Surface temperature modulation in molecular beam relaxation spectrometry applied to catalytic decomposition of CH3OH on NiO

A new modification of molecular beam relaxation spectrometry (MBRS) of surface processes is described making use of partial modulation in order to study nonlinear processes: a constant particle beam is directed towards the catalyst surface, the surface temperature is modulated due to absorption of a modulated beam of UV light, reaction products are analyzed by use of phase sensitive mass spectrometric detection. The application of the method is shown by a study of catalytic decomposition of methanol on polycrystalline NiO. Formation of CO was found to be a monomolecular, formation of H₂ and H₂O bimolecular processes. The resulting mechanism may be described as follows:

Rate constants in dependence from surface temperature T₀ are η = 1.8 × 10³$\text{exp(?}\text{46}\text{RT}{}_{\mathrm{o}}\text{kJ}\text{mol}\text{)}$; k_d1 = 1.8 × 10¹⁰$\text{exp(?}\text{92}\text{RTl}{}_0\text{kJ}\text{mol}\text{)}$ s^?1; k_d2 = 1.2 × 10^?2$\text{exp (?}\text{88}\text{RT}{}_0\text{kJ}\text{mol}\text{)}$ cm² particles^?1 s^?1; k_d3 = 3.5 × 10^?4$\text{exp(?}\text{88}\text{RT}{}_0\text{kJ}\text{mol}\text{)}$ cm² particles^?1 s^?1. Average surface residence times of the intermediates are: $\text{27 ?}{}^{\mathrm{\tau }}\text{HCO}\text{}\text{?}\text{1 ms}$ at 550 ? T₀ ? 650 K; $\text{42 ?}{}^{\mathrm{\tau }}\text{H ? 7 ms}$ at 540 ?T₀ ? 610 K; $\text{177 ?}{}^{\mathrm{\tau }}\text{OH ? 19 ms}$ at 550 ? T₀ ? 645 K.     

Residual gas and the optical properties of silver films

Silver films evaporated and maintained at room temperature in a variety of controlled residual gas environments are studied. Experimental techniques include: (1) precise in situ polarization-modulation ellipsometry $\text{(λ = 5461}\text{A}\text{?}\text{)}$ and dc resistivity as functions of time (after deposition) and pressure; (2) mass spectroscopy of residual gas; (3) optical spectra (1.8–3.8 eV) taken ellipsometrically on several films in situ; (4) Auger electron spectroscopy of the silver surfaces; and (5) absolute reflectance $\text{(λ = 5461}\text{A}\text{?}$ and 10.6 microm) and scattered light (5461 Å) measured after exposure of the films to air. H₂, N₂, CO, CO₂, or CH₄ present during deposition (p? 10^?5torr) shows no effects on the optical properties in the visible or infrared. Water vapor or oxygen present during deposition increases the scattered light (surface roughness) and optical absorption of the films in the visible. Films prepared in high partial pressures of O₂ (about 10^?5 torr) show significantly shorter optical and dc relaxation times and slightly enhanced infrared (λ = 10.6 microm) absorption. These results for stabilized films are understood better by considering the surface dynamical changes occurring in the films prior to stabilization. In the first few hours after deposition, the films anneal principally by surface self-diffusion. During this time the reflectance and dc conductivity increase typically by 0.4% and 10% respectively. The presence in the films of dissolved oxygen can enhance or inhibit self-diffusion depending on whether the oxygen is mobile or immobile. When dissolved oxygen diffuses to the surface, it resides there in an optically-absorbing chemisorbed state, inducing a surface state within about 2.5 eV of the Fermi level. These surface-dynamical changes often continue for more than a day before diminishing to a level beneath the sensitivity of the ellipsometer. Upon exposure to air the Ag films physisorb about 20 Å of water vapor and undergo further optically-absorbing oxygen chemisorption. All of the measurements are discussed in a consistent way to reveal further details of the mechanisms contributing to light scattering and optical absorption in the films as a function of oxygen partial pressure.     

Connections between thermodynamic quantities and vacancy and diffusion characteristics in binary metallic solid solutions

With the use of the similarity of interatomic potentials relations concerning vacancy and diffusion characteristics in disordered regular solid solutions have been derived. It has been shown that the vacancy concentration is constant along ifT_c(x) = T_A + (T_B ? T_A)x + 2ΔT_x(1?x), (T_A and T_B are the melting points of pure components A and B respectively, and ΔT is proportional to the excess enthalpy of mixing, x is the concentration of the atoms B) which is proportional to the binding energy of the crystal. The validity conditions of several empirical rules known in the literature are also analyzed. It has been found that the generalization of the well-known rule for self- and impurity diffusion in pure metals has the following form In $\text{D}{}_0{}^{\mathrm{z}}\text{(x) ～ p}\text{Q}{}^{\mathrm{z}}\text{(x)}\text{T}{}_{\mathrm{c}}\text{(x)}$ (Z = AorB) where p is a constant for alloys having identical structures (D₀^z(x) and Q^z(x) denote the preexponential factors and the activation energies respectively). The results calculated from the relations derived were compared with experimental data for tracer diffusion in the systems AgAu, CuNi (having slight deviation from regularity), Pb-Tl (showing ordering phenomena) and AlZn (clustering effects) and a good agreement was found.     

