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By studying the adsorption of CO on up to 30 layers of Pt deposited on Ru(0001) the influence of surface strain on the adsorption energy has been disentangled from the residual chemical interaction with the substrate. While the electronic influence of the substrate has largely vanished for three Pt layers, the effect of surface strain due to the 2.5% lattice mismatch of Pt and Ru remains initially intact and is only gradually released for $n\ensuremath{\ge}5$ Pt layers. Electronic structure calculations confirm the experimental observations, in particular, the dramatic decrease of the CO adsorption energy on a single Pt layer which is caused by the strong Pt-Ru interlayer coupling.

The local reactivity of Pd overlayers supported by Au has been studied by calculating atomic hydrogen and CO adsorption energies as a microscopic probe. The calculations are based on density functional theory within the generalized gradient approximation. The binding energies show a maximum on two Pd layers on Au, both for the (100) and (111) surfaces. We have furthermore analyzed local trends by considering different adsorption sites on the Pd overlayers. The results can be rationalized within the d-band model if also second-nearest-neighbor interactions and bond-length effects are taken into account.

Publisher Summary This chapter describes the dynamical studies of molecular adsorption and desorption from metal and semiconductor surfaces based on potential energy surfaces that were derived from first-principles electronic structure calculations. In many cases, these dynamical simulations are in quantitative agreement with available experiments. Still it is the advantage of simulations compared to the experiment that the time evolution of wave packets or trajectories can be followed in any moment. This makes determination and analysis of the crucial qualitative mechanisms governing the interaction dynamics possible. Thus, the high-dimensional simulations based on ab initio electronic structure calculations do not only yield a quantitative but sometimes also a novel qualitative understanding of the adsorption and desorption dynamics. The chapter also discusses the fundamental difference between atomic and molecular adsorption. Whether quantum or classical methods are appropriate for the simulation of the adsorption dynamics or not are discussed in the chapter followed by a short introduction to the determination of potential energy surfaces from first principles and their continuous representation by some analytical or numerical interpolation schemes. The dissociative adsorption and associative desorption of hydrogen at metal and semiconductor surfaces and the molecular trapping of oxygen on platinum are discussed in the chapter.

So far we have almost entirely dealt with surface structures and processes that correspond to the electronic ground state. This means that we have assumed that the Born-Oppenheimer approximation is justified. Although there are many important processes at surfaces that involve electronic transitions [225,383,384], the status of the theoretical treatment of processes with electronically excited states is not very satisfactory. Many factors still hamper the development of quantitative models incorporating electronic excitations. Neither the determination of the electronically excited states nor the calculation of coupling matrix elements between these excited states is trivial. But even if the excited states and the coupling between them is known, the simulation of the reaction dynamics with electronic transitions still represents a challenge. In the next sections I will illustrate why the treatment of excited states is so complicated, but I will also show that there are some promising approaches to overcome the problems. In addition, concepts to treat reaction dynamics with electronic transitions will be discussed.

The study of adsorption is of central importance in the field of surface science. Adsorption processes are involved in almost all technological processes in which surfaces play a crucial role. Often they are an important step in the preparation of a device as, e.g., in the growth of a semiconductor device. But adsorption can also be of significant importance in industrially relevant processes. The most prominent example is heterogeneous catalysis since usually the reactants have to adsorb on the catalyst before they can react. But of course, also from a fundamental point of view the physical and chemical factors determining adsorption processes are most interesting. In this chapter I will first introduce the basic quantities necessary to describe adsorption. After classifying the different types of adsorption systems the necessary theoretical tools to treat these systems will be addressed. Furthermore, reactivity concepts will be discussed and their usefulness will be demonstrated in some case studies.

In the preceding chapter we have been concerned with the solution of the electronic Schrodinger equation for fixed nuclear coordinates. By repeating total-energy calculations for many different nuclear configurations, energy minima and whole potential energy surfaces for chemical reactions at surfaces can be determined. However, this static information is often not sufficient to really understand how a reaction proceeds. Furthermore, in the experiment the potential energy surface (PES) is never directly measured but just reac­tion rates and probabilities. For a real understanding of a reaction mechanism a dynamical simulation has to be performed. In addition, this allows a true comparison between theory and experiment and thus provides a reliable check of the accuracy of the calculated PES on which the dynamics simulation is based.

This paper provides a perspective on the article "Hydrogen adsorption, absorption and diffusion on and in transition metal surfaces: A DFT study" by Ferrin et al. in this issue.

