Anomalously hindered E2 strength B(E2;2+(1)-->0+) in 16C

The electric quadrupole transition from the first 2(+) state to the ground 0(+) state in 16C is studied through measurement of the lifetime by a recoil shadow method applied to inelastically scattered radioactive 16C nuclei. The measured mean lifetime is 77+/-14(stat)+/-19(syst) ps. The central value of mean lifetime corresponds to a B(E2;2+(1)-->0(+)) value of 0.63e(2) fm(4), or 0.26 Weisskopf units. The transition strength is found to be anomalously small compared to the empirically predicted value.     

Observation of B+-->a1+(1260)K0 and B0-->a1-(1260)K+

We present branching fraction measurements of the decays B(+)-->a(1)(+)(1260)K(0) and B(0)-->a(1)(-)(1260)K(+) with a(1)(+/-)(1260)-->pi(-/+)pi(+/-)pi(+/-). The data sample corresponds to 383 x 10(6) BB pairs produced in e(+)e(-) annihilation through the Upsilon(4S) resonance. We measure the products of the branching fractions B(B(+)-->a(1)(+)(1260)K(0)B(a(1)(+)(1260)-->pi(-)pi(+)pi(+))=(17.4+/-2.5+/-2.2) x 10(-6) and B(B(0)-->a(1)(-)(1260)K(+)B(a(1)(-)(1260)-->pi(+)pi(-)pi(-)) = (8.2+/-1.5+/-1.2) x 10(-6). We also measure the charge asymmetries A(ch)(B(+)-->a(1)(+)(1260)K(0) = 0.12+/-0.11+/-0.02 and A(ch)(B(0)-->a(1)(-)(1260)K+) = -0.16+/-0.12+/-0.01. The first uncertainty quoted is statistical and the second is systematic.     

Effect of hydrogenation on B/Si(0 0 1)-(1 × 2)

Ab initio calculations, based on pseudopotentials and density functional theory, have been performed to investigate the effect of hydrogenation on the atomic geometries and the energetics of substitutional boron on the generic Si(0 0 1)-(1 × 2) surface. For a single B atom substitution corresponding to 0.5 ML coverage, we have considered two different sites: (i) the mixed Si-B dimer structure and (ii) boron substituting for the second-layer Si to form Si-B back-bond structure, which is energetically more favorable than the mixed Si-B dimer by 0.1 eV/dimer. However, when both of these cases are passivated by hydrogen atoms, the situation is reversed and the Si-B back-bond case becomes 0.1 eV/dimer higher in energy than the mixed Si-B dimer case. For the B incorporation corresponding to 1 ML coverage, among the substitutional cases, 100% interdiffusion into the third layer of Si and 50% interdiffusion into the second layer of Si are energetically similar and more favorable than the other cases that are considered. However, when the surface is passivated with hydrogen, the B atoms energetically prefer to stay at the third layer of the Si substrate.     

Anharmonic effects on B2-FeAl(1 1 0) surface: A molecular dynamics study

Using molecular dynamics simulations and the analytic embedded-atom method (AEAM), the surface anharmonicity of B2-FeAl(1 1 0) has been studied in the temperature range from 0 K to 1400 K. The temperature dependence of the interlayer spacing, mean square vibrational amplitudes, surface phonon frequencies and line-widths, and layer structure factor have been calculated. The obtained results indicate that the anharmonic effects are small in the temperature range from 0 K to 900 K. The temperature dependences of the interlayer spacing indicates that the rippling effect of the B2-FeAl(1 1 0) surface is exhibited by the contraction of Fe surface atoms and the expansion of Al atoms, which persists at high temperatures. The temperature dependence of the layer structure factors shows that the B2-FeAl(1 1 0) surface does not disorder until the temperature of 1300 K.     

