Orientation and bonding of adsorbed CH3NCO on Pt{110} and Cu{110} from vibrational and photoelectron spectroscopies

The bonding and orientation of CH{in3}NCO on Pt{110} and Cu{110} are studied by HREELS, ARUPS, AES, and LEED. CH{in3}NCO is found to adsorb nondissociatively on both Cu{110} and Pt{110} at T = 160 K, bonding primarily through the 2π a′ orbital with the NCO group lying down on the surface and the methyl group largely unperturbed. We propose that the absence of a strong HREELS band at about 2290 cm^?1, which is the liquid phase frequency for the NCO asymmetric stretching mode, combined with the presence of strong bands between 1000 and 1450 cm^?1 provides a “finger-print” for NCO bonded to the surface in a lying-down configuration.     

Effects of potassium on the reaction pathways of CH2 fragment over Mo2C/Mo(1 0 0)

The adsorption and surface reactions of CH₂I₂ on the K-dosed Mo₂C/Mo(1 0 0) have been studied by high resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy and thermal desorption spectroscopy. Potassium is an effective promoter for the rupture of C-I bond in the adsorbed CH₂I₂. A partial dissociation of this compound occurred even at 100 K and was completed at 190 K at monolayer K coverage. The dissociation was further promoted by the illumination of coadsorbed layer at 100 K. As revealed by HREELS and XPS measurements the primary products of the dissociation are CH₂ and I. Methylene was converted to π-bonded ethylene characterized by T_p = 160 K, and di-σ-ethylene with T_p = 350 K. Other products of the surface reaction are hydrogen and methane. The coupling reaction of CH₂ species was clearly facilitated by potassium. The effect of potassium was explained by the extended electron donation to adsorbed alkyl iodide in one hand, and by the direct interaction between potassium and I on the other hand.     

On the Number of Sums and Products

A new lower bound on max{|A + A|, |A · A|} is given,where A is a finite set of complex numbers. 2000 MathematicsSubject Classification 11B75 (primary), 52C10 (secondary).     

Adsorption of CH3Cl on clean and Cl-dosed Pd(100) surfaces

The adsorption of methyl chloride on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), electron energy loss spectroscopy (in the electronic range, EELS), temperature-programmed desorption (TPD) and work function change. CH₃Cl adsorbs with high sticking probability at 80–100 K. UPS and TDS spectra suggest that the adsorption of CH₃Cl is molecular at 100 K, with a little distortion of the corresponding gas-phase molecular electronic structure. No dissociation of CH₃Cl was observed even up to 550 K. By means of TPD, we distinguished two adsorption states with desorption energies of 46.9 and 33.4 kJ/mol. The formation of a condensed layer at 105–110 K was also observed. Adsorption of CH₃Cl caused a significant work function decrease, Δ = −0.91 eV, indicating a dipole with positive end pointed away from the surface. The effects of electronegative additives, preadsorbed Cl and O were also examined. Preadsorbed Cl caused a slight destabilization of adsorbed CH₃Cl at lower concentration, prevented the adsorption of CH₃Cl at higher concentration and facilitated the formation of a condensed layer. No such effect was experienced in the presence of preadsorbed O.     

On the Reactions Occurring in the CaO?Cr2O3?O2 system

The reactions occurring in the system CaO? Cr₂O₃? O₂ have been investigated. In the mixture of two oxides heated in air to higher temperature calcium chromate and chromite are being formed. The rate and the ratio of the two reactions is very sensitive to the composition of gaseous atmosphere, especially to the partial pressure of oxygen. The formation of spinel was accelerated by a small amount of oxygen but was partly or fully suppressed at a higher oxygen pressure on the expense of chromate formation. The oxidation of CaO? Cr₂O₃ was also studied kinetically. Detailed measurements in different gaseous atmospheres were performed with doped CaO and Cr₂O₃. The results were interpreted in the light of the defect structure of the reacting oxides.     

On the bargaining set, kernel and core of superadditive games

We prove that for superadditive games a necessary and sufficient condition for the bargaining set to coincide with the core is that the monotonic cover of the excess game induced by a payoff be balanced for each imputation in the bargaining set. We present some new results obtained by verifying this condition for specific classes of games. For N-zero-monotonic games we show that the same condition required at each kernel element is also necessary and sufficient for the kernel to be contained in the core. We also give examples showing that to maintain these characterizations, the respective assumptions on the games cannot be lifted. Received: March 1998/Revised version: December 1998     

Adsorption of hydrogen and deuterium on potassium-promoted Pd(100) surfaces

The adsorption of H₂ and D₂ has been studied on clean and K-promoted Pd(100) surfaces using thermal desorption, work function changes, ultraviolet photoelectron and Auger spectroscopy. The potassium adlayer significantly lowers the sticking coefficient (from 0.6 to 0.06 at θ_k = 0.2), and the uptake of hydrogen, but increases the desorption energy for H₂ desorption. Calculation showed that each potassium adatom blocks approximately 4–5 adsorption sites for H₂ adsorption. Atomization of hydrogen led to an increase of hydrogen uptake. The adsorption of potassium on the H-covered surface caused a significant decrease in the amount of hydrogen adsorbed on the surface (as indicated by less desorbing hydrogen below 500 K) and promoted the dissolution of H atoms into the bulk of Pd. The dissolved hydrogen was released only above 600–650 K. In the interpetation of the results the extended charge transfer from K-dosed Pd to the adsorbed H atoms and the direct interaction between adsorbed H and K adatoms are taken into account.     

On a Question of Erdős and Ulam

Ulam asked in 1945 if there is an everywhere dense rational set, i.e., 1 a point set in the plane with all its pairwise distances rational. Erdős conjectured that if a set S has a dense rational subset, then S should be very special. The only known types of examples of sets with dense (or even just infinite) rational subsets are lines and circles. In this paper we prove Erdős’ conjecture for algebraic curves by showing that no irreducible algebraic curve other than a line or a circle contains an infinite rational set.     

Thermal stability of adsorbed CH3NCO on Cu{110} AND Pt{110} from vibrational and photoelectron spectroscopies

The thermal stability of CH₃NCO adsorbed on Cu{110} and Pt{110} is investigated using HREELS, TPD, and ARUPS. CH₃NCO desorbs largely without fragmentation from Cu{110}, but on Pt{110} only about 20% of the adsorbed CH₃NCO desorbs intact, with 80% decomposing on the surface at T > 200 K into CO(a), H(a), CH_x(a), N(a) and NH_y(a) fragments. The kinetics of the surface decomposition were characterised for 220 < T < 300 K by HREELS and the activation energy for CH₃NCO decomposition was found to vary strongly as a function of coverage.