Adsorption of iso-butane on ZnO(0 0 0 1)-Zn

Presented are thermal desorption spectroscopy (TDS) and adsorption probability measurements of iso-butane on the Zn-terminated surface of ZnO. The initial adsorption probability, S₀, decreases linearly from 0.57 to 0.22 (±0.02) with impact energy, E_i = 0.74-1.92 eV, and is independent of adsorption temperature, T_s = 91-114 K (±5 K), indicating non-activated molecular adsorption. The coverage, Θ, dependent adsorption probabilities, S(Θ), show a cross-over from adsorbate-assisted adsorption (S increases with Θ) to Kisliuk-like dynamics at about the desorption temperature of iso-butane bi-layers (∼110 K). Thus, the adsorption dynamics are precursor-mediated. The enhanced (gas-surface) mass-match, caused by forming a second layer of the alkane, leads to adsorbate-assisted adsorption. A direct fitting procedure of the TDS data yields a pre-exponential factor of 2.5 × 10¹³/s and a coverage dependent heat of adsorption of E_d(Θ) = 39 − 6 ∗ Θ + 2.5 ∗ exp(−Θ/0.07) kJ/mol.     

Adsorption dynamics of CO2 on Cu(1 1 0): A molecular beam study

Molecular beam scattering measurements have been conducted to examine the adsorption dynamics of CO₂ on Cu(1 1 0). The initial adsorption probability, S₀, decreases exponentially from 0.43 ± 0.03 to a value close to the detection limit (∼0.03) within the impact energy range of E_i = (0.12-1.30) eV. S₀ is independent of the adsorption temperature, T_s, and the impact angle, α_i, i.e., the adsorption is non-activated and total energy scaling is obeyed. The coverage, Θ, dependent adsorption probability, S(Θ), agrees with precursor-assisted adsorption dynamics (Kisliuk type) above T_s ∼ 91 K. However, below that temperature adsorbate-assisted adsorption (S increases with Θ) has been observed. That effect is most distinct at large E_i and low T_s. The S(Θ) data have been modeled by Monte Carlo simulations. No indications of CO₂ dissociation were obtained from Auger Electron Spectroscopy or the molecular beam scattering data.     

CO2 adsorption on Cr(1 1 0) and Cr2O3(0 0 0 1)/Cr(1 1 0)

We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr₂O₃(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO₂ adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of E_i = 0.1-1.1 eV and adsorption temperatures of T_s = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy E_d = 33.3 kJ/mol, first order pre-exponential 1 × 10¹³ s⁻¹). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (E_d = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S₀, is independent of T_s for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing E_i, with S₀ smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low E_i is approx. independent of coverage (Kisliuk-shape) and increases initially at large E_i with coverage (adsorbate-assisted adsorption). CO₂ physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of E_d has been obtained by means of a Redhead analysis of the TDS curves.     

Relativistic comparison theorems for the Klein-Gordon equation with scalar and vector potentials in d-dimensions

Relativistic comparison theorems are established for discrete eigenvalues of Klein-Gordon equation with vector and scalar potentials in d-dimensions. Theorem 1: If V(λ) and S(λ) depend on a parameter λ, ∂S/∂λ?0, S?0, ∂V/∂λ?0, V?0, E>0, then it follows that ∂En/∂λ?0. Theorem 2: If S₂?S₁?0, 0?V₂?V₁, E>0, then the corresponding eigenvalues are ordered as En(2)?En(1). Theorem 3: If 0?V₂?V₁, S₂?|S₁|, En(1)>0, En(2)>0, then En(2)?En(1). Some illustrative examples are given.     

