterions in the high temperature vapor. In urea the three lowest energy molecular o are near-degenerate [σ(4b₁) ～ π(1a₂) ～ π(2b₂)] while in thiourea only the two lowest energy MO's are near-degenerate [π(1a₂ σ(4b₁) ～ π(2b₂)]. The first ionization potentials of urea and thiourea are 9.7 and 7.9 eV respectively.     

On the possibility of single-electron transfer during alkaline hydrolysis of sulfophthalides

The previously observed generation of radical products during the alkaline hydrolysis of diphenylsulfophthalide (DPSP) and polydiphenylenesulfophthalide in dimethyl sulfoxide (DMSO) is indicative of the involvement of single-electron transfer (SET) in thses processes. The possibility of SET from the hydroxide ion (HI) to the sulfophthalide molecule is determined by the ratio between the ionization potential (IP) of the HI and electron affinity (EA) of the sulfophthalide. According to B3LYP/6-311+G(d,p) calculations for DPSP, EA _ver = 0.06 eV and EA _ad = 0.58 eV, whereas EA _eff = 2.13 eV (with consideration of the C-O bond rupture in the sulfophthalide cycle). Such a value of the electron affinity cannot ensure SET from the HI, the ionization potential of which in DMSO reaches ∼5.25 eV. The EA of the carbocation formed from DPSP is 6.44 eV as calculated in the same approximation. A mode of SET from the HI to the carbocation intermediate formed during DPSP heterolysis in DMSO is proposed. Two possible modifications of the electron donor are considered. The possibility of occurrence of SET from the dimsyl ion and HI-DMSO complex is evaluated using the G3B3 method. The ionization potential of the dimsyl ion in DMSO is almost 1 eV lower than the IP of the HI, which makes the former a preferential donor in comparison with the HI. The ionization potential of the weak HI-DMSO complex even exceeds the IP of the HI; i.e., complexation does not improve the electron-donor properties of the hydroxide ion.     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

Effects of alkyl side chains on properties of aliphatic amino acids probed using quantum chemical calculations

Effects of alkyl side chains (R‐) on the electronic structural properties of aliphatic amino acids are investigated using quantum mechanical approaches. The carbon (C 1s) binding energy spectra of the aliphatic amino acids are derived from the C 1s spectrum of glycine (the parent spectrum) by the addition of spectral peaks, depending on the alkyl side chains, appearing in the lower energy region IP < 290 eV (where IP is the ionization potential). The two glycyl parent spectral peaks of the amide 291.0 eV [C₍₂₎] and carboxylic 293.5 eV [C₍₁₎] C atoms are shifted in the aliphatic amino acids owing to perturbations depending on the size and structure of the alkyl chains. The pattern of the N 1s and O 1s spectra in glycine is retained in the spectra of the other amino acids with small shifts to lower energy, again depending on the alkyl side chain. The Hirshfeld charge analyses confirm the observations. The alkyl effects on the valence binding energy spectra of the amino acids are concentrated in the middle valence energy region of 12–16 eV, and hence this energy region of 12–16 eV is considered as the `fingerprint' of the alkyl side chains. Selected valence orbitals, either inside or outside of the alkyl fingerprint region, are presented using both density distributions and orbital momentum distributions, in order to understand the chemical bonding of the amino acids. It is also observed that the HOMO–LUMO energy gaps of the aliphatic amino acids are reduced with the growth of the alkyl side chain.     

Lattice relaxation and charge-transfer optical transitions due to self-trapped holes in nonstoichiometric LaMnO3 crystal