Composition and structure of chemically prepared GaAs(1 1 1)A and (1 1 1)B surfaces

The (1 1 1)A and (1 1 1)B surfaces of GaAs chemically treated in HCl-isopropanol solution (HCl-iPA) and annealed in vacuum were studied by means of X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS). To avoid uncontrolled contamination, chemical treatment and sample transfer into UHV were performed under pure nitrogen atmosphere. The HCl-iPA treatment removes gallium and arsenic oxides, with about 0.5-3 ML of elemental arsenic being left on the surface, depending on the crystallographic orientation. With the increase of the annealing temperature, a sequence of reconstructions were identified by LEED: (1 × 1) and (2 × 2) on the (1 1 1)A surface and (1 × 1), (2 × 2), (1 × 1), (3 × 3), (√19 × √19) on the (1 1 1)B surface. These sequences of reconstructions correspond to the decrease of surface As concentration. The structural properties of chemically prepared GaAs(1 1 1) surfaces were found to be similar to those obtained by decapping of As-capped epitaxial layers.     

Measurement of the ratios of branching fractions B(B0s --> Ds- pi+ pi+ pi-)/B(B0-->D- pi+ pi+ pi-) and B(B0s --> Ds- pi+)/B(B0-->D- pi+)

Using 355 pb;{-1} of data collected by the CDF II detector in pp[over ] collisions at sqrt[s]=1.96 TeV at the Fermilab Tevatron, we study the fully reconstructed hadronic decays B_{(s)};{0}-->D_{(s)};{-}pi;{+} and B_{(s)};{0}-->D_{(s)};{-}pi;{+}pi;{+}pi;{-}. We present the first measurement of the ratio of branching fractions B(B_{s};{0}-->D_{s};{-}pi;{+}pi;{+}pi;{-})/B(B;{0}-->D;{-}pi;{+}pi;{+}pi;{-})=1.05+/-0.10(stat)+/-0.22(syst). We also update our measurement of B(B_{s};{0}-->D_{s};{-}pi;{+})/B(B;{0}-->D;{-}pi;{+}) to 1.13+/-0.08(stat)+/-0.23(syst), improving the statistical uncertainty by more than a factor of 2. We find B(B_{s};{0}-->D_{s};{-}pi;{+})=[3.8+/-0.3(stat)+/-1.3(syst)]x10;{-3} and B(B_{s};{0}-->D_{s};{-}pi;{+}pi;{+}pi;{-})=[8.4+/-0.8(stat)+/-3.2(syst)]x10;{-3}.     

The surface structure of BaO on Pt(1 1 1): (2 × 2)-reconstructed BaO(1 1 1)

The surface structure of BaO(1 1 1) has been determined using STM and computer modelling. The BaO(1 1 1) surface was prepared in thin film form on Pt(1 1 1) and presents a surface with twice the lattice parameter expected for that of the bulk termination, i.e. a (2 × 2) reconstruction. Computer modelling indicates that the bulk termination is unstable, but that the (2 × 2) reconstructed BaO(1 1 1) surface has a low surface energy and is hence a stable surface reconstruction. The (2 × 2) reconstruction consists of small, three-sided pyramids with (1 0 0) oriented sides and either oxygen or barium ions at the apices. Less regular surface reconstructions containing the same pyramids are almost equally stable, indicating that we may also expect less regular regions to appear with a fairly random distribution of these surface species. The simulations further suggest that a regular (4 × 4) reconstruction built up of bigger pyramids is even more energetically favourable, and some evidence is found for such a structure in the STM.     

High resolution analysis of the rotational levels of the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states of SO2

A high resolution (0.0018 cm⁻¹) Fourier transform instrument has been used to record the spectrum of an enriched ³⁴S (95.3%) sample of sulfur dioxide. A thorough analysis of the ν₂, 2ν₂ − ν₂, ν₁, ν₁ + ν₂ − ν₂, ν₃, ν₂ + ν₃ − ν₂, ν₁ + ν₂ and ν₂ + ν₃ bands has been carried out leading to a large set of assigned lines. From these lines ground state combination differences were obtained and fit together with the existing microwave, millimeter, and terahertz rotational lines. An improved set of ground state rotational constants were obtained. Next, the upper state rotational levels were fit. For the (0 1 0), (1 1 0) and (0 1 1) states, a simple Watson-type Hamiltonian sufficed. However, it was necessary to include explicitly interacting terms in the Hamiltonian matrix in order to fit the rotational levels of the (0 2 0), (1 0 0) and (1 0 1) states to within their experimental accuracy. More explicitly, it was necessary to use a ΔK = 2 term to model the Fermi interaction between the (0 2 0) and (1 0 0) levels and a ΔK = 3 term to model the Coriolis interaction between the (1 0 0) and (0 0 1) levels. Precise Hamiltonian constants were derived for the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states.     

