Co chemisorption on PtCu surfaces II. (1 × 1)-CO/Pt0.98Cu0.02(110)

The chemisorption of CO on the Pt atoms of an initially (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface at ~ 373 K can lead to the formation of a (1 × 1) surface. Comparisons are made with (1 × 3)-CO surfaces formed by CO exposures at 293 or 155 K. Thermal desorption shows that the (1 × 1)-CO surface has an enhanced population of high temperature CO peak ( ~ 543 K) from Pt sites. The CO-induced structural conversion also leads to a decrease in the subsequent CO uptake on the low temperature Pt sites and on the Pt-Cu “mixed” sites, with a concomitant increase in adsorption on the Cu-like sites. Such a reduction in the number of the Pt-Cu “ mixed” sites is also reflected in the CO-induced changes of the Cu 3d-derived states and the $\text{Cu 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core levels. A dynamic interplay between chemisorption and surface structure is thus demonstrated.     

A numerical estimate of deviations from the exponential decay law for the transition 2P12→1S12 in hydrogen

The deviations from the exponential decay law of the $\text{2}\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ states of the Dirac hydrogen atom with respect to the transition $\text{2}\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{→1}\text{S}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ are numerically estimated. We find |a₀(t) ? exp(?λt)|?2.5 × 10^?4 for all t, where a₀(t) is the “exact” decay amplitude and λ is a complex constant such that (2Re λ)^?1 is the “natural lifetime” of the $\text{2}\text{P}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ states with respect to the spontaneous transition to $\text{1}\text{S}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$.     

87Rb NMR lineshape study of the incommensurate phase in Rb2ZnBr4

On going from the paraelectric (P) to the incommensurate (I) phase the ⁸⁷Rb $\text{1}\text{2}$ → $\text{1}\text{2}$ NMR lines in Rb₂ZnBr₄ broaden into a continuum which is limited by two edge singularities. The observed line shape and its temperature dependence are well described by the frequency distribution function predicted by the “plane wave” modulation model of the I phase. The anomalous temperature dependence of T₁ shows the presence of the phason branch in the I phase.     

Interaction of CO,CO2 and O2 with nonpolar,stepped and polar Zn surfaces of ZnO

The nonpolar (10$\text{1}$0), stepped (40$\text{4}$1) and (50$\text{5}$1), and the polar (0001) surfaces of ZnO were prepared. Stable unreconstructed nonpolar and stepped surfaces were obtained. LEED analyses showed that the step height and the step width of the stepped surfaces were similar to the theoretical values. The polar surface showed a 1 × 1 LEED pattern of six-fold symmetry after annealing at 500°C, and evidence of a more complicated pattern at 300–400°C. Temperature programmed desorption of CO resulted in the desorption of CO from the stepped and the polar surfaces. However, desorption of CO₂ was observed from the stoichiometric nonpolar surface, and no desorption from the reduced nonpolar surface. CO₂ was also observed by interacting CO with all surfaces at elevated temperatures. A total of four temperature programmed desorption peaks of CO₂, α, β, γ, and δ were observed. The α and β peaks were observed on the nonpolar and the stepped surfaces, and the γ peak was observed on the polar surface. The α peak was assigned to adsorption on a surface ZnO pair, and the β peak was assigned to adsorption on an anion vacancy or a step. While adsorbed water enhanced the β, preadsorbed methanol reduced it. O₂ adsorption was similar on the nonpolar and the stepped surfaces, but was weak on the polar surface.     

Microwave spectrum of 2-propene-1-imine,CH2CHCHNH

The reactive species, 2-propene-1-imine, has been identified by its microwave spectrum as a pyrolysis product of diallylamine vapor (100 mTorr, 400°C). Two entirely planar forms are observed, both with an “s-trans” CCCN configuration. The lower energy rotamer has an “anti” CCNH configuration, with $\text{r}\text{CC = 1.453(3)}\text{A}\text{?}$, $\text{r}\text{CC = 1.336(4)}\text{A}\text{?}$, and ∠ CCC = 122.9(3)° and a dipole moment of 2.01(2) D with 1.13(1) D and 1.66(2) D “a” and “b” components. The higher energy rotamer has a “syn” CCNH configuration and a dipole moment of 2.51(2) D with 2.39(2) and 0.77(3) D the “a” and “b” components. From relative intensity measurements, the ground state energy difference is determined to be 0.9 ± 0.1 kcal mole^?1.     

Why are formaldehyde and (possibly) propynal nonplanar in their S1(π1, n) electronic states?