Computable upper bounds for the adiabatic approximation errors

For a given Hermitian Hamiltonian H(s)(s∈[0,1])with eigenvalues Ek(s)and the corresponding eigenstates|Ek(s)(1 k N),adiabatic evolution described by the dilated Hamiltonian HT(t):=H(t/T)(t∈[0,T])starting from any fixed eigenstate|En(0)is discussed in this paper.Under the gap-condition that|Ek(s)-En(s)|λ0 for all s∈[0,1]and all k n,computable upper bounds for the adiabatic approximation errors between the exact solution|ψT(t)and the adiabatic approximation solution|ψadi T(t)to the Schr¨odinger equation i|˙ψT(t)=HT(t)|ψT(t)with the initial condition|ψT(0)=|En(0)are given in terms of fidelity and distance,respectively.As an application,it is proved that when the total evolving time T goes to infinity,|ψT(t)-|ψadi T(t)converges uniformly to zero,which implies that|ψT(t)≈|ψadi T(t)for all t∈[0,T]provided that T is large enough.     

Violation apparente de la symétrie de charge dans les réactions de fusion D(d, p)T et D(d, n)3He aux énergies stellaires

The existing experimental data concerning the D(d, p)T and D(d, n)³He reactions at stellar energies are analysed using the WKB approximation. The s and p partial waves decomposition of the astrophysical S(E) factor is presented; the anisotropies are also investigated. An interpretation coherent with the charge symmetry of the p wave astrophysical factor, S₁ⁱ(E), and the anisotropy, A_i(E) (i=n, p), for these fusion reactions is given in terms of two known 1⁻ levels in ⁴He at E_x=23.64 MeV (T=1) and 24.25 MeV (T=0). To cite this article: F. Nebia et al., C. R. Physique 3 (2002) 733–739.     

Dynamics of interaction of H2 and D2 with Ni(110) and Ni(110) surfaces

Elastic and direct-inelastic scattering as well as dissociative adsorption and associative desorption of H₂ and D₂ on Ni(110) and Ni(111) surfaces were studied by molecular beam techniques. Inelastic scattering at the molecular potential is dominated by phonon interactions. With Ni(110), dissociative adsorption occurs with nearly unity sticking probability s₀, irrespective of surface temperature T_s and mean kinetic energy normal to the surface 〈 E_⊥ 〉. The desorbing molecules exhibit a cos θ_e angular distribution indicating full thermal accommodation of their translation energy. With Ni(111), on the other hand, s₀ is only about 0.05 if both the gas and the surface are at room temperature. s₀ is again independent of T_s, but increases continuously with 〈 E⊥ 〉 up to a value of ～0.4 for 〈 E⊥ 〉 = 0.12 eV. The cos⁵θ_e angular distribution of desorbing molecules indicates that in this case they carry off excess translational energy. The results are qualitatively rationalized in terms of a two-dimensional potential diagram with an activation barrier in the entrance channel. While the height of this barrier seems to be negligible for Ni(110), it is about 0.1 eV for Ni(111) and can be overcome through high enough translational energy by direct collision. The results show no evidence for intermediate trapping in a molecular “precursor” state on the clean surfaces, but this effect may play a role at finite coverages.     

Reactivity screening of silica

As a screening of the chemical activity of silica [SiO₂/Si(1 0 0)], which is one of the most often used supports for nanostructures, thermal desorption spectroscopy data have been gathered for a variety of gases such as n-nonane, n-hexane, n-butane, iso-butane, ethane, CO₂, CO, O₂, and H/H₂. Whereas, the alkanes with chain lengths larger than three adsorb with large binding energies (E_d = 50-70 kJ/mol), the activity towards the other probe molecules is negligible (<24 kJ/mol) down to adsorption temperatures of 95 K. The adsorption of n- and iso-butane has additionally been studied by molecular beam scattering and follows standard precursor mediated adsorption dynamics.     

The adsorption of PH3 on the Si(111)-(7 × 7) surface: an example of autocatalytic dissociative chemisorption

The dissociative chemisorption of phosphine, PH₃, on the Si(111)-(7 × 7) surface has been examined employing supersonic molecular beam techniques. The initial probability of reaction, S_R,0, has been found to be sensitive to substrate temperature, T_s, where S_R,0 increases sharply by approximately a factor of 4–5 as T_s is increased above 800°C, which corresponds well with the (7 × 7) ↔ “(1 × 1)” phase transition. The reaction probability, S_R, measured as a function of dose for PH₃ reacting on Si(111)-(7 × 7) at T_s < 800°C, exhibits a dramatic increase as the surface is exposed to the PH₃ molecular beam. This unique autocatalytic behavior is consistent with a mechanism in which submonolayer coverages of P(a) are capable of lifting the (7 × 7) reconstruction thus giving rise to a more reactive “(1 × 1)-like” phase. The reaction probability of Si₂H₆ on Si(111)-(7 × 7) is also observed to pass through a maximum with increasing P(a) coverages, and can be explained by considering similar changes in surface structure and reactivity.     