Diffusion of oxygen vacancies in yttrium iron garnet investigated by dynamic conductivity measurements

In yttrium iron garnet, Y₃Fe₅O₁₂, the oxygen vacancy concentration at high temperatures depends on the partial oxygen pressure. Due to the electron donating nature of the vacancies, changes in the oxygen vacancy concentration can be measured by electrical conductivity measurements. We discuss a dynamic method for studying the diffusion of oxygen vacancies by measurements of the time dependence of the electrical conduction after a change in the oxygen partial pressure has taken place. It is shown that the interpretation of the measurements is straightforward if the relative change in conductivity remains small (? 10%). Measurements were performed on single crystals and on polycrystalline samples at temperatures 900–1400°C. The samples were made n-type by substitution with Si or p-type by substitution with Ca, Zn or Pb. The partial oxygen pressure was changed between 1 and 0.1 atm. For all samples the diffusion coefficient D of the oxygen vacancies can be represented by $\text{D = A}\text{exp}\text{(?}\text{Q}\text{kT}\text{)}$, where A = 8400 cm²s^?1 and Q = 2.90 eV. It is shown that the activation energy of 2.90 eV is due to the migration enthalpy of the vacancies only.     

Observation of magnetization reversals during relaxation measurements on superfluid 3HeA

A reversal of the recovery of the magnetization following an rf pulse has been observed in superfluid ³HeA in 306 Oe. The time difference between the end of the rf pulse and the start of the magnetization reversal is found to be proportional to both (1 ? T/T_c)^{?$\text{1}\text{2}$} and the magnitude of the initial rotation angle θ.     

Electrical conductivity and chemical diffusion coefficient measurements in single crystalline nickel oxide at high temperatures

Electrical conductivity measurements on nickel oxide have been performed at high temperatures (1273 K<T< 1673 K) and in partial pressures of oxygen ranging from Po₂ = 1.89 × 10^?4 atm to Po₂ = 1 atm. The $\text{po}{}_2{}^{\mathrm{<mtext>1</mtext><mtext>n</mtext>}}$ dependence of the conductivity decreases from about $\text{1}\text{4}$ for Po₂ = 1 atm to smaller values for lower partial pressures of oxygen. The activation enthalpy for conduction increases for decreasing oxygen partial pressures (from 22.5 kcal mol^?1 at Po₂ = 1 atm to 26.0 kcal mol^?1 for Po₂ = 1.89 × 10^?4 atm). This behaviour can be explained by the simultaneous presence of singly and doubly ionized nickel vacancies, with different energies of formation.Furthermore, chemical diffusion coefficient measurements have been performed in the same temperature range, using the conductivity technique, and leading to the result:

$\text{D}\text{?}\text{= 0.244 exp (?}\text{36,600}\text{RT}\text{) cm}{}^2\text{s}{}^{\mathrm{?1}}$

.     

The low temperature electrical properties of 1T-TaS2

Measurements of the electrical resistivity of 1T-TaS₂ to 0.03 K show that the increase in resistivity below ～ 50 K is extrinsic.Below 2 K the resistivity is described by ? = ?₀ exp $\text{T}{}_0\text{/T)}\text{1}\text{3}$. Because of this fractional power law behavior, we conclude that the increase is due to Anderson localization by random impurity and/or defect potentials. Other difficulties in understanding the properties of 1T-TaS₂ are also pointed out.     

