Spatial distributions of desorbing products in steady-state NO and N2O reductions on Pd(110).
The angular and velocity distributions of desorbing product N(2) were examined over the crystal azimuth in steady-state NO+CO and N(2)O+CO reactions on Pd(110) by cross-correlation time-of-flight techniques. At surface temperatures below 600 K, N(2) desorption in both reactions splits into two directional lobes collimated along 41 degrees -45 degrees from the surface normal toward the [001] and [001] directions. Above 600 K, the normally directed N(2) desorption is enhanced in the NO reduction. Each product desorption component, as well as CO(2), shows a fairly asymmetric distribution about its collimation axis. Two factors, i.e., the anisotropic site structures and the reactant orientation and movements, are operative to induce such asymmetry, depending on the product emission mechanism.
Interactions of Pd(110) with (N2+N)-EELS, AES and LEED studies
The interactions of a Pd(110) surface with (N2+N) have been studied by means of high-resolution vibrational electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). At 300 K, double- and triple-bonded N2 admolecules and N adatoms (located most probably in the long-bridge, short-bridge and on-top sites of the unreconstructed Pd(110) surface) are formed; some N atoms penetrate into the “bulk”. The double- and triple-bonded N2 admolecules are identified by the N-N stretching energies of 199 and 265 meV, respectively; the N adatoms in the long-bridge, short-bridge and on-top sites are identified by the Pd-N stretching energies of 55, 90 and ∼130 meV, respectively. By heating at ∼500 K, the triple-bonded N2 and most of the double-bonded N2 admolecules are desorbed. The Pd clean surface is recovered by heating at ∼900 K. The (2×3)-N structure is formed by a large (N2+N) exposure followed by heating at ∼500–800 K or by a large N2 exposure at ∼700 K. It is proposed that N atoms located in the long-bridge sites of the “distorted” Pd(110) surface form the (2×3)-N structure. Molecular nitrogen is not adsorbed on the (2×3)-N surface at 300 K.
The Invariance of the Vacuum is the Invariance of the World
The following theorem is demonstrated: If the vacuum is invariant under the group generated by the space integral of the time component of a local vector current, then the Hamiltonian is invariant also. A similar theorem holds for the group generated by the space integral of a space component of a local axial-vector current.