Studies of molecular oxygen adsorbed on Cu surfaces

Molecular oxygen adsorbed on (110) and polycrystalline Cu surfaces has been investigated by UPS, XPS, AES, HREELS and LEED. Molecularly adsorbed O₂ on the (110) surface shows the characteristic three-peak He II spectrum due to π_g, π_u and σ_g orbitals, accompanied by an O-O stretching frequency at 660 cm⁻¹. On the polycrystalline Cu surface, adsorbed O₂ shows the three peak He II spectra with a considerably smaller separation between the π_u and σ_g band and two O-O stretching bands at 610 and 880 cm⁻¹. O₂ adsorbed on the Cu(110) surface gives rise to a (1 × 1) LEED pattern and characteristic K π¹π¹ transition in the Auger spectrum.     

Photoelectron spectra of adsorbed carbonates

In order to provide photoelectron spectra for analytical purposes, X-ray and ultraviolet photoelectron spectra for known surface species have been measured. The surface carbonate on Ag(110), formed by reacting gaseous CO₂ with pre-adsorbed atomic oxygen, yields C 1s and O 1s peaks at 287.7 and 529.9 eV, respectively. The UPS results show features due to surface CO₃ which scale well with those for bulk carbonates. These observations, coupled with the increase in work function that accompanies carbonate formation, suggest strongly that this surface species is negatively charged.     

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.     

Ultraviolet photoemission studies of chemisorption and point defect formation on ZnO nonpolar surfaces

Ultraviolet photoemission (He I and He II) studies are reported on point defect formation as well as CO₂ and O₂ interaction with ZnO(101̄0) under thermodynamically well defined conditions. Physisorption spectra can be correlated with UPS spectra in the gas phase if a uniform relaxation/polarization shift is taken into account. CO₂ chemisorption may be characterized by formation of carbonate like surface complexes. Small concentrations of intrinsic point defects lead to significant changes in the surface electronic structure and provide specific CO₂ chemisorption sites which are intermediates during heterogenous oxidation of CO on ZnO.     

Reactivity of carbon dioxide at nickel (110)

Adsorption and reactivity of carbon dioxide at the clean and oxygen precovered Ni(110) surface has been studied by means of EELS and LEED. On the clean surface two different types of CO₂ molecules have been observed by EELS at 135 K, one being the undisturbed linear configuration. With increasing temperature the linear molecule changes into a different species which precedes dissociation at 220 K into CO and O. EELS and LEED data of the intermediate species support the assumption that it is a bent CO⁻₂ anion adsorbed in C_2v symmetry with twofold oxygen coordination to the surface. Oxygen preadsorption stabilizes the linear CO₂ molecule up to higher temperatures which does not convert into a bent species in this case. Instead, a reaction product of CO₂ and O is found and interpreted as a carbonate species.     

异核双原子和三原子分子在红外强激光场中的电离抑制

Ionization is the fundamental process in interaction of atoms/molecules with femtosecond strong laser fields. Comparing to atoms, molecules exhibit peculiar behaviors in strong-field ionization because of their diverse geometric structures, molecular electronic orbitals as well as extra nuclear degrees of freedom. In this study, we investigate strong field single and double ionization of carbon monoxide (CO) and carbon dioxide (CO₂) in linearly polarized 50-fs, 800-nm laser fields with peak intensity in the range of 2×10¹³ W/cm² to 2×10¹⁴ W/cm² using time-of-flight mass spectrometer. By comparing the ionization yields with that of the companion atom krypton (Kr), which has similar ionization potential to the molecules, we investigate the effect of molecular electronic orbitals on the strong-field ionization. The results show that comparing to Kr, no significant suppression is observed in single ionization of both molecules and in non-sequential double ionization (NSDI) of CO, while the NSDI probability of CO₂ is strongly suppressed. Based on our results and previous studies on homonuclear diatomic molecules (N₂ and O₂), the mechanism of different suppression effect is discussed. It is indicated that the different structure of the highest occupied molecular orbitals of CO and CO₂ leads to distinct behaviors in two-center interference by the electronic wave-packet and angular distributions of the ionized electrons, resulting in different suppression effect in strong-field ionization.     