Abnormally low frequencies observed for the out-of-plane vibration (b₁) of the $\text{A}\text{?}{}^1\text{A}{}_2$ electronic state of formaldehyde (H₂CO) and for the analogous carbonyl hydrogen vibration (a″) of $\text{A}\text{?}{}^1\text{A″}$ propynal (HCCCHO) are modeled by means of two-state calculations of vibronic coupling with higher singlet states, ¹B₂ and ¹A′, respectively. In each case, the active vibration is the out-of-plane hydrogen motion. The same vibronic calculations reproduce also the large positive anharmonicities of the active vibrations in the $\text{A}\text{?}\text{(π}{}^1\text{, n)}$ states; for H₂CO the calculated vibrational spacing alternates as observed, consistent with the known nonplanar structure, while in propynal the calculated spacing increases regularly, thus predicting an effectively planar structure. The nonplanarity of H₂CO is caused mainly by a vibronic coupling constant nearly twice that of propynal. The H₂CO coupling constant is near the value estimated independently by means of the intensity “borrowed” by the S₁-S₀ transition from the much stronger S₂-S₀ transition. Brief consideration is given to analogous vibrational levels of the ¹A₂ state of H₂CS and of the ³A₂ state of D₂CO in the vibronic context of this paper.     

Successive Jahn-Teller transitions and the canted pseudospin state in K2PbCu(NO2)6

The successive phase transitions in K₂PbCu(NO₂)₆ has been studied by precise X-ray measurements. The unit cell in Phase III is doubled along each of the cubic principal axes. The experimental results can be interpreted as sequencial cooperative Jahn-Teller transitions where the higher transition is due to the coupling with the uniform distortion, while the lower transition is caused by the coupling with the uniform distortion, while the lower transition is caused by the coupling with the zone boundary phonon mode with $\text{k}{}_0\text{= (}\text{1}\text{2}\text{,}\text{1}\text{2}\text{,}\text{1}\text{2}$. The electronic state in Phase III is characterized as the “canted pseudospin” state.     

Pseudoscalar mesons in the c-c and b-b systems

It is shown that the pseudoscalar η_c and “η_c” (η_b and “η_b”) with M ≈ 2.80 and 3.51 GeV (9.17 and 9.88 GeV) can be predicted by using the radial mixing model, where the “η_c” and “η_b” are the excited 0^- states in the $\text{c-}\text{c}$ and $\text{b-}\text{b}$ systems, respectively, and the former would correspond to the observed 0^- meson with M ≈ 3.45 GeV in charmonium.     

Dynamical basis for spin-12 as well as generation stability of leptons and quarks

A three-way preon interaction with a spin-dependence of the form σ_μν⁽¹⁾ σ_νγ⁽²⁾ σ_λμ⁽³⁾, is proposed for a two-flavor (t,v) preon model, within a Bethe-Salpeter framework. This mechanism not only eliminates all $\text{S =}\text{3}\text{2}$ states but severely restricts the spatial symmetries of allowed $\text{S =}\text{1}\text{2}$ states to at most threedistinct “generations” with very strong selection rules forbidding transitions among them.     

Infrared absorption studies in the vapor phase of CF3Br: Spectra between 1800 and 400 cm−1

An extensive investigation of the ir spectrum in the gas phase of CF₃Br was made under medium resolution in the range 1800 to 400 cm^?1. The results of this study enabled an unambiguous and self-consistent assignment of a high number of fundamentals, overtones, combinations, and “hot” bands absorbing in different regions to be made. The vibrational origin of the low-lying ν₃ fundamental was evaluated by applying the Ritz Combination Principle; for bands where this mode is somehow involved the ${}^{\mathrm{<mtext>79</mtext><mtext>81</mtext>}}\text{Br}$ isotopic shift was observed. Molecular parameters obtained from the rotational structure of the perpendiculars ν₅ and ν₂ + ν₅ are also presented.     