Electrical properties of metal-piezoelectric semiconductor interface under stress

The effects of the mechanical stress on the current-voltage characteristics of metal-piezoelectric semiconductor interface are investigated for both AuCdS and AuGaP Schottky diodes. The changes in currents due to the approximately uniaxial stress, which is applied by bending the crystal wafers attached to the cantilever, are measured. Under the compressive stress parallel to the interface, the current of the diode fabricated on the (0001)Cd or that on the $\text{(}\text{1}\text{?}\text{1}\text{?}\text{1}\text{?}\text{)P}$ surface increases but that of the diode on the $\text{(000}\text{1}\text{?}\text{)S}$ or on the (111)Ga surface decreases. The direction of the changes in current is opposite under the tensile stress. The relative current change $\text{ΔI}\text{I}{}_0$ is proportional to the applied stress and its magnitude at the surface strain 3 × 10^?4is 2.4 × 10^?2, 8.0 × 10^?2, 1.0 × 10^?2 and $\text{0.5 × 10}{}^{\mathrm{?2}}\text{for diodes made on (0001)Cd, (000}\text{1}\text{?}\text{)S, (111)Ga}$ and $\text{(}\text{1}\text{?}\text{1}\text{?}\text{1}\text{?}\text{)P}$ surfaces respectively. These values are also nearly constant over the small bias region in which I₀ is proportional to exp($\text{eV}{}_{\mathrm{a}}\text{KT}$). In this region, the changes in current are transformed to the changes in barrier height. In order to explain this effect, the changes in barrier height are calculated according to the piezoelectricity in the depletion layer of the Schottky diodes. The predicted value agrees with the one obtained experimentally.     

A monte-carlo/molecular dynamics study of the diffusional recombination kinetics of C(a) + O(a) → CO(g) on Pt(111)

The diffusion constants for C and O adsorbates on Pt(111) surfaces have been calculated with Monte-Carlo/Molecular Dynamics techniques. The diffusion constants are determined to be D_C(T)=(3.4 × 10^?3e^{$\text{?13156}\text{T}$})cm²s^?1 for carbon and D_O(T) = (1.5×10^?3 e^{$\text{?9089}\text{T}$}) cm² s^?1 for oxygen. Using a recently developed diffusion model for surface recombination kinetics an approximate upper bound to the recombination rate constant of C and O on Pt(111) to produce CO_(g) is found to be (9.4×10^?3 e^{$\text{?9089}\text{T}$}) cm² s^?1.     

Iron diffusion and electrical conductivity in magnesio-wüstite solid solutions (Mg,Fe)O

Diffusion of ⁵⁹Fe and electrical conductivity in magnesio-wüstite solid solution (Mg_xFe_1?x)O with x = 0.26 and 0.5 have been measured as a function of temperature and oxygen partial pressure. For both solid solutions, the results show that at 1100°C the diffusivity D of ⁵⁹Fe is directly proportional to $\text{p}{}_{\mathrm{o2}}{}^{\mathrm{<mtext>1</mtext><mtext>6</mtext>}}$, whereas the electrical conductivity σ is directly proportional to $\text{p}{}_{\mathrm{o2}}{}^{\mathrm{<mtext>1</mtext><mtext>3.4</mtext>}}$. At a given temperature and oxygen partial pressure, the value of D decreases with an increase in MgO concentration in the solid solution. The results are discussed in terms of the coexistence of variously ionized cation vacancies and their change in concentration with MgO additions.     

The four-pion final state in pp annihilations at 5.7 GeV/c

577 events from $\text{p}\text{p}\text{→2π}{}^{\mathrm{+}}\text{2π}{}^{\mathrm{?}}$ annihilations at 5.7 GeV/c are analyzed. Single-particle spectra are studied and an effective matrix element in the form Π_i=1⁴ exp(?Bp_Ti²) is determined. The rates for production of ?° and f⁰ mesons are estimated and the influence of resonances on the angular distributions of pions and dipions is investigated. Transverse momenta correlations can be explained using peripheral phase space.     

Variable range hopping conduction in the lowest temperature region

Experiment on doped semiconductor indicates that there exists a temperature region of hopping conduction where the resistivity varies as T^-2, deviating from the Mott formula exp $\text{[}\text{(T}{}_1\text{T}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{]}$. A new version of the variable range hopping is proposed taking into account the effect of random nature of the system on the correlation of the energy levels of localized states, and is shown to lead to T^-2 dependence of the resistivity in the lowest temperature region.     

The bonded water molecule3ĪII: 180° Flips and diffusion

The proton spin-lattice relaxation time, T₁, is measured as a function of temperature in α -(COOH)₂·-2H₂O, K₂HgCl₄· H₂O and LiCHO·H₂O. The relaxation is caused by 180° flips of the water molecules about their 2-fold axes and good agreement is obtained between calculated and observed values of T₁. Empiricly the flip rate follows a classical Arrhenius equation: P· exp $\text{(? ΔH}\text{(RT))}$. A literature survey of values of P and ΔH obtained from similar investigations on other hydrates is given. The survey shows that the preexponential factor, P, is a function of the activation enthalpy, ΔH. P increases from 10¹² to 10¹⁷ Hz when ΔH changes from 2 to 17 $\text{kcal}\text{mole}$. Using a dynamical rate theory as formulated by Feit, we find the flip rate is given by: K₂· √(ΔH)· exp (K₁ΔH)· exp $\text{(?ΔH}\text{(RT>))}$. This expression can be fitted to the observed data using K₁ = 0.69 $\text{mole}\text{kcal}$ and K₂ = 2 × 10¹¹ Hz · $\text{(kcal}\text{mole)}$$\text{?1}\text{2}$. Thus both the frequency factor, K₂√ (ΔH), and the entropic factor, exp (K₁ΔH), have been obtained for flipping water molecules in hydrates. The values of K₁ and K₂ are shown to be physically reasonable.     