We explore the role of electronic and ionic polarization energies in the physics of “colossal” magnetoresistive (CMR) materials. We use the Mott-Littleton approach to evaluate polarization energies in the LaMnO₃ lattice associated with holes localized on both the Mn³⁺ cation and the O^2?anion. The full (electronic and ionic) lattice relaxation energy for a hole localized at the O site is estimated at 2.4 eV, which is appreciably greater than that of 0.8 eV for a hole localized at the Mn site, indicating a strong electron-phonon interaction in the former case. The ionic relaxation around the localized holes differs for anion and cation holes. The relaxation associated with Mn⁴⁺ is approximately isotropic, whereas ionic displacements around O^? holes show axial symmetry with the axis directed towards the apical oxygens. Using the Born-Haber cycle, we examine thermal and optical energies of the hole formation associated with the electron ionization from Mn³⁺, O^2?, and La³⁺ions in the LaMnO₃ lattice. For these calculations, we derive a phenomenological value for the second electron affinity of oxygen in the LaMnO₃ lattice by matching the optical energies of the La⁴⁺ and O^? hole formation with maxima of binding energies in the experimental photoemission spectra. The calculated thermal energies predict that the electronic hole is marginally more stable in the Mn⁴⁺ state in the LaMnO₃ host lattice, but the energy of a hole in the O^? state is only higher by a small amount, 0.75 eV, suggesting that both possibilities should be treated seriously. We examine the energies of a number of fundamental optical transitions, as well as those involving self-trapped holes of Mn⁴⁺ and O^? in the LaMnO₃ lattice. The reasonable agreement of our predicted energies, linewidths, and oscillator strengths with experimental data leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5 eV is associated with the O(2p)-Mn(3d) transition of a charge-transfer character, whereas the band near 2.3 eV is rather associated with the presence of Mn⁴⁺ and/or O^? self-trapped holes in the nonstoichiometric LaMnO₃ compound.     

Bi2Sr2CaCu2-xLixOy体系的光电子能谱研究

采用X射线光电子能谱对Bi₂Sr₂CaCu_2-xLi_xO_y体系在真空中获得的清洁表面进行了研究。结果表明:Li的掺入对Bi和Sr的化学键性质几乎没有影响,而Ca,Cu,O的化学键性质有较大的变化,其结合能随着Li含量x的增加向高能方向移动。同时对真空中获得的清洁表面和普通表面样品O1s进行了比较,结果发现真空中获得的清洁样品O1s有一个峰(528.4eV),而普通表面样品O1s有两个峰(528.4eV)和(531eV),这表明O1s的高能峰是由污染产生的。 关键词：     

Electron spectroscopic studies of clean and oxidized iron

X-ray photoelectron spectroscopy (XPS) and soft X-ray appearance potential spectroscopy (APS) together with Auger electron spectroscopy (AES) were used to study the electronic properties of clean and oxidized (Fe₃O₄) iron surfaces. The features arising from excitations of electrons from Fe 2p core levels are discussed consistently within the common one-electron picture (i.e. neglecting final state effects). For pure Fe the shape of the APS L₃ peak is evaluated taking into consideration the theoretical density of states above the Fermi level and is found to agree well with that observed. As a consequence it is shown that in this case the appearance potential is about 1 eV larger than the threshold energy for the excitation of a core electron to the Fermi level. Thus for $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ electrons this quantity results to be 704.8 eV from both XPS and APS techniques. Successive oxidation at 500°C leads to an increase of the appearance potentials of the Fe 2p levels by only 0.5 eV, whereas the positions of the corresponding XPS peaks are shifted by as much as 3.5 eV. However this apparent disagreement can be eliminated by taking into account the above mentioned effect concerning the appearance potentials from pure Fe and the fact that the threshold energies (which determine the appearance potentials) of the XPS signals are shifted only by 1.7 eV. This example demonstrates that considerable care has to be taken in discussing “binding energies” or “chemical shifts” as derived from different electron spectroscopic techniques. The observed splitting of the MVV Auger transition of Fe at 47 eV upon oxidation is interpreted in terms of the qualitative features of the valence band structure of Fe₃O₄ and ascribed to the participation of a cross-transition between O 2p and Fe 3p states.     

