Oscillatory oxidation of Co over a Pt catalyst

We report measurements of the temporal oscillatory oxidation rates of CO over a polycrystalline Pt wire. The experiments were conducted near atmospheric pressure in a clean flow reactor system. Reproducible oscillations in both the temperature of the Pt wire and in the rate of CO₂ production were found over a wide range of gas compositions, $\text{0.001 <}\text{P}{}_{\mathrm{CO}}\text{P}{}_{\mathrm{O}}{}_2\text{< 0.045,}$ and temperatures, 150°C < T_g < 350°C, where PCO and po₂ are the respective partial pressures of carbon monoxide and oxygen in the gas stream, and T_g is the temperature of the gas. The oscillations are believed to occur between two branches of a Langmuir-Hinshelwood reaction mechanism. It is suggested that the slow formation and removal of subsurface oxygen drives the reaction between the two branches. A simple kinetic model based on this hypothesis gives excellent qualitative agreement with the observed oscillations.     

Average polarization of 12B in 12C(μ, ν)12B(g.s.) reaction: Helicity of the π-decay muon and nature of the weak coupling

The helicity, h^?, of μ^? in π-decay has been determined as positive (h^??+0.90) from the average polarization, P_av≡〈J_B·s_μ〉, of ¹²B produced in the $\text{μ}{}^{\mathrm{?}}\text{+}{}^{12}\text{C}\text{→ν}{}_{\mathrm{\mu }}\text{+}{}^{12}\text{B}$ reaction. We obtain also dynamical information on μ-capture: (i) the weak magnetism form factor, μ=4.5±1.1, and (ii) the sum of the induced pseudoscalar (g_p) and the 2nd class induced tensor (g_T) couplings versus g_A, ($\text{g}{}_{\mathrm{<mtext>P</mtext>}}\text{+g}{}_{\mathrm{<mtext>T</mtext>}}\text{)}\text{g}{}_{\mathrm{<mtext>A</mtext>}}\text{=7.1±2.7}$. The latter result, adopting the “canonical” value of $\text{g}{}_{\mathrm{<mtext>P</mtext>}}\text{g}{}_{\mathrm{<mtext>A</mtext>}}$, leads to $\text{g}{}_{\mathrm{<mtext>T</mtext>}}\text{g}{}_{\mathrm{<mtext>A</mtext>}}\text{=+1±2.7}$ which is compatible with zero and in strong contradiction with the value ?—6 recently advocated by Kubodera, Delorme and Rho.     

Magnetic circular dichroism of transition metal ions in solids—II K2NaGaF6: Cr3+

The absorption and MCD spectra of the ${}^4\text{A}{}_{\mathrm{2g}}\text{→}{}^4\text{T}{}_{\mathrm{2g}}$, ${}^4\text{A}{}_{\mathrm{2g}}$, ${}^4\text{A}{}_{\mathrm{2g}}\text{→}{}^4\text{T}{}_{\mathrm{1g}}{}^{\mathrm{a}}$ and ${}^4\text{A}{}_{\mathrm{2g}}\text{→}{}^4\text{T}{}_{\mathrm{1g}}{}^{\mathrm{b}}$ spin-allowed transitions of Cr³⁺ in K₂NaGaF₆ are reported. It is shown that transitions to the ${}^4\text{T}{}_{\mathrm{1g}}$. states are induced by T_1u vibratio the other spin-allowed transition, ${}^4\text{A}{}_{\mathrm{2g}}\text{→}{}^4\text{T}{}_{\mathrm{2g}}$, there are three competing intensity mechanisms: electric dipole induced by T_1u vibrations, electric dipole induced by T_2u vibrations and magnetic dipole, and an estimate is made of the relative importance of these. The magnetic dipole ${}^4\text{A}{}_{\mathrm{2g}}\text{→}{}^2\text{E}{}_{\mathrm{g}}$ zero-phonon line is observed to be accompanied by a vibrational sideband for which the coupling is predominantly with T_2u vibrations. Other weak transitions are observed in MCD spectra and their origin briefly discussed.     

