First Principles Study of Adsorption of O2O_{2} on Al Surface with Hybrid Functionals

Adsorption of $O_{2}$ molecule on Al surface has been a long standing puzzle for the first principles calculation. We have studied the adsorption of $O_{2}$ molecule on the Al(111) surface using hybrid functionals. In contrast to the previous LDA/GGA, the present calculations with hybrid functionals successfully predict that $O_{2}$ molecule can be absorbed on the Al(111) surface with a barrier around 0.2$\thicksim$0.4 eV, which is in good agreement with experiments. Our calculations predict that the LUMO of $O_{2}$ molecule is higher than the Fermi level of the Al(111) surface, which is responsible for the barrier of the $O_{2}$ adsorption.Comment: 14 pages, 5 figure

The shortcomings of semi-local and hybrid functionals: what we can learn from surface science studies

A study of the adsorption of CO on late 4d and $5d$ transition metal (111) surfaces (Ru, Rh, Pd, Ag, Os, Ir, and Pt) considering atop and hollow site adsorption is presented. The applied functionals include the gradient corrected PBE and BLYP functional, and the corresponding hybrid Hartree-Fock density functionals HSE and B3LYP. We find that PBE based hybrid functionals (specifically HSE) yield, with the exception of Pt, the correct site order on all considered metals, but they also considerably overestimate the adsorption energies compared to experiment. On the other hand, the semi-local BLYP functional and the corresponding hybrid functional B3LYP yield very satisfactory adsorption energies and the correct adsorption site for all surfaces. We are thus faced with a Procrustean problem: the B3LYP and BLYP functionals seem to be the overall best choice for describing adsorption on metal surfaces, but they simultaneously fail to account well for the properties of the metal, vastly overestimating the equilibrium volume and underestimating the atomization energies. Setting out from these observations, general conclusions are drawn on the relative merits and drawbacks of various semi-local and hybrid functionals. The discussion includes a revised version of the PBE functional specifically optimized for bulk properties and surface energies (PBEsol), a revised version of the PBE functional specifically optimized to predict accurate adsorption energies (rPBE), as well as the aforementioned BLYP functional. We conclude that no semi-local functional is capable to describe all aspects properly, and including non-local exchange also only improves some, but worsens other properties.Comment: 12 pages, 6 figures; to be published in New Journal of Physic

Alpha-induced reactions for the astrophysical p-process: the case of 151Eu

The cross sections of 151Eu(alpha,gamma)155Tb and 151Eu(alpha,n)154Tb reactions have been measured with the activation method. Some aspects of the measurement are presented here to illustrate the requirements of experimental techniques needed to obtain nuclear data for the astrophysical p-process nucleosynthesis. Preliminary cross section results are also presented and compared with the predictions of statistical model calculations.Comment: Accepted for publication in Journal of Physics Conference Series, proceeding of the Nuclear Physics in Astrophysics IV. conferenc

Alpha-induced cross sections of 106Cd for the astrophysical p-process

The 106Cd(alpha,gamma)110Sn reaction cross section has been measured in the energy range of the Gamow window for the astrophysical p-process scenario. The cross sections for 106Cd(alpha,n)109Sn and for 106Cd(alpha,p)109In below the (alpha,n) threshold have also been determined. The results are compared with predictions of the statistical model code NON-SMOKER using different input parameters. The comparison shows that a discrepancy for 106Cd(alpha,gamma)110Sn when using the standard optical potentials can be removed with a different alpha+106Cd potential. Some astrophysical implications are discussed.Comment: 10 pages, 9 figures, accepted for publication in Phys. Rev

Hydrogen Production From Photolysis of Steam Adsorbed Onto Platinized SrTiO{Sub 3}

Photodissociation of high presssure (1-8 atm) water vapor adsorbed onto the partially platinized surface of single crystal SrTiO{sub 3} has been observed. The reaction is catalytic; several thousands of monolayers of hydrogen have been produced. The reaction requires light of greater than bandgap (3.1 eV) energy, and proceeds vigorously only if elevated temperatures (375- 450 K) and high pressures are used. Recombination of products and poisoning of the metal appears to diminish the reaction rate after several hours. This reaction differs from the photolysis of water in the analogous short-circuit electrochemical cell in that we use no electrolyte and we are able to use non-conducting, stoichiometric SrTiO{sub 3} because bulk electrical conductivity has little influence on localized surface reactions. We have observed evidence for the following steps during photolysis of water on platinized SrTiO{sub 3} : (1) generation of electron-hole pairs by light of greater than bandgap energy, (2) dissociative chemisorption of water vapor on the hydroxylated semiconductor surface, (3) diffusion of a surface hydrogen species to platinum islands and desorption of hydrogen from the islands, and (4) recombination of hydrogen and oxygen which limits the amount of hydrogen that can be generated in a batch reactor