We present self-consistent density-functional calculations of the adsorption of atomic and molecular hydrogen on the (210) surface of palladium using a plane-wave basis set with optimized ultrasoft pseudopotentials. The layer relaxations of the (210) surface and the preferred adsorption sites for atomic hydrogen adsorption are determined. Furthermore, we show that on the rather open Pd(210) surface a molecular ${\mathrm{H}}_{2}$ adsorption state becomes stabilized by the presence of atomic hydrogen on the surface. This provides a consistent explanation of recent experiments. An analysis of the bonding situation in terms of the local density of states is also presented.

Recent theoretical progress in gas-surface reaction dynamics, a field relevant to heterogeneous catalysis, is described. One of the most fundamental reactions, the dissociative chemisorption of H2 on metal surfaces, can now be treated accurately using quantum mechanics. Density functional theory is used to compute the molecule-surface interaction, and the motion of the hydrogen atoms is simulated using quantum dynamics, modeling all six molecular degrees of freedom. Theory is in good quantitative agreement with molecular beam experiments, offering useful interpretations, and allowing reliable predictions. The success of the approach calls for extensions to larger systems, such as dissociative chemisorption of polyatomic molecules.

The structure of liquid water has been addressed by ab initio molecular dynamics simulations based on density functional theory. Exchange-correlation effects have been described by the popular PBE and RPBE functionals within the generalized gradient approximation as these functionals also yield satisfactory results for metals which is important to model electrochemical interfaces from first principles. In addition, dispersive interactions are included by using dispersion-corrected schemes. It turns out that the dispersion-corrected RPBE functional reproduces liquid water properties quite well in contrast to the PBE functional. This is caused by the replacement of the over-estimated directional hydrogen-bonding in the PBE functional by non-directional dispersive interactions.

A novel H{sub 2} molecular adsorption state on metal surfaces has been detected by temperature-programmed desorption and electron energy loss spectroscopy experiments of the H{sub 2}/Pd (210) system. The molecular nature of this state has been verified by isotope exchange experiments. This molecular state leads to a decrease of the surface work function while atomic hydrogen on Pd(210) causes an increase. Ab initio total-energy calculations have confirmed all experimental findings. Through these calculations the microscopic nature of this novel molecular adsorption state could be identified; it turns out that this state is stabilized by the presence of atomic hydrogen on the Pd(210) surface.

The rotational and vibrational state distribution of ${\mathrm{D}}_{2}$ molecules desorbing from Pd(100) is determined by tunable vacuum ultraviolet laser ionization spectroscopy. For the deuterium supply a permeation source is used operating at surface temperatures in the range from 400 to 850 K. The molecules are detected in rovibrational states up to ${(v}^{\ensuremath{''}}{,J}^{\ensuremath{''}})=(0,12)$ and (1,8). A significant rotational cooling is observed together with a strong vibrational heating. These experimental results are found to be in good agreement with quantum-mechanical calculations based on an ab initio potential-energy surface. A consistent microscopic picture of the hydrogen reaction dynamics on Pd(100) is established and thus a recent controversy about the application of the principle of detailed balance in the system ${\mathrm{D}}_{2}/\mathrm{Pd}(100)$ can be settled.