Intensities and pressure-broadened widths of CO2 R-branch lines at 15 μm from tunable laser measurements

Intensities and half-widths of individual lines, over the temperature range 200–325°K in the 15 μm bands of ¹²C¹⁶O₂, have been determined with a tunable diode laser spectrometer. Measurements were made on pure CO₂ and on dilute CO₂-in-N₂ mixtures on the R-branches of the 01¹0-00⁰0 and 02²0-01¹0 transitions. Intensities are approximately equal to those listed in the AFGL compilation. The pressure-broadened half-widths follow the general relationship $\text{b}{}_{\mathrm{L}}{}^0\text{(T) = b}{}_{\mathrm{L}}{}^0\text{(T}{}_0\text{) [}\text{T}{}_0\text{T}\text{]}{}^{\mathrm{n}}$ where n varies considerably from line to line but is always greater than $\text{1}\text{2}$.     

Study on vacancy motion in Y2O3-doped CeO2 by 17O NMR technique

The ¹⁷O NMR measurement was made to elucidate the microscopic nature of vacancy motion in Y₂O₃-doped CeO₂. Spin-lattice relaxation rate, T^?1₁, spin-spin relaxation rate, T^?1₂, and resonance intensity were measured at v₀ = 8.13 MHz as a function of temperature [385 < T (K) < 1110] and composition [$\text{0.06 <}\text{Y}{}_2\text{O}{}_3\text{(}\text{m}\text{o}\text{) < 6}$]. The static electric field gradient associated with lattice defects resulted in the composition dependences of the line width and the intensity. In low dopant concentrations, doubly peaked temperature dependence of T^?1₁ was found, while a single and asymmetric peak was observed in high concentrations. T^?1₁ of 4.0 and 6.0 $\text{m}\text{o}$ doped samples were analyzed using a barrier height distribution model for the oxygen vacancy jump. The mean value of the activation energy was found to increase with the Y₂O₃ concentration.     

A thermodynamic study of nonstoichiometric cerium dioxide

Thermogravimetric measurements were performed on nonstoichiometric CeO_2?x in the temperature range 750–1500°C and from oxygen pressures of 10^?2 to 10^?26 atm. From this data the deviation from stoichiometry x = x(T, P_o2) was determined. The thermodynamic quantities Δ$\text{H}$_o2 and Δ$\text{S}$_o2 were calculated in the region 0.001? x ? 0.3 and found to be independent of temperature.In the composition region 0.001< x < 0.006, the variation of Δ$\text{S}$_o2 with x is consistent with a defect model involving randomly distributed doubly ionized oxygen vacancies. The experimental dependence of x and σ (electrical conductivity) is shown to be consistent with this model as Δ$\text{H}$_o2 (≈ -10 eV) exhibits a slight dependence on x. It is postulated that the variation in Δ$\text{H}$_o2 may result from lattice parameter increases with x, while the defects remain essentially randomly distributed.In the composition region 0.006 < x < 0.1, x ∝ with 1 < n < 5, and in the region 0.1 < x < 0.3, x ∝ with n increasing rapidly with x to n? 30. This behavior is believed to result from increasing defect interaction with increasing departures from stoichiometry. It is interesting to note that the ordered phase observed by Bevan and Kordis between CeO_1·72 and CeO_1·70 was not observed in this study at temperatures between 1300° and 1500°C.     

A model for phase transitions in Tetrathiafulvalene-Tetracyanoquinodimethane (TTF-TCNQ)

The observed phase transitions in Tetrathiafulvalene-Tetracyanoquinodimethane (TTF-TCNQ) are discussed using a simple model for the interchain coupling of charge density waves. Estimates based on Coulomb energies show that for 38 K < T < 49 K the components $\text{q}{}_{\mathrm{x}}\text{=}\text{π}\text{a}\text{+ q′}{}_{\mathrm{x}}$ and q_z of the wave vector associated with the charge density wave satisfy $\text{q}{}_{\mathrm{z}}\text{c}\text{q′}{}_{\mathrm{x}}\text{a?0.1}$, with $\text{q′}{}_{\mathrm{x}}\text{a～(T}{}_2\text{? T)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and T₂ = 49 K. A possible mechanism for the first order transition at 38 K is proposed. The results are compared with neutron and X-ray scattering and with isotope shifts of the transition temperatures.     