STM study of the Mo(1 1 2) and Mo(1 1 1) surfaces

The Mo(1 1 2) and Mo(1 1 1) surfaces have been studied by STM and DFT/GGA modeling. Due to high quality and cleanness of the surfaces, for the first time good STM images of large fragments of the Mo(1 1 2) and Mo(1 1 1) have been obtained. Lack of atomic resolution in the rows of the Mo(1 1 2) surface is attributed to flatness of distribution of density of the electronic states along the rows. This suggestion is illustrated by comparison of STM images for Mo(1 1 1) and Mo(1 1 2) and model calculations of STM pictures for these surfaces.     

LCAO study of the Cu(1 1 0)-p(2 × 1)O surface

We have performed semi-empirical LCAO calculations of the electronic structure of the Cu(1 1 0)-p(2 × 1)O surface. This has been done accounting for the Cu-Cu interactions by means of a recently proposed set of parameters, which give very good results for the bulk as well as for the surfaces of lowest Miller indices. Furthermore, the O-O interactions, which have been neglected in the preceding similar studies, have been taken into account. The resulting surface bands are in very good agreement with the overall set of the available experimental data. Several issues concerning the physical properties of this surface are addressed in the present paper: the changes induced on the clean surface bands by the adsorption and the reconstruction; the arrangement of the Cu and O atoms in the added rows; the position of the p_y antibonding band of the oxygen. In particular, we have found that the latter has an energy of −0.2 eV at the point. This result confirms an experimental indication in the same direction previously reported by Courths et al. [R. Courths, S. Hüfner, P. Kemkes, G. Wiesen, Surf. Sci. 376 (1997) 43].     

O2 dissociation on Pd(2 1 1) and Cu(2 1 1) surfaces

We have performed ab initio Density Functional Theory (DFT) based calculations to observe the reactivity of the Pd(2 1 1) and Cu(2 1 1) surfaces towards O₂. In order to properly address the adsorption dynamics, the static potential energy surface calculations have been complemented with first principles molecular dynamics calculations, which reveal interesting steering effects that complicate the dissociation dynamics. We have found that on both surfaces the step microfacets are very reactive and the dissociation of the O₂ molecule at room temperature occurs mostly on those sites.     

The CuGaSe2(0 0 1) surface: A (4 × 1) reconstruction

We report on the formation of a stable (4 × 1) reconstruction of the chalcopyrite CuGaSe₂(0 0 1) surface. Using Ar⁺ ion-bombardment and annealing of epitaxial CuGaSe₂ films grown on GaAs(0 0 1) substrates it was possible to obtain flat, well-ordered surfaces showing a clear (4 × 1) reconstruction. The cleanliness and structure were analyzed in situ by AES and LEED. AES data suggest a slight Se-enrichment and Cu-depletion upon surface preparation. Our results demonstrate that (0 0 1) surfaces of the Cu-III-VI₂(0 0 1) material can show stable, unfacetted surfaces.     

Optical solitons in (1 + 1) and (2 + 1) dimensions

This paper addresses the nonlinear Schrödinger's equation that serves as the model to study the propagation of optical solitons through nonlinear optical fibers. The main focus of this paper is the aspect of integrability. There are a couple of integration tools that are employed to obtain the exact solutions to the model. Fan's F-expansion approach is applied to extract several forms of solutions to the model. This integration mechanism displays cnoidal waves, snoidal waves and several other solutions; needless to mention that these solutions, in the limiting case, leads to bright, dark and singular soliton solutions. The study then rolls over to the (2 + 1)-dimensions where, in addition, the semi-inverse variational principle is applied to extract a bright soliton solution, along with the necessary constraint conditions. There is also a display of several numerical simulations.     