Heat capacity of the Pb<Subscript>5</Subscript>(Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>x</Subscript>)<Subscript>3</Subscript>O<Subscript>11</Subscript> ferroelectric system

The temperature dependences of the molar heat capacity at constant pressure, C_p, of Pb₅(Ge_1?xSi_x)₃O₁₁ crystals with x=0, 0.39, and 0.45 in the range 5–300 K, as well as of their permittivity, dielectric losses, and the pyroelectric effect, have been measured. Experimental data on the temperature behavior of the heat capacity are presented in the form of a sum of two Debye and one Einstein terms, C_p(T)=0.405C_D1(Θ_D1=160 K, T)+0.53C_D2(Θ_D2=750 K, T)+0.046C_E(Θ_E=47 K, T). Besides a peak in the region of the ferroelectric Curie point T_c=450 K for crystals with x=0, the temperature dependences of the heat capacity did not reveal any other pronounced anomalies.     

Solvatochromic Studies of Fluorescein Dianion in N,N-Dimethylformamide/Water and Dimethylsulphoxide/Water Mixtures

Abstract

The absorption, excitation and fluorescence spectra of the fluorescein dianion (FL^2?) in N, N-Dimethylformamide (DMF)/water (H₂O) and Dimethylsulphoxide (DMSO)/H₂O solvent mixtures have been investigated. It is found that the absorption λ_max and emission maxima EX, _nax are both hypsochromic shifted when the H₂O content in the solvent mixtures increases. However, the shoulder peaks λ_s remain constant at 483. 5nm within the range of H₂O mole fraction, x= 0 to 0. 518 in a DMF/H₂O solvent mixture and at 484. 4nm within the range, x=0 to 0. 304, in a DMSO/H₂O mixture. Further increases in H₂O content, cause a hypsochromic shift in λ_s. The molar energies for the electronic transition from the ground state (S₀) to S₁, the first excited singlet state, (E_T(1)) and to S₂, the 2nd excited singlet state, (E_T(2)) of FL^2?in the solvent mixtures are also plotted against x. Linear straight lines and intersection points are observed at x=0. 73 for E_T(1) and at x=0. 51 for E_T(2) in the DMF/H₂O mixture and at x=0. 71 for E_T(1) and at x=0. 31 &; 0. 69 for E_T(2) in the DMSO/H₂O mixture. Hydrogen-bonding stabilization effects are used to explain the above observations. The variation in relative fluorescence quantum yields of S₁ and S₂ of FL^2? with x in the aprotic solvent-H₂O mixtures to FL^2? in pure aprotic solvent are determined and discussed.     

Gas-surface scattering from W(100): Kinetic energy dependence of the effective corrugation

Angular and velocity distributions have been obtained for the scattering of argon and N₂ from a W(100) surface for incidence energies, E_i, in the range 0.03–5.5 eV, and surface temperatures, T_s, from 90 to 1700 K. For E_i < 0.1 eV, we find broad angular distributions (> 60° FWHM) which are relatively insensitive to T_s and velocity distributions which are inconsistent with parallel momentum conservation, indicating a relatively corrugated interaction potential. Increasing E_i first causes a rapid narrowing in these distributions, but as E_i exceeds ~ 2 eV, they broaden again, as the effective corrugation again becomes large.     