Test of statistical model cross section calculations for α -induced reactions on Ag 107 at energies of astrophysical interest

Background: Astrophysical reaction rates, which are mostly derived from theoretical cross sections, are necessary input to nuclear reaction network simulations for studying the origin of p nuclei. Past experiments have found a considerable difference between theoretical and experimental cross sections in some cases, especially for (α,γ) reactions at low energy. Therefore, it is important to experimentally test theoretical cross section predictions at low, astrophysically relevant energies. Purpose: The aim is to measure reaction cross sections of Ag107(α,γ)In111 and Ag107(α,n)In110 at low energies in order to extend the experimental database for astrophysical reactions involving α particles towards lower mass numbers. Reaction rate predictions are very sensitive to the optical model parameters and this introduces a large uncertainty into theoretical rates involving α particles at low energy. We have also used Hauser-Feshbach statistical model calculations to study the origin of possible discrepancies between prediction and data. Method: An activation technique has been used to measure the reaction cross sections at effective center of mass energies between 7.79 MeV and 12.50 MeV. Isomeric and ground state cross sections of the (α,n) reaction were determined separately. Results: The measured cross sections were found to be lower than theoretical predictions for the (α,γ) reaction. Varying the calculated averaged widths in the Hauser-Feshbach model, it became evident that the data for the (α,γ) and (α,n) reactions can only be simultaneously reproduced when rescaling the ratio of γ to neutron width and using an energy-dependent imaginary part in the optical α+Ag107 potential. Conclusions: The new data extend the range of measured charged-particle cross sections for astrophysical applications to lower mass numbers and lower energies. The modifications in the model predictions required to reproduce the present data are fully consistent with what was found in previous investigations. Thus, our results confirm the previously suggested energy-dependent modification of the optical α+nucleus potential.Peer reviewedFinal Accepted Versio

Nucleosynthesis in Massive Stars With Improved Nuclear and Stellar Physics

We present the first calculations to follow the evolution of all stable nuclei and their radioactive progenitors in stellar models computed from the onset of central hydrogen burning through explosion as Type II supernovae. Calculations are performed for Pop I stars of 15, 19, 20, 21, and 25 M_sun using the most recently available experimental and theoretical nuclear data, revised opacity tables, neutrino losses, and weak interaction rates, and taking into account mass loss due to stellar winds. A novel ``adaptive'' reaction network is employed with a variable number of nuclei (adjusted each time step) ranging from about 700 on the main sequence to more than 2200 during the explosion. The network includes, at any given time, all relevant isotopes from hydrogen through polonium (Z=84). Even the limited grid of stellar masses studied suggests that overall good agreement can be achieved with the solar abundances of nuclei between 16O and 90Zr. Interesting discrepancies are seen in the 20 M_sun model and, so far, only in that model, that are a consequence of the merging of the oxygen, neon, and carbon shells about a day prior to core collapse. We find that, in some stars, most of the ``p-process'' nuclei can be produced in the convective oxygen burning shell moments prior to collapse; in others, they are made only in the explosion. Serious deficiencies still exist in all cases for the p-process isotopes of Ru and Mo.Comment: 53 pages, 17 color figures (3 as separate GIF images), slightly extended discussion and references, accepted by Ap

Surface energy and stability of stress-driven discommensurate surface structures

A method is presented to obtain {\it ab initio} upper and lower bounds to surface energies of stress-driven discommensurate surface structures, possibly non-periodic or exhibiting very large unit cells. The instability of the stressed, commensurate parent of the discommensurate structure sets an upper bound to its surface energy; a lower bound is defined by the surface energy of an ideally commensurate but laterally strained hypothetical surface system. The surface energies of the phases of the Si(111):Ga and Ge(111):Ga systems and the energies of the discommensurations are determined within $\pm 0.2$ eV.Comment: 4 pages RevTeX. 2 Figures not included. Ask for a hard copy (through regular mail) to [email protected]

Odd p isotope 113In: Measurement of alpha-induced reactions

One of the few p nuclei with an odd number of protons is 113In. Reaction cross sections of 113In(alpha,gamma)117Sb and 113In(alpha,n)116Sb have been measured with the activation method at center-of-mass energies between 8.66 and 13.64 MeV, close to the astrophysically relevant energy range. The experiments were carried out at the cyclotron accelerator of ATOMKI. The activities were determined by off-line detection of the decay gamma rays with a HPGe detector. Measured cross sections and astrophysical S factor results are presented and compared with statistical model calculations using three different alpha+nucleus potentials. The comparison indicates that the standard rates used in the majority of network calculations for these reactions were too fast due to the energy dependence of the optical alpha potential at low energy.Comment: 9 pages, 9 figures, Accepted for publication in Phys. Rev. C. Minor changes mad