We report on the effects of chemical bonding on high-resolution transmission electron microscopy images of covalently bonded light elements (B, C, N). We follow the procedure of Deng and Marks [1] (with some modifications) to calculate the accurate electrostatic potential of the bonded configuration using the WIEN2k DFT code [2]. The conventional TEM simulation, based on the independent atom model (IAM), is shown for comparison. We have studied single-layer hexagonal boron nitride and nitrogen-doped graphene membranes by calculation and experiment. Nitrogen doped graphene membranes are prepared by CVD methods [3] with the addition of ammonia into the reaction [4]. TEM imaging is carried out using an image-side Cs-corrected Titan 80-300, operated at 80kV. The spherical aberration is 20 m and a defocus of f1 = −90 A (Scherzer defocus) and f2 = −180 A is used.Image calculations for nitrogen doped graphene are based on a DFT relaxed atomic configuration. This configuration was obtained with the VASP DFT code [5], and agrees with previous works [6, 7] in that the bond length changes after replacing C by N are less than 2pm. According to conventional (IAM) TEM simulation, a nitrogen substitution in graphene would be invisible (a contrast of less than 0.1%) for all these relaxed configurations. Image simulations based on the electrostatic potentials from the DFT calculation, however, predict a detectable signal. Our charged defect produces predominantly low -er spatial frequency information in the projected potentials. This is mostly cut off by the CTF in the Scherzer-focus image of the Cs-corrected microscope (f1 , top row in Fig. 1), but visible at larger defo -cus. A further analysis of the projected potentials and electron distribution shows that the chemical shift here is not localized on the nitrogen substitution, but is actually spread onto the neighbouring car -bons. We note that a physical phase plate (not present in the current experiment) would be highly useful to study this non-local bonding effect to include all frequencies in one image. The third row in Fig. 1 shows an example experimental image from nitrogen doped single layer graphene membranes recorded at f2 (a drift-compensated average of 34 individual exposures). Two defects in excellent agreement with expectations (a dark contrast of ca. 0.5%) are visible in this image. In in-focus (f1) images of the same noise level of the same defects, the substitutions could not be detected above the noise (in agreement with the calculation). We also briefly consider the IAM and DFT calculation results for single-layer hexagonal boron nitride (h BN). Fig. 2 shows the projected electrostatic potentials for IAM and DFT, withdifferent resolutionlimiting apertures. The IAM simulation predicts a small but significant contrast difference for the two

The dissociative adsorption of hydrogen on Pd(100) has been studied by ab initio quantum dynamics and ab initio molecular-dynamics calculations. Treating all hydrogen degrees of freedom as dynamical coordinates implies a high dimensionality and requires statistical averages over thousands of trajectories. An efficient and accurate treatment of such extensive statistics is achieved in a three-step approach: In a first step we evaluate the ab initio potential-energy surface (PES) for a number of appropriate points in configuration space. Then (as step 2) we determine an analytical representation that serves as an interpolation between the actually calculated points. In an independent third step dynamical calculations are performed on the analytical representation of the PES. Thus the dissociation dynamics is investigated without any crucial assumption except for the Born-Oppenheimer approximation which is anyhow employed when density-functional-theory calculations are performed. The ab initio molecular dynamics is compared to detailed quantum-dynamical calculations on exactly the same ab initio PES. The occurence of quantum oscillations in the sticking probability as a function of kinetic energy is addressed. They turn out to be very sensitive to the symmetry of the initial conditions. At low kinetic energies sticking is dominated by the steering effect, which is illustrated using classical trajectories. The steering effect depends on the kinetic energy, but not on the mass of the molecules, as long as no energy transfer to the substrate atoms is considered. The comparison between quantum and classical calculations of the sticking probability shows the importance of zero-point effects in the hydrogen dynamics. The dependence of the sticking probability on the angle of incidence is analyzed; it is found to be in good agreement with experimental data. The results show that the determination of the potential-energy surface combined with high-dimensional dynamical calculations, in which all relevant degrees of freedom are taken into account, leads to a detailed understanding of the dissociation dynamics of hydrogen at a transition metal surface.

The interaction of hydrogen with many transition metal surfaces is characterized by a coexistence of activated with non-activated paths to adsorption with a broad distribution of barrier heights. By performing six-dimensional quantum dynamical and classical molecular dynamics calculations using the same potential energy surface derived from ab initio calculations for the system H2/Pd(100) we show that these features of the potential energy surface lead to strong steering effects in the dissociative adsorption dynamics. The adsorption dynamics shows only a small isotope effect which is purely due to the quantum nature of hydrogen.

We report calculations of the dissociative adsorption of H_2 at Pd (100) covered with 1/4 monolayer of sulfur using quantum dynamics as well as molecular dynamics and taking all six degrees of freedom of the two H atoms fully into account. The ab initio potential-energy surface (PES) is found to be very strongly corrugated. In particular we discuss the influence of tunneling, zero-point vibrations, localization of the nuclei's wave function when narrow valleys of the PES are passed, steering of the approaching H_2 molecules towards low energy barrier configurations, and the time scales of the center of mass motion and the other degrees of freedom. Several ``established'' concepts, which were derived from low-dimensional dynamical studies, are shown to be not valid., Comment: 4 pages, 3 figures, submitted to Surf. Sci. Lett. Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.html