Satellites in ESCA inner-shell spectra of 3d0 transition metal complexes

The occurrence of satellites to inner shell ESCA spectral lines is discussed for ions in the beginning of the transition metal series. It is found that the satellites correspond to e′_g → e″_g and t′_2g → t″_2g shake-up transitions, where e″_g and t″ _2g have mainly metal ion 3d-character. The intensity is found to increase with relaxation charge transfer and to decrease with the 3d occupancy in the neutral ground state.     

A numerical study of the convergency of second and approximate second-order multiconfiguration Hartree-Fock procedures

A detailed numerical study has been made of the convergency of second-and approximate second-order multiconfigurational Hartree-Fock procedures. Calculations were performed on the excited 2p ^{2 1} S state of Be and on the lowest states of ³Σ _g ^-, ¹δ _g , ¹Σ _g ⁺, ¹Δ _g and ³Δ _g symmetry in O₂. The O₂ calculations included all configurations that could be formed from doubly occupied core orbitals with eight electrons in the valence orbitals, 3σ _g , 1π _u , 1π _g and 3σ _u . All second-order calculations converged in between 4 and 6 iterations even for a case where approximate second-order procedures did not converge.     

Time-resolved EPR and ODMR studies on the lowest excited triplet states of α-silyl and α-germyl ketones

Sublevel properties of the lowest excited triplet (T¹) nπ* states of α-silyl and α-germyl ketones were examined by means of ODMR, time-resolved EPR and optical spectroscopy. The EPR parameters, D, E and g, population ratios, and triplet lifetimes were obtained. The D value and the triplet lifetime varied among the molecules. In contrast the E value and population ratio remains nearly the same. These properties together with their solvent dependence and emission properties are interpreted in terms of spin-orbit couplings between T₁ (nπ*) and higher S₁ (nπ*), T₂ (ππ*) and S₂ (ππ*) states. An origin of the remarkable red-shifts of ^1,3 (π*) are discussed based on a model of delocalized n, π and π* electrons over the Si and Ge atoms. This model is also consistent with all the triplet properties obtained.     

C2H221分子转动角动量定向分布(Orientation)及其碰撞弛豫和转移

利用圆偏振激光受激Raman抽运,以 C₂H₂分子为样品选择性地制备了它的电子基态单一转动态(X¹Σ⁺_g,ν″₂＝1,J″的角动量定向布居(orientation)．并从圆偏振紫外激光诱导的A¹A_u(ν′₃＝1)←X¹Σ⁺_g(ν″₂＝1)的荧光（谱）,直接测定了 C₂H₂(X¹Σ^＋_g,ν″₂＝1,J″＝4,7,8,…,13）的角动量定向布居值．从时间分辨的荧光信号谱测定了角动量定向布居的碰撞弛豫速率常数,同时还研究了由各初始激励的转动态向其他邻近转动态碰撞诱导的角动量定向布居转移． 关键词：     

Application of a cw quantum cascade laser CO2 analyser to catalytic oxidation reaction monitoring

Catalytic oxidation reaction monitoring has been performed for the first time with a trace gas carbon dioxide analyser based on a continuous wave (cw), thermoelectrically cooled (TEC), distributed feedback (DFB) quantum cascade laser (QCL) operating at around 2307 cm^?1. The reaction kinetics for carbon monoxide oxidation over a platinum catalyst supported on yttria-stabilised zirconia were followed by the QCL CO₂ analyser and showed that it is a powerful new tool for measuring low reaction rates associated with low surface area model catalysts operating at atmospheric pressures. A detection limit was determined of 40 ppb (1 standard deviation) for a 0.1 s average and a residual absorption standard deviation of 1.9×10^?4.     

Threshold energy electron impact excitation of carbon dioxide and carbon disulphide

The threshold energy electron impact excitation spectra of CO₂ and CS₂ have been studied using the sulfur hexafluoride scavenger technique. The main results are triplet state excitation and autoionisation of negative ions associated with resonant excited states of the molecules. This confirms previous data concerning diatomic molecules. Furthermore, transitions such as ¹Π_g?X¹Σ_g⁺ and ¹Π_u?X ¹Σ_g⁺ are only weakly induced by low energy electrons, while the corresponding triplet excitations are probably more easily produced. Structures at 5.6, 6.1 and 6.6 eV observed in CS₂ are due to negative ions and/or to ³Π_u, ³Π_g excitation.The autoionisation of CO₂^?(X²Π_u) proceeds also by ejection of a thermal energy electron and leads to highly excited vibrational levels (3–5 eV) of the ground electronic state of CO₂.     