Electron-vibration splitting of the vibronic levels of the 1T2g state of MgO:Ni2+

Magnetic circular dichroism (MCD) and magnetic linear dichroism (MLD) measurements of the ${}^3\text{A}{}_{\mathrm{2g}}\text{→}{}^1\text{T}{}_{\mathrm{2g}}$ absorption band in MgO:Ni²⁺ are reported. It is shown that for the lowest energy vibrational peak there is considerable evidence of splitting of the T_2g × T_1u vibronic levels by the electron-vibration interaction.     

An Auger spectroscopic study of the kinetic behavior of adsorbed oxygen on palladium

The adsorption of oxygen on polycrystalline palladium, the kinetics of the reaction of adsorbed oxygen with carbon monoxide and the amount of adsorbed oxygen present during the catalyzed reaction, $\text{CO +}\text{1}\text{2}\text{O}{}_2\text{→ CO}{}_2$, were studied by Auger electron spectroscopy. At temperatures below 783 K, the initial sticking probability is high (~0.8). Adsorbed oxygen and CO react with high probability and low activation energy to form carbon dioxide. The reaction is first order with respect to carbon monoxide pressure and zero order in oxygen coverage. Oxygen coverages measured during the CO-oxidation reaction decrease sharply around P_CO ? P_O2 and are very small when P_CO >P_O2. The reaction kinetics are discussed using a modified Eley-Rideal mechanism involving strongly adsorbed oxygen atoms and surface carbon monoxide in a short-lived state. The oxygen adsorption phenomena are correlated with the reaction kinetics.     

Temperature shift of the CdxHg1−xTe energy gap

The temperature coefficient of the Cd_xHg_1?xTe energy gap dE_g/dT as a sum of lattice dilatation and the phonon-electron interaction terms has been calculated as the function of molar composition x, for 0?x?0.3, in the temperature range 4.2–300 K. A non-linear dependence of $\text{d}\text{E}{}_{\mathrm{g}}\text{d}\text{T}$ vs x and a strong effect of temperature on $\text{d}\text{E}{}_{\mathrm{g}}\text{d}\text{T}$ values have been obtained and a comparison with experimental data is discussed.     

Measurement of the parameter η&#x0304; in the radiative decay of the muon as a test of the v-a structure of the weak interaction

The parameter η&#x0304; of muon decay has been measured in the radiative decay $\text{μ}{}^{\mathrm{+}}\text{→}\text{e}{}^{\mathrm{+}}\text{ν}{}_{\mathrm{<mtext>e</mtext>}}\text{ν}\text{?}{}_{\mathrm{\mu }}\text{γ}$ of unpolarized positive muons. The result $\text{η}\text{?}\text{?0.083}$ (68% confidence) or $\text{η}\text{?}\text{= ?0.03±0.10}$ with ρ fixed at $\text{3}\text{4}$ yields an improvement of the previous value by more than a factor of two. An analysis of all data on muon decay that are presently available slightly improves the constraints on the weak coupling constants to: g_s?0.29g_v, g_p?0.27g_v, g_T?0.23g_v and 0.92g_v?g_A?1.2g_v     

The Grüneisen parameter γ of KBr,RbCl and Bi through high pressure phase transitions

High pressure values for the adiabatic pressure derivative of temperature $\text{(}\text{?T}\text{?P}\text{)}{}_{\mathrm{s}}$ have been obtained by measuring the temperature change caused by a small rapid increase in pressure. Values for KBr and RbCl in phases B1 and B2 and for Bi in phases I, II and III are given for T = 295 K. The Grüneisen parameter γ is given by $\text{γ =}\text{B}{}_{\mathrm{s}}\text{(}\text{?T}\text{?P}\text{)}{}_{\mathrm{s}}\text{T}$ where B_s is the adiabatic bulk modulus. Ultrasonic and statk compressibility data are used to estimate the pressure and phase dependence of B_s. Dramatic increases in both γ and (?T/?P)_s are observed as the pressure increases through a phase transition. Values for the logarithmic derivate $\text{q ≡ (? ln}\text{γ}\text{?}\text{ln V}{}_{\mathrm{T}}$ are given.     