Adsorption of thiophene on silica-supported Mo clusters

The adsorption/decomposition kinetics/dynamics of thiophene has been studied on silica-supported Mo and MoS_x clusters. Two-dimensional cluster formation at small Mo exposures and three-dimensional cluster growth at larger exposures would be consistent with the Auger electron spectroscopy (AES) data. Thermal desorption spectroscopy (TDS) indicates two reaction pathways. H₄C₄S desorbs molecularly at 190–400 K. Two TDS features were evident and could be assigned to molecularly on Mo sites, and S sites adsorbed thiophene. Assuming a standard preexponential factor (ν = 1 × 10¹³/s) for first-order kinetics, the binding energies for adsorption on Mo (sulfur) sites amount to 90 (65) kJ/mol for 0.4 ML Mo exposure and 76 (63) kJ/mol for 2 ML Mo. Thus, smaller clusters are more reactive than larger clusters for molecular adsorption of H₄C₄S. The second reaction pathway, the decomposition of thiophene, starts at 250 K. Utilizing multimass TDS, H₂, H₂S, and mostly alkynes are detected in the gas phase as decomposition products. H₄C₄S bond activation results in partially sulfided Mo clusters as well as S and C residuals on the surface. S and C poison the catalyst. As a result, with an increasing number of H₄C₄S adsorption/desorption cycles, the uptake of molecular thiophene decreases as well as the H₂ and H₂S production ceases. Thus, silica-supported sulfided Mo clusters are less reactive than metallic clusters. The poisoned catalyst can be partially reactivated by annealing in O₂. However, Mo oxides also appear to form, which passivate the catalyst further. On the other hand, while annealing a used catalyst in H/H₂, it is poisoned even more (i.e., the S AES signal increases). By means of adsorption transients, the initial adsorption probability, S₀, of C₄H₄S has been determined. At thermal impact energies (E_i = 0.04 eV), S₀ for molecular adsorption amounts to 0.43 ± 0.03 for a surface temperature of 200 K. S₀ increases with Mo cluster size, obeying the capture zone model. The temperature dependence of S₀(T_s) consists of two regions consistent with molecular adsorption of thiophene at low temperatures and its decomposition above 250 K. Fitting S₀(T_s) curves allows one to determine the bond activation energy for the first elementary decomposition step of C₄H₄S, which amounts to (79 ± 2) kJ/mol and (52 ± 4) kJ/mol for small and large Mo clusters, respectively. Thus, larger clusters are more active for decomposing C₄H₄S than are smaller clusters.     

Passenger's fluctuation and chaos on ferryboats

We study the fluctuation of shipping passengers on a few ferryboats which shuttle between an origin and a destination repeatedly. We present the dynamical model for the ferryboats. The model is described in terms of nonlinear maps defined from the vectors T_i(n) and W_i(n), i=1, 2, …, N for N ferryboats where T_i(n) is the arrival time of ferryboat i at the origin on trip n and W_i(n) the number of passengers waiting at the origin on trip n. We clarify the fluctuations of shipping passengers and tour time for the ferry schedule. It is found that the dynamical transitions among the regular, periodic, and chaotic motions occur by varying the ferry's capacity F_max, headway T_min, and loading parameter ΓΠ. Even if the second ferryboat follows the leader (first ferryboat) keeping the constant headway, the passengers shipping on the second fluctuate highly when the parameters take specific values.     

Contribution of polaron hopping to the electron paramagnetic resonance linewidth in La1−xCaxMnO3 and related materials

We analyze the contribution of polaron hopping to the electron paramagnetic resonance linewidth in La_1−xCa_xMnO₃ and related materials. The material is assumed to be in the paramagnetic phase and the conductivity is associated with the activated polaron hopping. It is also assumed that the adiabatic, small polaron picture is appropriate so that the conductivity varies as exp[−E_a/T]/T, where E_a denotes the polaron activation energy. The polaron contribution to the linewidth is given by the expression C[χ₀(T)/χ(T)]exp[−E_a/T] where χ₀(T) is the Curie susceptibility (∼1/T), χ(T) is the measured susceptibility and C is a material-dependent parameter. Various experimental studies reporting polaron contributions to the linewidth are discussed. It is pointed out that fitting the linewidth to the functional form ΔH₀+(A/T)exp[−E_a/T] is not physically justified. In the high temperature–mean field regime, the exchange narrowed width, (1−Θ/T)k(∞), where Θ is the paramagnetic Curie temperature, replicates the exponential functional form with reasonable values for the activation energy. From previous measurements of the conductivity that showed activated polaron hopping as the leading transport mechanism, we concluded that the linewidth in La_0.7Ca_0.3MnO₃ is a sum of exchange narrowing and one-phonon spin–lattice terms with no evidence of a contribution from polaron hopping or band transport as had been previously proposed. A similar conclusion is reached for La_0.8Ca_0.2MnO₃, nanometer-sized La_0.9Ca_0.1MnO₃, and La_0.9Te_0.1MnO₃.     