CO chemisorption on PtCu surfaces I. CO on (1 × 3) Pt0.98Cu0.02(110)

Thermal desorption and photoemission spectroscopy (PES) have been used to investigate the chemisorption of CO on an annealed Pt_0.98Cu_0.02(110) surface. The clean surface shows 9.1 ± 2.6% Cu within the top 4 Å, and is (1 × 3) reconstructed. Thermal desorption of CO has revealed the existence of various adsorption states with these respective heats of adsorption: (α) 35.2 to 37.8 kcal/mol and (β) 24.5 to 26.3 kcal/mol on Pt sites, (γ) 16.0 to 17.2 kcal/mol on PtCu “mixed” sited, and (δ) 12.9 to 13.9 kcal/mol on Cu sites. PES observation of Cu 3d-derived states (using hv = 150 eV) and the $\text{Cu 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core levels (using Mg Kα radiation) shows that the electronic structure of the Cu constituent is changed only when CO adsorbs on the Pt-Cu “mixed” sites or the Cu sites. Furthermore, the CO states associated with Pt sites reflect the structural difference between the (1 × 3) alloy surface and the (1 × 2) pure Pt(110) surface: α-CO on the alloy surface desorbs at a temperature 17 to 21 K. higher than the maximum desorption temperature of CO from pure Pt(110), and the ratio of β-CO to α-CO desorption from the alloy surface is larger than the ratio of low temperature to high temperature peaks in the desorption of CO from pure Pt(110).     

Chlorine K X-ray absorption-edge structures of CIS-[Pt(NH3)2Cl2], trans-[Pt(NH3)2Cl2], trans-[Pd(NH3)2Cl2] and (NH4)2PdCl4

The K absorption-edge spectra of the ligand chlorine ion in square-planar complex compounds cis- and trans-[Pt(NH₃)₂Cl₂], trans-[Pd(NH₃)₂Cl₂], and (NH₄)₂PdCl₄ are reported and discussed in connection with the chlorine K absorption spectra of K₂PtCl₄ and K₂PdCl₄, reported previously. The observed chemical shift of a white line at the absorption threshold is interpreted in terms of the difference of the ligand-field splitting of electronic states for metal ions. The white line is attributed to the electronic transition from the Cl^? ls level to the lowest unoccupied antibonding molecular orbital (MO), which is specified by a $\text{MO}\text{b}{}_{\mathrm{1g}}\text{(σ}{}^1\text{)}$ in the square-planar complex with D_4h symmetry. The other absorption structures are regarded as continuum “shape resonances” of the outgoing electron trapped by the cage of the surrounding atoms. The effect of geometrical isomerism is found in the chlorine K absorption spectra of cis- and trans-[Pt(NH₃)₂Cl₂].     

In-out semi-classical gravity and 1N quantum gravity

Classical gravity with higher derivatives, when coupled to the in-out matrix element of T_μν: (a) appears to be free from instabilities of the in-in theory; (b) may be considered as the “underlying classical theory” to Tomboulis' $\text{1}\text{N}\text{-}\text{resummed}$ quantum gravity. I explain how these two facts may be adduced as evidence for the viability both of the former semi-classical approximation and of the latter approach to full quantum gravity.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

The Ã1B2-X̃1A1 two-photon fluorescence excitation spectrum of 1,3-difluorobenzene

The $\text{A}\text{?}{}^1\text{B}{}_2\text{-}\text{X}\text{?}{}^1\text{A}{}_1$ system of 1,3-difluorobenzene has been observed using the technique of two-photon fluorescence excitation obtained with a pulsed dye laser. Calibration was achieved by a combination of the neon optogalvanic spectrum and etalon fringes. In circular, compared to linear, polarization the bands divide into two groups, those which are B₂-A₁ and which retain their intensity with circular polarization, and those which are A₁-A₁ and lose about 60% of their intensity under the same conditions. These two kinds of bands also show characteristic rotational contours. All of the A₁-A₁ bands whose assignments are established obtain their intensity through vibronic interaction in which the vibration ν′₂₅ (ν′₁₄ in the Wilson numbering) mixes the $\text{A}\text{?}$ with, presumably, the $\text{X}\text{?}$ state. There is an important Fermi resonance between the 9¹ and 10¹11¹ levels. Parts of the one-photon absorption spectrum have been photographed to identify sequences associated with the 0₀⁰ band for comparison with those observed in the two-photon spectrum, and to search for bands involving odd quanta of b₂ vibrations, including ν′₂₅ (ν′₁₄); none was found.     

The Ã1A1-X̃1A1 two-photon fluorescence excitation spectrum of 1,2-difluorobenzene

The two-photon fluorescence excitation spectrum of 1,2-difluorobenzene was obtained with a tunable dye laser calibrated using a combination of the neon optogalvanic spectrum and etalon fringes. The spectrum consists only of A₁-A₁ bands but the use of linear and circular polarization divides the bands into two types. The 0⁰₀ type retains its intensity in circular polarization and, rotationally, shows little or no zero-rank contribution. The 5¹₀ (or 14¹₀) type loses much of its intensity in circular polarization and, rotationally, shows a large zero-rank contribution. These observations all accord with the trace of the two-photon transition tensor being close to zero for the 0⁰₀ type and large for the 5¹₀ type, the latter type being involved in vibronic interaction which mixes the $\text{A}\text{?}$ and $\text{X}\text{?}$ states. There is strong evidence for Fermi resonance between the 5¹ and 6¹10¹ levels. Parts of the one-photon absorption spectrum have been photographed to aid sequence identification and also to look for the 5¹₀, A₁-A₁ transition. This transition is not observed: nor is there any evidence for intensity stealing by b₂ vibrations.     