CO<Subscript>2</Subscript> activation on single crystal based ceria and magnesia/ceria model catalysts

Novel multifunctional ceria based materials may show an improved performance in catalytic processes involving CO₂ activation and reforming of hydrocarbons. Towards a more detailed understanding of the underlying surface chemistry, we have investigated CO₂ activation on single crystal based ceria and magnesia/ceria model catalysts. All model systems are prepared starting from well-ordered and fully stoichiometric CeO₂(111) films on a Cu(111) substrate. Samples with different structure, oxidation state and compositions are generated, including CeO_2-x/Cu(111) (reduced), MgO/CeO_2-x/Cu(111) (reduced), mixed MgO-CeO₂/Cu(111) (stoichiometric), and mixed MgO-CeO_2-x/Cu(111) (reduced). The morphology of the model surfaces is characterized by means of scanning tunneling microscopy (STM), whereas the electronic structure and reactivity is probed by X-ray photoelectron spectroscopy (XPS). The experimental approach allows us to compare the reactivity of samples containing different types of Ce³⁺, Ce⁴⁺, and Mg²⁺ ions towards CO₂ at a sample temperature of 300 K. Briefly, we detect the formation of two CO₂-derived species, namely carbonate (CO₃ ^2-) and carboxylate (CO₂ ^-) groups, on the surfaces of all investigated samples after exposure to CO₂ at 300 K. In parallel to formation of the carbonate species, slow partial reoxidation of reduced CeO_2-x/Cu(111) occurs at large doses of CO₂. The reoxidation of the reduced ceria is largely suppressed on MgO-containing samples. The tendency for reoxidation of Ce³⁺ to Ce⁴⁺ by CO₂ decreases with increasing degree of intermixing between MgO and CeO_2-x. Additionally, we have studied the stability of the formed carbonate species as a function of annealing temperature.     

CO-H2-O2 reaction on a catalytic surface: A computer simulation study

The oxidation of carbon monoxide to form carbon dioxide and the oxidation of hydrogen to form water are the reactions of environmental and industrial importance. These two reactions have been studied independently by Monte Carlo computer simulation using Langmuir-Hinshelwood mechanism but no effort has been made to study the combined CO-H₂-O₂ reaction on these lines. Keeping in view the importance of this 3-component system, the surface coverages and production rates are studied as a function of CO partial pressure for different ratios of H₂ and O₂. The diffusion of reacting species on the surface as well as their desorption from the surface is also introduced to include temperature effects. The phase diagrams of the system are drawn to observe the behavior of these atoms/molecules on the surface and the production of CO₂ and H₂O are determined at different concentrations of H₂. The results are compared with 2-component systems.     

Rydberg states of carbon dioxide and carbon disulfide

The electronic absorption spectra of carbon dioxide and carbon disulfide have been reexamined. Model potential calculations have been used to calculate the energies of excited states in Rydberg approximation, and (npσ) and (npπ) Rydberg series have been assigned. For both molecules, the lowest excited ¹Π_g and ¹Π_u states are identified as Rydberg states. The lowest ${}^1\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ state is mainly Rydberg for CO₂, but contains some valence character for CS₂, There is no evidence for transitions to additional valence states of these symmetries.It is shown that $\text{LCAO}\text{MO}$ predictions about excited states can be misleading because of near-linear dependencies which arise in multicenter expansions. A consideration of the united atom orbitals for CO₂ and CS₂ predicts that there should be only the number of low-energy excited states which are found from the spectral analysis.     

Ab initio MRD-CI investigation of the electronic spectrum of glyoxal (CHO)2

Multireference configuration interaction (MRD-CI) calculations are used to compute the electronic spectrum of glyoxal (CHO)₂, a key species in atmospheric chemistry. The calculations place the first dipole-allowed transition 1¹A_u←X¹A_g at 2.80?eV (442.8?nm) with an oscillator strength of 0.0002 and the dipole forbidden 1¹B_g?←?X¹A_g transition at 4.22?eV (293.8?nm), in accordance with prior experiments. In addition, a much stronger transition (3¹B_u?←?X¹A_g) at 8.51?eV (145.7?nm) is predicted, which has not yet been reported in the literature. This transition corresponds to 1b_g?→?2a_u excitation and can be characterized as π(CO)?→?π?(CO) type in accordance with the computed relatively large oscillator strength f?=?0.38. The corresponding triplet states are also computed.     