Electric field dependence of the thermal conductivity of a granular superconductor: Giant field-induced effects predicted

The temperature and electric-field dependence of the electronic contribution to the thermal conductivity (TO) of a granular superconductor is considered within a 3D model of inductive Josephson junction arrays. In addition to a low-temperature maximum of zero-field TC κ(T, 0) (controlled by mutual inductance L ₀ and normal state resistivity R _n), the model predicts the two major effects in the applied electric field: (i) the decrease in the linear TC and (ii) the giant enhancement of the nonlinear (i.e. ∇T-dependent) TC with Δκ(T, E)/κ(T, 0), reaching 500% for parallel electric fields E≃ E _T(E _T=S ₀|∇T| is an “intrinsic” thermoelectric field). The possibility of experimentally observing the predicted effects in granular superconductors is discussed.     

Criteria for selection of components for surrogates of natural gas and transportation fuels

The present paper addressed the production of soot precursors, acetylene, benzene and higher aromatics, by the paraffinic (n-, iso-, and cyclo-) and aromatic components in fuels. To this end, a normal heptane mechanism compiled from sub-models in the literature was extended to large normal-, iso-, and cyclo-paraffins by assigning generic rates to reactions involving paraffins, olefins, and alkyl radicals in the same reaction class. Lumping was used to develop other semi-detailed sub-models. The resulting mechanism for components of complex fuels (named the Utah Surrogate Mechanism) includes detailed sub-models of n-butane, n-hexane, n-heptane, n-decane, n-dodecane, n-tetradecane and n-hexadecane, and semi-detailed sub-models of i-butane, i-pentane, n-pentane, 2,4-dimethyl pentane, i-octane, 2,2,3,3-tetramethyl butane, cyclohexane, methyl cyclohexane, tetralin, 2-methyl 1-butene, 3-methyl 2-pentene and aromatics. Generic rates of reaction classes were found adequate to generate reaction mechanisms of large paraffinic components. The predicted maximum concentrations of the fuel, oxidizer, and inert species, major products and important combustion intermediates, which include critical radicals and soot precursors, were in good agreement with the experimental data of three premixed flames of composite fuels under various conditions. The relative importance in benzene formation of each component in the kerosene surrogate was found to follow the trend aromatics > cyclo-paraffins > iso-paraffins > normal-paraffins. In contrast, acetylene formation is not that sensitive to the fuel chemical structure. Therefore, in formulation of surrogate fuels, attention should be focused on selecting components that will yield benzene concentrations comparable to those produced by the fuel, with the assurance that the acetylene concentration will also be well approximated.     

Study of the field dependence of the magnetocaloric effect in Nd1.25Fe11Ti: A multiphase magnetic system

The field dependence of magnetic entropy change ΔS_M(T,H) has been studied in the crystalline sample Nd_1.25Fe₁₁Ti, a multiphase system constituted by three phases: Fe₁₇Nd₂, Fe₇Nd and Fe₁₁TiNd. The magnetic entropy change has been calculated from the numerical derivative of magnetization curves M(T,H) with respect to temperature and subsequent integration in field. To determine the field dependence of the experimental ΔS_M, a local exponent n(T,H) can be calculated from the logarithmic derivative of the magnetic entropy change vs. field. In contrast with the results for single phase materials, where n at the Curie temperature T_C is field independent, it is shown that for a multiphase system n evolves with field both at the Curie temperature of the system and the Curie temperatures of the constituent phases. This is in agreement with numerical simulations using the Arrott-Noakes equation of state.