According to the Heisenberg uncertainty principle of quantum mechanics, particles which are localized in space by a bounding potential must have a finite distribution of momenta. This leads, even in the lowest possible energy state, to vibrations, and thus, to the so-called zero-point energy. For chemically bound hydrogen the zero-point energy can be quite substantial. For example, for a free H2 molecule it is 0.26 eV, a significant value in the realm of chemistry, where often an energy of the order of 0.1 eV/atom (or 2.3 kcal/mol) decides whether or not a chemical reaction takes place with an appreciable rate. Yet, in many theoretical studies the dynamics of chemical reactions involving hydrogen has been treated classically or quasiclassically, assuming that the quantum mechanical nature of H nuclei, i.e., the zero-point effects, will not strongly affect the relevant physical or chemical properties. In this article we show that this assumption is not justified. We will demonstrate that for very basic and fundamental catalytic reaction steps, namely the dissociative adsorption of molecular hydrogen at transition metal surfaces and its time reverse process, the associative desorption, zero-point effects cannot only quantitatively but even qualitatively affect the chemical processes and rates. Our calculations (treating electrons as well as H nuclei quantum mechanically) establish the importance of additional zero-point effects generated by the H2 surface interaction and how energy of the H–H stretch vibration is transferred into those and vice versa.

We report six-dimensional quantum dynamical calculations of dissociative adsorption and associative desorption of the system H_2/Pd(100) using an ab-initio potential energy surface. We focus on rotational effects in the steering mechanism, which is responsible for the initial decrease of the sticking probability with kinetic energy. In addition, steric effects are briefly discussed., RevTeX, 11 pages, 4 figures

Experimental and theoretical results on the dynamics of dissociative adsorption and recombinative desorption of hydrogen on silicon are presented. Using optical second-harmonic generation, extremely small sticking probabilities in the range 10 29 210 25 could be measured for H 2 and D 2 on Si~111!737 and Si~100!231. Strong phonon-assisted sticking was observed for gases at 300 K and surface temperatures between 550 K and 1050 K. The absolute values as well as the temperature variation of the adsorption and desorption rates show surprisingly little isotope effect, and they differ only little between the two surfaces. These results indicate that tunneling, molecular vibrations, and the structural details of the surface play only a minor role for the adsorption dynamics. Instead, they appear to be governed by the localized H‐Si bonding and Si‐Si lattice vibrations. Theoretically, an effective five-dimensional model is presented taking lattice distortion, corrugation, and molecular vibrations into account within the framework of coupled-channel calculations. While the temperature dependence of the sticking is dominated by lattice distortion, the main effect of corrugation is a reduction of the preexponential factor by about one order of magnitude per lateral degree of freedom. Molecular vibrations have practically no effect on the adsorption/desorption dynamics itself, but lead to vibrational heating in desorption with a strong isotope effect. Ab initio calculations for the H 2 interaction with the dimers of Si~100!2 31 show properties of the potential surface in qualitative agreement with the model, but its dynamics differs quantitatively from the experimental results. @S0163-1829~96!01732-8#

Several theoretical and experimental results on the dynamics of dissociative adsorption and recombinative desorption of hydrogen on silicon are reviewed.

The influence of molecular vibrations on dissociative adsorption is studied by six-dimensional quantum dynamical calculations. For the system H_2 at Pd(100), which possesses non-activated pathways, it is shown that large vibrational effects exist and that they are not due to a strongly curved reaction path and a late dissociation-hindering minimum barrier, as was previously assumed. Instead, they are caused by the lowering of the H-H vibrational frequency during the dissociation and the multi-dimensionality of the potential energy surface. Still there are quantitative discrepancies between theory and experiment identified., Comment: RevTex, 14 pages, 5 figures, in uufiles form, to appear in Chem. Phys. Lett

Six-dimensional quantum dynamical calculations of the scattering of H_2 from a Pd(100) surface using a potential energy surface derived from density-functional theory calculations are presented. Due to the corrugation and anisotropy of the PES strong off-specular and rotationally inelastic diffraction is found. The dependence of the diffraction intensitities on the incident kinetic energy is closely examined. In particular we focus on the quantum oscillations for normal and off-normal incidence., Comment: RevTeX, 5 pages, 5 figures, to appear in Chem. Phys. Lett

Among other things (e.g. steering and steric effects in dissociative adsorption) we had predicted that the initial sticking probability of H_2 molecules impinging at clean Pd(100) exhibits oscillations, reflecting the quantum nature of the scattering process. In the preceding comment Rettner and Auerbach (RA) analyze experimental results and conclude that these oscillations are not detectable and thus either not existing or at least very small. In this reply we argue that the experimental study of RA is not conclusive to rule out the existence of quantum oscillations in the scattering of H_2 and note several problems and incongruities in their study., Comment: RevTexX, 1 page, Phys. Rev. Lett