Absorption spectra of Ni2+ in (NH4)2Mg(SO4)2·6H2O and Co2+ in Na2Zn(SO4)2·4H2O

Optical absorption spectra of Ni²⁺ in (NH₄)₂Mg(SO₄)₂·6H₂O and Co²⁺ in Na₂Zn(SO₄)₂·4H₂O single crystals have been studied at room and liquid nitrogen temperatures. From the nature and position of the observed bands, a successful interpretation could be made assuming octahedral symmetry for both the ions in the crystals. The splitting observed for ${}^3\text{T}{}_{\mathrm{1g}}\text{(F)}$ band in Ni²⁺ and ${}^4\text{T}{}_{\mathrm{2g}}\text{(F)}$ band in Co²⁺ at liquid nitrogen temperature have been explained as due to spin-orbit interaction. The extra band observed at 16,325 cm^-1 in the case of Ni²⁺ at low temperature has been interpreted to be the superposition of vibrational mode of SO^2-₄ radical on ${}^3\text{T}{}_{\mathrm{1g}}\text{(F)}$ band. The observed band positions in both the crystals have been fitted with four parameters B, C, Dq and ζ.     

Electrical conductivity and defect structure of Ni-doped and “CO-reduced” Ni-doped SrTiO3 single crystals

The electrical conductivities of Ni-doped and “CO-reduced” Ni-doped SrTiO₃ single crystals were measured at temperatures 700–1200°C and P_o2's of 10^?7–10^?1 atm. Plots of log σ vs $\text{1}\text{T}$ at constant P_o2's were found to be linear, and the activation energies appeared to be 0.92 eV for Ni-doped SrTiO₃ and 0.50 eV for “CO-reduced” Ni-doped SrTiO₃ single crystals, respectively. Plots of log σ vs log P_o2 at constant temperature were found to be linear with an average slope of ?$\text{1}\text{4}$ for SrTiO₃:Ni and of ?$\text{1}\text{6}$ for “CO-reduced” SrTiO₃:Ni single crystals, respectively. The electrical conductivity dependencies on P_o2 indicate that a triply ionized titanium interstitial and an oxygen vacancy model are applicable to Nidoped and “CO-reduced” Ni-doped SrTiO₃ single crystals, respectively. The small polaron conduction was suggested on “CO-reduced” Ni-doped SrTiO₃ single crystal from the temperature dependence of conductivity.     

Photoelectron emission from Mg(OH)2 single crystals in the valence band region

The x-ray stimulated photoelectron emission from a) the (10$\text{1}$0) and b) the (0001) surface of Mg(OH)₂ single crystals was studied. A marked anisotropy was observed in the valence band region. For a) three maxima were found at 4.6, 8.8 and 12.0 eV which can be assigned to the T_1u, E_g and A_1g states respectively, if regular O_h symmetry is assumed for the ligand field. For b) the A_1g band disappears which is attribu to the fact that in Mg(OH)₂ the octahedral symmetry is reduced to D_3d by the special orientation of the OH dipoles parallel to the c-axis.     

Neutral heavy lepton candidate in e+e− interactions

An event of the form e⁺e^? → μ⁺μ^? + (2 jets) recently observed at √s = 43.45 GeV is interpreted as production of a pair of neutral heavy leptons N, each with mass 20.5 ± 1.0 Gev/c². Two possibilities are explored: (i) the lepton is a wak isodoublet neutrino, produced in pairs by virtual Z⁰ decay. In this case, one expects $\text{B(}\text{Z}{}^0\text{→}\text{N}\text{N}\text{) ≈ 5\%}$; (ii) the lepton is a “right-handed neutrino”, produced in pairs via a new vector boson Z_χ. In this case, in one model, the Z_χ must lie between about 50 and 67 GeV/c². More generally, it must be very weakly coupled to ordinary quarks and leptons in order not to conflict with low-q² neutral-current data. Suggestions are made for further observation of N$\text{N}$ pairs and other effects of Z_χ in forthcoming e⁺e^? and p$\text{p}$ collisions.