Vibrations, coverage, and lateral order of atomic nitrogen and formation of NH₃ on Ru(10̅̅10)

The dissociative chemisorption of nitrogen on the Ru(10̅10) surface has been studied using high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS) and low-energy electron diffraction (LEED). To prepare a surface covered by atomic nitrogen we have used ionization-gauge assisted adsorption. A saturation coverage of θN=0.6 is achieved of which about 30% is in the subsurface region. At saturation coverage a (-1/2 1/1) pattern is observed. Then v ǁ(Ru–N) mode at 41 meV and the v_l_(Ru–N) mode at 60 meV are identified. Upon exposing the nitrogen covered surface to hydrogen at 300 K we have observed the formation of NH3 which is characterized by its symmetric bending mode δs(NH3) at 149 meV. At 400 K, NH3 could not be detected. The reaction intermediate NH is stable up to 450 K and has been identified by its vibrational losses ν(Ru–NH) at 86 meV, and ν(N–H) at 408 meV. The TD spectra of mass 14 show three desorption states of nitrogen, Nα at 740 K (from subsurface N), Nβ shifting from 690 to 640 K with increasing coverage, and Nϒ at 550 K. The activation energy for desorption via the Nβ state is 120±10 kJ/mol. The TD spectra of mass two showed three desorption states at 450, 550, and 650 K due to the decomposition of NHx

Coverage, lateral order, and vibrations of atomic nitrogen on Ru(0001)

The N/Ru(0001) system was studied by thermal desorption spectroscopy (TDS), low‐energy electron diffraction (LEED), and high‐resolution electron energy‐loss spectroscopy (HREELS). Atomic nitrogen was prepared by NH3 decomposition at sample temperatures decreasing from 500 to 350 K during NH3 exposure. A maximum N coverage of θN=0.38 could thus be achieved. ∛, split 2×2 and 2×2 LEED patterns were observed for decreasing θN. After NH3 decomposition and before annealing the sample to a temperature above 400 K, the surface is composed of adsorbed N, H, and NH species. This composite layer exhibits a split ∛ LEED pattern due to domains of size 4 with heavy walls. This phase decays through dissociation of NH leading to sharp first‐order type desorption peaks of H2 and N2. From the weak intensity of the ν(Ru–NH) stretch mode it is concluded that NH is adsorbed at threefold‐hollow sites. The energy of the ν(Ru–N) mode shifts from 70.5 to 75.5 meV when θN is increased from 0.25 to 0.38

The Adsorption of Atomic Nitrogen on Ru(0001): Geometry and Energetics

The local adsorption geometries of the (2x2)-N and the (sqrt(3)x sqrt(3))R30^o -N phases on the Ru(0001) surface are determined by analyzing low-energy electron diffraction (LEED) intensity data. For both phases, nitrogen occupies the threefold hcp site. The nitrogen sinks deeply into the top Ru layer resulting in a N-Ru interlayer distance of 1.05 AA and 1.10 AA in the (2x2) and the (sqrt(3)x sqrt(3))R30^o unit cell, respectively. This result is attributed to a strong N binding to the Ru surface (Ru--N bond length = 1.93 AA) in both phases as also evidenced by ab-initio calculations which revealed binding energies of 5.82 eV and 5.59 eV, respectively.Comment: 17 pages, 5 figures. Submitted to Chem. Phys. Lett. (October 10, 1996

Sticking coefficient for dissociative adsorption of N₂ on Ru single‐crystal surfaces

The dissociative chemisorption of N2 on Ru(0001), Ru(101̄0), and Ru(112̄1) surfaces at 300 K was studied by means of high‐resolution electron energy loss spectroscopy and thermal desorption spectroscopy. The initial sticking coefficient was determined to s0=(1±0.8)×10−12, within the limits of error independent of surface orientation. On Ru(101̄0) and Ru(112̄1) small amounts of N can be dissolved into the subsurface region

Diffusion on a solid surface: Anomalous is normal

We present a numerical study of classical particles diffusing on a solid surface. The particles' motion is modeled by an underdamped Langevin equation with ordinary thermal noise. The particle-surface interaction is described by a periodic or a random two dimensional potential. The model leads to a rich variety of different transport regimes, some of which correspond to anomalous diffusion such as has recently been observed in experiments and Monte Carlo simulations. We show that this anomalous behavior is controlled by the friction coefficient, and stress that it emerges naturally in a system described by ordinary canonical Maxwell-Boltzmann statistics

Three-dimensional Models of Core-collapse Supernovae From Low-mass Progenitors With Implications for Crab

We present 3D full-sphere supernova simulations of non-rotating low-mass (~9 Msun) progenitors, covering the entire evolution from core collapse through bounce and shock revival, through shock breakout from the stellar surface, until fallback is completed several days later. We obtain low-energy explosions [~(0.5-1.0)x 10^{50} erg] of iron-core progenitors at the low-mass end of the core-collapse supernova (LMCCSN) domain and compare to a super-AGB (sAGB) progenitor with an oxygen-neon-magnesium core that collapses and explodes as electron-capture supernova (ECSN). The onset of the explosion in the LMCCSN models is modelled self-consistently using the Vertex-Prometheus code, whereas the ECSN explosion is modelled using parametric neutrino transport in the Prometheus-HOTB code, choosing different explosion energies in the range of previous self-consistent models. The sAGB and LMCCSN progenitors that share structural similarities have almost spherical explosions with little metal mixing into the hydrogen envelope. A LMCCSN with less 2nd dredge-up results in a highly asymmetric explosion. It shows efficient mixing and dramatic shock deceleration in the extended hydrogen envelope. Both properties allow fast nickel plumes to catch up with the shock, leading to extreme shock deformation and aspherical shock breakout. Fallback masses of <~5x10^{-3} Msun have no significant effects on the neutron star (NS) masses and kicks. The anisotropic fallback carries considerable angular momentum, however, and determines the spin of the newly-born NS. The LMCCSNe model with less 2nd dredge-up results in a hydrodynamic and neutrino-induced NS kick of >40 km/s and a NS spin period of ~30 ms, both not largely different from those of the Crab pulsar at birth.Comment: 47 pages, 27 figures, 6 tables; minor revisions, accepted by MNRA

A LEED structural analysis of the Co(100) surface

The structure of the clean Co(1010) surface has been analysed by LEED. Application of a recently developed computational scheme reveals the prevalence of the termination A in which the two topmost layers exhibit a narrow spacing of 0.62 Å, corresponding to a 12.8(±0.5)% contraction with respect to the bulk value, while the spacing between the second and third layer is slightly expanded by 0.8(±0.2)%

Emission of exoelectrons during oxidation of Cs via thermal activation of a metastable O^-₂ surface species

Exposure of Cs surfaces to O2 causes the emission of exoelectrons. With a Cs monolayer on Ru(0001) the maximum yield is observed with an already partly oxidized surface on which a metastable O-2 species could be identified. Thermally activated transformation (with an activation energy of 0.8 eV) of this phase leads to dissociation accompanied by exoelectron emission via Auger deexcitation

All stationary axi-symmetric local solutions of topologically massive gravity

We classify all stationary axi-symmetric solutions of topologically massive gravity into Einstein, Schr\"odinger, warped and generic solutions. We construct explicitly all local solutions in the first three sectors and present an algorithm for the numerical construction of all local solutions in the generic sector. The only input for this algorithm is the value of one constant of motion if the solution has an analytic centre, and three constants of motion otherwise. We present several examples, including soliton solutions that asymptote to warped AdS.Comment: 42 pages, 9 figures. v2: Changed potentially confusing labelling of one sector, added references. v3: Minor changes, matches published versio