Formation and hydrogenation of p(2 × 2)-N on Pt(1 1 1)

The formation of a well-ordered p(2 × 2) overlayer of atomic nitrogen on the Pt(1 1 1) surface and its reaction with hydrogen were characterized with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The p(2 × 2)-N overlayer is formed by exposure of ammonia to a surface at 85 K that is covered with 0.44 monolayer (ML) of molecular oxygen and then heating to 400 K. The reaction between ammonia and oxygen produces water, which desorbs below 400 K. The only desorption product observed above 400 K is molecular nitrogen, which has a peak desorption temperature of 453 K. The absence of oxygen after the 400 K anneal is confirmed with AES. Although atomic nitrogen can also be produced on the surface through the reaction of ammonia with an atomic, rather than molecular, oxygen overlayer at a saturation coverage of 0.25 ML, the yield of surface nitrogen is significantly less, as indicated by the N₂ TPD peak area. Atomic nitrogen readily reacts with hydrogen to produce the NH species, which is characterized with RAIRS by an intense and narrow (FWHM ∼ 4 cm⁻¹) peak at 3322 cm⁻¹. The areas of the H₂ TPD peak associated with NH dissociation and the XPS N 1s peak associated with the NH species indicate that not all of the surface N atoms can be converted to NH by the methods used here.     

Soft X-ray appearance potential spectroscopy (SXAPS) study of TiO2(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces

A soft X-ray appearance potential spectroscopy (SXAPS) apparatus with high sensitivity was built to measure non-derivative spectra. SXAPS spectra (non-derivative) of Ti 2p and O 1s for TiO₂(1 1 0)-1 × 2 and (0 0 1)-1 × 1 surfaces have been measured using low incident currents (about 10 μA/cm²) and a photon counting mode. Density of empty states on Ti and O sites are deduced by self-deconvoluting the spectra. The self-deconvoluted SXAPS spectra are qualitatively similar to those measured by X-ray absorption spectroscopy (XAS). The Ti 2p_3/2 spectrum shows two strong peaks which correspond to t_2g and e_g states. For the O 1s spectrum two strong peaks near the threshold are also found which can be ascribed to O 2pπ and O 2pσ states. These results suggest that the spectra almost obey the dipole selection rule, so-called the “approximate dipole selection rule”. The SXAPS spectra of Ti 2p and O 1s for the (1 1 0) and (0 0 1) surfaces resemble qualitatively, which is consistent with the XAS results. The spectra measured on the (1 1 0)-1 × 2 surface at an incident angle of 45° off normal to the surface and on the (1 1 0) surface sputtered by Ar ions indicate that SXAPS is very sensitive to the surface electronic states.     

para-Sexiphenyl thin film growth on Cu(1 1 0) and Cu(1 1 0)-(2 × 1)O surfaces

We present a reflectance difference spectroscopy (RDS) study of para-sexiphenyl (p-6P) thin film growth on Cu(1 1 0) and Cu(1 1 0)-(2 × 1)O substrates. The RDS spectra show pronounced anisotropies for p-6P films formed on both substrates at room temperature, demonstrating that the molecules are uniaxially aligned within the films. Based on the RD spectra and the evolution of the optical transitions with p-6P coverage the growth mode on both substrates could be identified. From the dominating RDS feature, assigned to the lowest energy HOMO-LUMO transition, the orientation of the molecular chain can be determined. On Cu(1 1 0), the p-6P molecular chains align in the direction, i.e., along the Cu atomic rows, whereas on the Cu(1 1 0)-(2 × 1)O surface, the molecules are oriented in the orthogonal [0 0 1] direction, i.e., along the “added” Cu-O rows of the Cu(1 1 0)-(2 × 1)O surface. The energetic position and line shape of the main RDS feature differs for the two substrates and varies with p-6P coverage. This fine structure is discussed in terms of different molecular conformations, adlayer structure and vibronic replicas.