Prediction of strong magnetic quantum oscillations of the nuclear spin relaxation in a quasi-two-dimensional metal

The nuclear spin lattice relaxation rate in a quasi-two-dimensional (2-D) metal under strong magnetic fields is studied in a special case where the electronic cyclotron mass is small compared with the free electron mass. In the pure limit (ω_cτ ? 1) and for sufficiently low temperatures ($\text{h}\text{?}\text{ω}{}_{\mathrm{c}}\text{> 2π}{}^2\text{k}{}_{\mathrm{B}}\text{T}$) we find remarkable quantum oscillations of the relaxation rate as a function of the magnetic field. The period of these oscillations is identical to that of the de Haas-van Alphen oscillations and their amplitude grows linearly with the magnetic field. The possibility of observing such oscillations experimentally in the quasi-2-D mercury chain compound Hg_3?δAsF₆ is discussed.     

The application of XPS to the determination of the kinetics of the co oxidation reaction over the Ir(111) surface

The coverages of adsorbed oxygen and CO on an Ir(111) surface have been determined using X-ray photoelectron spectroscopy (XPS) during the steady-state catalytic production of CO₂. Correlating the coverages of the reacting adsorbates with the rate of CO₂ production allows the kinetics of the CO oxidation reaction to be determined. The reaction is found to obey a Langmuir-Hinshelwood rate expression of the form , where R_CO2 is the rate of CO₂ production, k⁰ is the pre-exponential factor of the reaction rate coefficient, [CO] and [O] are the surface coverages of CO and oxygen, respectively, and E_a is the activation energy for the oxidation reaction. The activation energy for this catalytic oxidation reaction is found to be approximately 9 $\text{kcal}\text{mole}$.     

A phenomenological investigation of the shower particle pseudorapidity fluctuations in hadron-nucleus collisions

From the experimental matrix $\text{?(N}{}_{\mathrm{<mtext>g</mtext>}}\text{; n}{}_{\mathrm{<mtext>s</mtext>}}\text{, Δη}{}_{\mathrm{i}}\text{)}$ (the number of events with a given number N_g of particles with velocity 0.3c<v<0.7c, and n_s, the number of shower particles (v > 0.7c) in a pseudo-rapidity interval Δη_i) obtained from the Heidelberg-Igel hadron-nucleus experiment at CERN, we extract the shower particle multiplicity distributions as a function of ν, the number of nucleons that interacted with the projectile. We use a theoretically justified statistical correlation between ν and N_g (recoil protons) and the assumption that the correlation between n_s and N_g is given merely through ν. The data used are for incoming energies 50 and 150 GeV with a proton or pion as the projectile and Cu or Pb as the target. We find that the shower particle multiplicity distributions are of a Poissonian character in each η bin and for each value of ν. The average multiplicities $\text{〈n}{}_{\mathrm{<mtext>s</mtext>}}\text{〉(ν)}\text{Δη}{}_{\mathrm{i}}$ are approximately the same for the two targets and for the different projectile, possibly with a small excess for the pion projectile.The results are compared to a rather general theoretical model of the soft interaction type. The predictions from this model for the shower particle pseudorapidity distributions are very close to the derived values.     

γ-decay of the deeply-bound hole states in 101Pd, 105Pd, 107Pd, 111Sn, 117Sn observed in the (3He, αγ) reaction at 70 MeV