Electrical conductivity at high temperature and thermodynamic study of point defects in single crystalline cuprous oxide

In order to follow the variation of point defect interactions in relatively concentrated solutions we have studied the electrical conductivity of cuprous oxide in the range of temperature 650–1100°C and for oxygen partial pressures greater than 10^?6 atm. The P^{$\text{1}\text{π}$}_O2 dependence of the conductivity varied non linearly from about n = 8 for P_o2 close to 10^?6 atm. to lower values of n with increasing oxygen partial pressure. The activation enthalpy of conductivity determined in these ranges of temperature and oxygen partial pressure has been found to be also a function of temperature and varied between 12 and 17 kcal mol^?1.The interpretation of these results has permitted us to show that the departure from linearity of the plots of log or log $\text{σ = ?(}\text{l}\text{T}\text{)}$ excludes the existence of an ideal solution of ionized and non-ionized copper vacancies as was proposed previously in the literature. To explain these results it is possible to take into account a partially disordered distribution of these defects. It is shown that the increase of interactions and consequently the variation of the electrical conductivity as a function of the thermodynamic parameters, may be simulated by an ideal solution including new charged species of the type (V'_CuV^x_Cu).     

Optical absorption spectrum of hematite, αFe2O3 near IR to UV

Optical absorption spectra have been measured on thin (011) single crystal platelets and on highly oriented (110) thin films of αFe₂O₃. We have observed and assigned some of the absorption bands predicted by ligand field theory and SCF-Xα calculations. The temperature dependence of the 11760 cm^?1 single crystal band has been fitted to the function $\text{? = ?}{}_0\text{(1 + exp (?}\text{θ}\text{T}\text{))}$ with $\text{?}{}_0\text{= 0.85 × 10}{}^{\mathrm{?4}}$ and θ = 200 K (139 cm^?1). We have measured the photocurrent as a function of wavelength and have found several peaks that coincide with optical absorption bands.     

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.     

A thermo-chemical study of the defect structure of barium oxide

A study of BaO has been made by use of thermogravimetric analysis, oxygen concentration analysis, and X-ray lattice parameter measurements in the temperature range 850°C ? T ? 1420°C and oxygen pressure range 7 × 10^-6 atm ? p_O2 ? 0·820 atm. Both the weight gain by the BaO samples and subsequently determined excess oxygen concentration were found to be directly proportional to . The enthalpy of incorporation oxygen in the lattice

$\text{1}\text{2}\text{O}{}_2\text{(g)=O(excess)}$

was determined to be ?0·395 ± 0·034 eV. Creation of vacancies on cation sites or of oxygen interstitials are consistent with the experimental results. As an alternative, the formation of O₂^2? ions, (as in BaO₂) as a result of incorporation of excess oxygen in the lattice, has been suggested.     

The impurity resistivity and resistivity minima in AlCr and AlMn alloys

The impurity resistivity of $\text{Al}$Cr between 1.5 and 50°K was determined with a characteristic temperature for the T² variation θ₁=960±40°K. The behaviour of the resistivity minimum both in $\text{Al}$Cr and $\text{Al}$Mn alloys with impurity concentration provides evidence that a T³ phonon resistivity is found also in aluminium with anomalous impurity resistivity.     

Etude de l'autodiffusion dans la phase cubique centree de l'europium et du gadolinium

The self-diffusion coefficient follows a relation of the form : D = (1,0_?0.4^+0.7)exp (?shape=case>34400RT±700)

$\text{cm}{}^2\text{sec}$

for b.c.c. europium D = (1,0_?0.3^+0.5) × 10^?2 exp(?

$\text{32700 ±4000}\text{RT}\text{)}$

$\text{cm}{}^2\text{sec}$

for β-b.c.c. gadolinium.Whereas europium has normal self-diffusion parameters, β-b.c.c. gadolinium must be set in the class of the anomalous b.c.c. rare-earth metals.From these results we conclude that there exists no evident connexion between the instability of the 4f shell and the activation energies anomalously low in the b.c.c. phases of the rare-earth metals.