On the bonding and reactivity of CO2 on metal surfaces

We present ab-initio valence-bond calculations on free and coordinated CO₂. By analogy to transition-metal complexes with coordinated CO₂ three different coordination geometries for the CO₂ molecule are considered: (a) pure carbon coordination, (b) pure oxygen coordination, (c) mixed carbon-oxygen coordination. It is shown that pure oxygen ((b) geometry) and mixed carbon-oxygen coordination ((c) geometry) are more favourable than pure carbon coordination. In all cases studied, the bonding between the CO₂ moiety and the metal atom is described best as a CO₂⁻ anion interacting with a Ni cation. On the basis of the theoretical calculations, many observed and expected features (some already known and some predicted) of the interaction of CO₂ and metal surfaces can be discussed. The electron transfer to the CO₂ moiety drives the observed bent geometry of the coordinated CO₂ molecule and is accompanied by an elongation of the CO bond distance with respect to the free molecule. The bond elongation leads to a drastic lowering of the asymmetric CO stretching frequency, and a change in the relative energy position of the photoelectron peaks. We also consider intermolecular interaction between the CO₂⁻ anion and surrounding neutral CO₂ molecules via “solvation” in analogy to results of recent gas-phase cluster experiments. On the basis of the deduced metal-CO₂ bonding scheme the reactivity of coordinated CO₂ is investigated. Three reaction channels are considered: (a) dissociation into CO and O⁻, (b) oxidation to CO₃⁻ and CO₃²⁻, (c) disproportionation of CO₂⁻ + CO₂ to CO₃⁻ and CO. On the basis of energetic considerations we argue that dissociation is likely to occur on transition-metal surfaces, while oxidation to carbonate species is more likely on noble metals due to the low binding energies for the dissociation products, namely oxygen and CO on these surfaces.     

In-beam study of74Br and a first cascade in75Kr

An in-beam study of⁷⁴Br has been carried out using γ-ray spectroscopy in connection with the reactions (³He,p2in) and (d,2n) on⁷⁴Se targets. Separate level schemes on top of the two β-decaying isomers of⁷⁴Br are proposed. Two cascades connecting highspin states are assigned to populate theI ^π=4⁽⁺⁾ isomer. In this level scheme, the excited states at 72.1, 188.3 and 472.0 keV are found to have half-lives of 13.3, 0.7 and 0.7 ns, respectively. The low-spin isomer of⁷⁴Br is tentatively assigned asI ^π=(0^?). This state is populated by two other cascades. For the low-spin levels two-particle configurations of proton and neutron Nilsson orbitals are proposed. The high-spin states are tentatively interpreted as arising from the configurations (πg _9/2,νg _9/2) and (πg _3/2,νg _9/2). As a by-product, a cascade of four transitions has been observed during the³He bombardment of⁷⁴Se and tentatively assigned to negative-parity states in⁷⁵Kr.     

Shifts of g Values in the Excited Triplet States of Metal Complexes Studied by Time-Resolved W-band EPR

A highly time-resolved high-frequency/high-field W-band electron paramagnetic resonance (EPR) (ν ~ 94 GHz) is a powerful technique to determine small g anisotropies of transient paramagnetic species. We applied this method to studies of the lowest excited triplet (T₁)³ ππ* states in metal complexes such as a platinum (Pt) diimine complex and metal (Zn and Mg) porphines in rigid glasses. From the analyses of time-resolved EPR spectra, g anisotropies were obtained as g _z  = 2.0048, g _x  = g _y  = 2.0035 for Pt(b-iq)(CN)₂ (b-iq = 3,3′bi-isoquinoline) and g _z  = 1.9968, g _x  = g _y  = 2.0022 for zinc tetraphenylporphine (ZnTPP). No measurable anisotropies were found for magnesium (Mg) TPP. The g values of the Pt complex are larger than g _e (=2.0023, g value of free electron) and that g _z of ZnTPP is smaller than g _e. These results were interpreted in terms of the nature of the perturbed states: the higher triplet ππ′* state mixes with T₁(ππ*) via spin–orbit coupling in ZnTPP. In contrast, the higher triplet dπ* state is involved in this coupling for the Pt complex. Thus, the nature of the perturbed state can be distinguished from the anisotropic g values of the T₁(ππ*) state.