The adsorption of SO2 on iron surfaces studied by x-ray photoelectron spectroscopy

The interaction of SO₂ with evaporated iron surfaces in the temperature range 80–450 K was investigated by using X-ray photoelectron spectroscopy. At 300 K, SO₂ decomposed at the initial stage of the interaction and gave adsorbed S with the S2p peak at 161.9 eV and adsorbed O with the O1s at 530.0 eV. Further exposure of SO₂ gave adsorbed SO₄ with S2p at 166.8 eV O1s at 531.3 eV, being different in binding energies from ionic SO₄^2?. This indicates the two stage reaction Of SO₂ with iron surface; SO₂(gas) → S(ads) + 20(ads), SO₂(gas) + 2O(ads) → SO₄(ads). The first reaction did not occur at low temperature or in the presence of adsorbed O. The adsorbed SO₄ formed at 80 K showed a quantitative decomposition reaction into S(ads) and O(ads) in the temperature range 200–350 K.     

Ab initio calculation of the first ionization potential in linear alkanes using exciton theory

An ab initio calculation is used to give the form of the ionization potential in the alkane series and to explain the origin of the various contributions to this potential. The question of whether the ionization relates to a σ-type orbital, composed mainly of carbon atomic orbitals, or to a π-type, composed of CH bonds, is considered. The results show a gradual evolution from a CH ionization (13·26 eV) in CH₄ to a CC ionization (10·0 eV) at the farthest limit of the series.     

Cross sections for ionization and ionic fragmentation of pyrimidine molecules by electron collisions

The electron impact mass spectroscopy and the total ion collection measurements were used to investigate the ionization and ionic fragmentation of the pyrimidine, C₄H₄N₂, molecules in the gas phase. The cation mass spectra were measured in the 10−85 amu range and the observed mass peaks assigned to corresponding ionic fragments. The most abundant cation in the mass spectra is the parent cation, C₄H₄N \hbox{₂⁺_{\mathrm{2}}^{\mathrm{+}}} + 2 , at 80 amu. The appearance energies of the ionic fragments were determined and the possible fragmentation processes are discussed. The electron impact absolute total and partial ionization cross sections in pyrimidine were obtained over the energy range from the respective ionization thresholds up to 150 eV.     

XF3 (X=N,P,As)价层电离势的运动方程耦合团簇理论计算

采用运动方程单双取代耦合团簇理论(EOM-CCSD)对XF₃(X=N,P,As)的价层垂直离势(VIP)进行了系统计算,同时对称匹配团簇组态相互作用(SAC-CI)、外价层格林函数(OVGF)以及部分三阶近似(P3)方法也被应用到目前计算.与已有的实验结果比较表明:EOM-CCSD计算的价层垂直电离势整体上与SAC-CI结果相近,而优于OVGF和P3理论结果,在整个价层上,EOM-CCSD结果与实验值总体差距约0.2 eV, 在外价层这种差距相对较小,在内 关键词： XF₃(X=N;P;As)')" href="#">XF₃(X=N;P;As) 垂直电离势 运动方程耦合团簇理论     

Near-threshold electron-impact ionization of calcium atoms

The electron-impact ionization of calcium atoms is studied in the near-threshold energy range (from 6.11 to 16 eV). Experiments were performed by the method of intersecting electron and atomic beams with the recording of formed positive calcium ions. The electron beam (ΔE _1/2 = 0.15 eV) was formed using a hypocycloidal electron monochromator. An analysis of the specific features of ionization cross sections revealed a contribution from the excitation and decay of low-lying autoionization atomic states, which converge to the excitation thresholds of the 3d, 4p, and 5s ionic levels, and resonances (long-lived states of negative ions). The specific features of cross sections are identified using the experimental and theoretical data on photoionization (photoabsorption).     

The photoelectron and absorption spectra of allyl halides

The ultraviolet photoelectron spectra (5–20 eV) absorption spectra (1400–2500 Å) of allyl C₃H₅X (X = F, Cl, Br, I) have been measured. The ionisation potentials between 5–16 eV are determined and the type of orbital is interpreted with the aid of the vibration pattern and the band form.From these spectra several conclusions can be drawn concerning the iodide: (1) the ionisation potential of the π_C_C system of allyl iodide is lower than expected when compared to the ionisation potential of the corresponding band of the other halides. (2) The splitting of its np lone pair photoelectron bands is not in accordance with the expected splitting associated with spin-orbit coupling. (3) One of the two components of the lone pair bands has been broadened. These observations can be explained by a through-space interaction of one of the iodine “lone pairs” with the π-system.     