The role of the configuration of metal surface atoms in the interaction between individual large, planar organic molecules and a metal substrate was investigated by low-temperature scanning tunneling microscopy and density functional theory calculations, including a semi-empirical correction scheme to account for dispersion effects. As test case, we used the adsorption of the oligopyridine derivative 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)pyridine-2-yl)pyrimidine (2,4'-BTP) on a stepped Ag(100) surface. Both experiment, via statistical evaluation of the adsorption site and orientation of 2,4'-BTP admolecules, and theory indicate distinct structural effects. The results are compared with the adsorption behavior of pyridine derivatives and benzene on metal surfaces. Consequences on the understanding of the interaction between heteroatoms or functional groups in large organic adsorbates and metal atoms in typical nano-scaled surface defects and hence of the interaction with more realistic metal surfaces are discussed.

We have studied the interaction of CH(4) with Al(2) and Al(3) neutral and charged clusters in the two lowest lying spin states using density functional theory. These calculations, via extended search, are used to determine the stable positions of H and CH(3) near the cluster, and the transition state to break the H-CH(3) bond. In all cases, stable methyl-aluminum-hydrides are possible. The H desorption is studied by means of vibration analysis and application of transition state theory. A common observed trend is that, in breaking the H-CH(3) bond, the interacting H atom is attached to the "surface" of the clusters attracting some negative charge of approximately 0.2e. The charge transfer is illustrated using the corresponding orbitals near the transition state in conjunction with the computed Mulliken population analysis. Thermal vibrations, generally, do not enhance the reaction. In all exothermic cases, the binding energy toward CH(3)+HAl(n) (charge) increases with increasing charge of the original Al(n) ((q=-1,0,1)) cluster. Although Al lacks occupied d-orbitals, the small Al clusters reduce the (free methane) CH(3)-H dissociation barrier except for Al(3) ((q=-1,0)). The relevant reactions in desorption require approximately 400-700 degrees C.

Self assembled monolayers (SAMs) made from an aromatic organodithiol, 2-mercaptomethylbenzenethiol (C(6)H(4)SHCH(2)SH) on Au(111) have been investigated in the context of a combined experimental and theoretical approach. The SAMs prepared by immersion of an Au-substrate in corresponding ethanolic solutions were characterized using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS) and thermal desorption spectroscopy (TDS). Adapted from the experimentally obtained unit cell of the SAM, density functional theory (DFT) calculations were applied to get a deeper insight into the structure of these dithiolate based SAMs. On the basis of the experimental and theoretical findings we provide a detailed structural model for this aromatic SAM consisting of the phenyl-group rigidly anchored to the substrate by two thiolate-bonds.

Properties of the metal–water interface have been addressed by periodic density functional theory calculations, in particular with respect to the electronic and geometric structures of water bilayers on several transition metal surfaces. It will be demonstrated that the presence of the metal substrate leads to a significant polarization of the water bilayer. This causes a substantial water-induced reduction of the work function in spite of the weak water–metal interaction, but it is not associated with a significant change of the electronic structure of the metal substrates. The structure and the vibrational spectra of water bilayers at room temperatures have been studied performing ab initio molecular dynamics simulations. The simulations suggest that the water bilayer structure on noble metals is not stable at room temperature, whereas on more strongly interacting metal surfaces some ordering of the water layer persists. In addition, metal–water interfaces under electrochemical conditions, i.e. for charged metal substrates, are addressed. Our simulations show that the charging of the surface leads to characteristic changes in the wall–oxygen distribution and the vibrational spectra.

Changing the composition and structure of a bimetallic surface system modifies its electronic properties and thus its catalytic activity. On the basis of density functional theory calculations, the electronic factors underlying the modified properties of bimetallic surfaces such as overlayer systems and, in particular, surface alloys will be discussed. It will be demonstrated that by mixing two metals a new metallic compound can result whose properties are not intermediate but beyond those of both constituents, so that for example by adding a relatively inert metal a more reactive surface can result. Besides the reactivity, the stability of the bimetallic systems will also be briefly discussed.