The γ-decay of deep-hole states in ^{101, 105, 107}Pd was studied via the (³He, αγ) reaction at E_³he = 70 MeV and supplemented by data from ^{112, 118}Sn targets to investigate the deep-hole spreading mechanism. The γ-decay pattern for the $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ deep-hole state shows a strong dependence on the spreading width: if the deep-hole state is observed as a sharp peak, it mainly decays to the low-lying $\text{7}\text{2}{}^{\mathrm{+}}$ state by a spin-flip M1 transition with a large M1-E2 mixing ratio; if the deep-hole state is observed as a broad bump, it decays statistically indicating the complete spreading of the hole strength over the underlying states; if the deep-hole state is observed with a structure intermediate between a sharp peak and broad bump, its γ-decay shows both decay patterns.A sharp peak at E_x = 2.396 MeV in ¹⁰¹Pd which carries a large fraction of the $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ hole strength (C²S = 2.0) was found to be a single state having a width of less than 2.5 keV.For the spin-flip M1 transition the destructive interference between the $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ component and the coupled components of the deep-hole state was found in heavily spread states.A quasiparticle-plus-rotor (QPR) model was applied to calculate the fragmentation in the doorway stage for the $\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ neutron deep-hole state in the Pd isotopes. A reasonable agreement between the calculation and the experimental results was obtained for the strength fragmentation, for the nucleus ¹⁰¹Pd. However, the large M1-E2 mixing ratio experimentally observed was not reproduced.     

Deuteron optical model calculations including a Tp tensor interaction

Deuteron-nucleus optical model calculations which include two types of tensor spin-orbit interactions are compared. The interactions considered are of the forms $\text{V}{}_{\mathrm{<mtext>T</mtext><mtext>R</mtext>}}\text{(r)T}{}_{\mathrm{r}}\text{and}\text{1}\text{2}\text{[V}{}_{\mathrm{<mtext>T</mtext><mtext>P</mtext>}}\text{(r)T}{}_{\mathrm{p}}\text{+ T}{}_{\mathrm{p}}\text{V}{}_{\mathrm{<mtext>T</mtext><mtext>P</mtext>}}\text{(r)]}$ where the operators T_r and T_p are defined as $\text{T}{}_{\mathrm{r}}\text{ (}\text{s}\text{·}\text{r}\text{?}\text{)}{}^2\text{?}\text{2}\text{3}\text{and}\text{T}{}_{\mathrm{p}}\text{ (}\text{s}\text{·}\text{p}\text{)}{}^2\text{?}\text{2}\text{3}\text{p}{}^2$. Here, $\text{s}\text{,}\text{r}\text{and}\text{p}$ are the deuteron spin, position, and momentum operators, respectively. If a physically reasonable radial dependence V_TP(r) is used, the S-matrix elements (and hence all observables) calculated using a T_p tensor potential are found to be similar to those calculated using a suitably chosen T_r potential, provided that the incident deuteron energy is small compared to the real central potential. At higher deuteron energies, the differences between the effects of T_r and T_p potentials are more pronounced.     

Ni2+ emission in MgO,KMgF3, KZnF3 and MgF2

The emission of Ni²⁺ ions in MgO, KMgF₃, KZnF₃ and MgF₂ crystals has been investigated. The fine structure on the bands at about 20 000 cm^-1 and 13 000 cm^-1 has been studied in detail and from this and the excitation spectra these bands are assigned to ${}^1\text{T}{}_{\mathrm{2g}}\text{→}{}^3\text{A}{}_{\mathrm{2g}}$ and ${}^1\text{T}{}_{\mathrm{2g}}\text{→}{}^3\text{T}{}_{\mathrm{2g}}$ transitions respectively.     

Phase transitions in ferroelectric ceramics of the type Sr0.5Ba0.5Nb2O6

The ferro-paraelectric transition in Sr_0.5Ba_0.5Nb₂O₆ ceramics has been studied by measuring the dielectric pennitivity as a function of temperature at different frequencies. Experimental results in the transition region over ?'_max fit the relation $\text{1}\text{?'}\text{?}\text{1}\text{?'}{}_{\max }\text{= A(T ?}\text{T}{}_0\text{)}{}^{\mathrm{\gamma }}$ with γ > 1, as is typical for dif transitions. In an interval of temperatures of 17°C over T₀ (=Tfor?'_max), γ = 2. Using the relaxors' model of Kirillov-Isupov an equation is inferred in order to compute the activation energy of the relaxors as a function of parameters directly obtained from the experimental results.