Enhanced optical nonlinearity based on silicon ring

The photoelectron spectrum of ethylene is studied using coupled cluster methods, including an existing ambiguity in what are reported to be its experimental vertical ionization potentals. Two complementary methods are used for generating the ionization potentials: δE CCSD(T) and IP-EOM-CCSD. The adiabatic IP of the neutral molecule in the ground state is well known and widely accepted to be 10.5122eV. The basis set extrapolated adiabatic IPS with zero-point corrections are 10.46 eV and 10.56 eV, respectively, but a vibronic coupling between the ground state cation and its first excited state can reduce these values by ～0.03 eV. From an exponential basis set extrapolation the vertical ionization potentials are predicted to be 10.8 eV (B_3u, 13.2eV (B_3g 15.0eV (A_g), 16.4eV (B_2u), and 19.6eV (B_1u) ±0.1 eV.     

Fabrication of large-scale ultra-smooth metal surfaces by a replica technique

The IR double-resonance techniques IR/R2PI (infrared/resonant 2-photon ionization), IR/PIRI (infrared-photo-induced Rydberg ionization) and IR-photodissociation spectroscopy are valuable tools to investigate structure, vibrations, and dynamical processes of neutral and ionic hydrogen-bonded clusters containing aromatic molecules. In this paper we report on the application of the IR double-resonance techniques to determine the NH and OH stretching vibrations of 4-aminophenol and 4-aminophenol(H₂O)₁, both in the neutral (S₀) and ionic (D₀) ground state. All vibrational frequencies obtained for 4-aminophenol and the cluster are compared with the values obtained from ab initio and DFT calculations. In the S₀ state, a trans-linear arrangement of 4-aminophenol(H₂O)₁ is obtained containing an O-H·· O hydrogen bond. In the D₀ state an overlay of two spectra can be observed resulting from the trans-linear structure and a second structure which contains a N-H·· O hydrogen bond. The observation of these two structures within the ion is an interesting example of a rearrangement reaction in the ionic state. Received: 3 November 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

The interaction of Cs and O2 on the basal plane of MoS2

The interaction of Cs and O₂ on MoS₂(0001) has been studied both in the alternate adsorption and the codeposition mode by LEED, AES, TDS and WF measurements at 170 and 300 K. Oxygen does not interact with Cs when θ_Cs?0.04 at 300 K or θ_Cs?0.08 at 170 K, where Cs is known to adsorb as strongly ionized, individual adatoms. The interaction at higher θ_Cs, where Cs is known to form clusters on MoS₂(0001), leads to clusters of a Cs/O complex characterized by a Cs(563 eV)/O(512 eV) Auger peak ratio of 1.1–1.3. The minimum WF is 2.1 eV at 300 and 170 K upon alternate adsorption, and 1.7 eV at both T upon codeposition. Upon heating, oxygen and Cs desorb independently, as no oxide desorption is observed. The Cs TDS spectrum is shifted to lower T in the presence of oxygen and a new desorption peak appears at ～ 880 K. The differences in the Cs/O interaction between MoS₂(0001) and other semiconductors and metals are attributed to the Cs clustering and the inertness of MoS₂(0001) to O₂ adsorption.     

多晶镍表面上氧的电子诱导脱附研究

长期暴露于大气的多晶镍样品,经700℃2小时真空退火及常温和500℃暴露氧20—60L后,发现其表面将产生两种电子诱导脱附(ESD)氧正离子。所对应的电子能量阈值分别为29eV和23eV。我们认为,两种氧正离子的产生机制为俄歇诱导脱附。29eV阈能者对应于镍表面化学吸附氧的2s能级电子向真空中电离;而23eV阈能者则对应于表面层中未与镍化合的自由氧的2s电子向体内费密能级跃迁。 关键词：