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The adsorption, dissociation, diffusion, and desorption of oxygen interacting with the Pt(111) surface have been studied using kinetic Monte Carlo simulations. This study has been motivated by uncertainties in the theoretical and the experimental derivations of O(2)Pt(111) reaction barriers. The simulations reproduce all known experimental data within basically one set of parameters, thus yielding microscopic insights into the elementary reaction steps occurring in the interaction of oxygen with Pt(111) and providing reliable estimates for adsorption energies and diffusion and desorption barriers. In particular, we confirm that the distance of oxygen atoms directly after dissociation is caused by ballistic hot atom motion rather than by diffusive motion. We address the equilibrium structure of oxygen atoms at high coverages. At low temperatures, chains of oxygen pairs are formed. We show that this mechanism can be explained by a lowered dissociation in the vicinity of already adsorbed atoms. Finally we discuss the role of the lateral interaction between the oxygen atoms in the oxygen desorption process.

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The sticking and scattering of O(2)Pt(111) has been studied by tight-binding molecular dynamics simulations based on an ab initio potential energy surface. We focus, in particular, on the sticking probability as a function of the angle of incidence and the energy and angular distributions in scattering. Our simulations provide an explanation for the seemingly paradox experimental findings that adsorption experiments suggest that the O(2)Pt(111) interaction potential should be strongly corrugated while scattering experiments indicate a rather small corrugation. The potential energy surface is indeed strongly corrugated which leads to a pronounced dependence of the sticking probability on the angle of incidence. The scattered O(2) molecules, however, experience a rather flat surface due to the fact that they are predominantly scattered at the repulsive tail of the potential.

Due to a computational error the results presented in Fig. 2 in Ref. [1] are wrong. The correct results are shown below. Fig. 3 remains unchanged. The conclusion drawn from the erroneous results in Ref. [1], that the experimental findings about surface temperature effects in dissociative adsorption cannot reproduced if only normal recoil is considered, is no longer justified. The implications of the corrected results will be treated in a forthcoming paper. I like to thank George Darling and Stephen Holloway for informing me about their own results which led to the discovery of the error.

Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields. This is the result of the debates of Symposium I "Atomic-scale design protocols towards energy, electronic, catalysis, and sensing applications", which took place within the 2018 European Materials Research Society fall meeting.

Contains fulltext : 97339.pdf (Publisher’s version ) (Closed access) We report on the search for electromagnetic and hadronic showers ("cascades") produced by a diffuse flux of extraterrestrial neutrinos in the AMANDA neutrino telescope. Data for this analysis were recorded during 1001 days of detector livetime in the years 2000-2004. The observed event rates are consistent with the background expectation from atmospheric neutrinos and muons. An upper limit is derived for the diffuse flux of neutrinos of all flavors assuming a flavor ratio of v(e):v(mu):v(tau) = 1:1:1 at the detection site. The all-flavor flux of neutrinos with an energy spectrum Phi proportional to E(-2) is less than 5.0 x 10(-7) GeV s(-1) sr(-1) cm(-2) at a 90% C.L. Here, 90% of the simulated signal would fall within the energy range 40 TeV to 9 PeV. We discuss flux limits in the context of several specific models of extraterrestrial and prompt atmospheric neutrino production. (C) 2010 Published by Elsevier B.V.

Point source searches with the IceCube neutrino telescope have been restricted to one hemisphere, due to the exclusive selection of upward going events as a way of rejecting the atmospheric muon background. We show that the region above the horizon can be included by suppressing the background through energy-sensitive cuts. This approach improves the sensitivity above PeV energies, previously not accessible for declinations of more than a few degrees below the horizon due to the absorption of neutrinos in Earth. We present results based on data collected with 22 strings of IceCube, extending its field of view and energy reach for point source searches. No significant excess above the atmospheric background is observed in a sky scan and in tests of source candidates, including the Active Galactic Nuclei Centaurus A and 3C279. Upper limits are reported, which for the first time cover point sources in the southern sky up to EeV energies., 6 pages, 4 figures. Updated to match version published in Phys. Rev. Lett

The first dedicated search for ultrahigh-energy (UHE) tau neutrinos of astrophysical origin was performed using the IceCube detector in its 22-string configuration with an instrumented volume of roughly 0.25 km3. The search also had sensitivity to UHE electron and muon neutrinos. After application of all selection criteria to approximately 200 live-days of data, we expect a background of 0.60±0.19(stat)+0.56−0.58(syst) events and observe three events, which after inspection, emerge as being compatible with background but are kept in the final sample. Therefore, we set an upper limit on neutrinos of all flavors from UHE astrophysical sources at 90% C.L. of E2νΦ90(νx)