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R A Olsen

Publications and source records attributed to R A Olsen.

At least 19 recordsLinked to original sources
PubMed2006

Reactive and nonreactive scattering of N2 from Ru(0001): a six-dimensional adiabatic study.

We have studied the dissociative chemisorption and scattering of N(2) on and from Ru(0001), using a six-dimensional quasiclassical trajectory method. The potential energy surface, which depends on all the molecular degrees of freedom, has been built applying a modified Shepard interpolation method to a data set of results from density functional theory, employing the RPBE generalized gradient approximation. The frozen surface and Born-Oppenheimer [Ann. Phys. (Leipzig) 84, 457 (1927)] approximations were used, neglecting phonons and electron-hole pair excitations. Dissociative chemisorption probabilities are found to be very small even for translational energies much higher than the minimum reaction barrier, in good agreement with experiment. A comparison to previous low dimensional calculations shows the importance of taking into account the multidimensional effects of N(2) rotation and translation parallel to the surface. The new calculations strongly suggest a much smaller role of nonadiabatic effects than previously assumed on the basis of a comparison between low dimensional results and experiments [J. Chem. Phys. 115, 9028 (2001)]. Also in agreement with experiment, our theoretical results show a strong dependence of reaction on the initial vibrational state. Computed angular scattering distributions and parallel translation energy distributions are in good agreement with experiments on scattering, but the theory overestimates vibrational and rotational excitations in scattering.

Journal Article↗
PubMed2006

Predicting catalysis: understanding ammonia synthesis from first-principles calculations.

Here, we give a full account of a large collaborative effort toward an atomic-scale understanding of modern industrial ammonia production over ruthenium catalysts. We show that overall rates of ammonia production can be determined by applying various levels of theory (including transition state theory with or without tunneling corrections, and quantum dynamics) to a range of relevant elementary reaction steps, such as N(2) dissociation, H(2) dissociation, and hydrogenation of the intermediate reactants. A complete kinetic model based on the most relevant elementary steps can be established for any given point along an industrial reactor, and the kinetic results can be integrated over the catalyst bed to determine the industrial reactor yield. We find that, given the present uncertainties, the rate of ammonia production is well-determined directly from our atomic-scale calculations. Furthermore, our studies provide new insight into several related fields, for instance, gas-phase and electrochemical ammonia synthesis. The success of predicting the outcome of a catalytic reaction from first-principles calculations supports our point of view that, in the future, theory will be a fully integrated tool in the search for the next generation of catalysts.

Journal Article↗
PubMed2006

Multidimensional effects on dissociation of N2 on Ru(0001).

The applicability of the Born-Oppenheimer approximation to molecule-metal surface reactions is presently a topic of intense debate. We have performed classical trajectory calculations on a prototype activated dissociation reaction, of N2 on Ru(0001), using a potential energy surface based on density functional theory. The computed reaction probabilities are in good agreement with molecular beam experiments. Comparison to previous calculations shows that the rotation of N2 and its motion along the surface affect the reactivity of N2 much more than nonadiabatic effects.

Journal Article↗
PubMed2005

Six-dimensional potential energy surface for H2 at Ru(0001).

The six-dimensional (6D) potential energy surface (PES) for the H(2) molecule interacting with a clean Ru(0001) surface has been computed accurately for the first time. Density functional theory (DFT) and a pseudopotential based periodic plane-wave approach have been used to calculate the electronic interactions between the molecule and the surface. Two different generalized gradient approximation (GGA) exchange-correlation functionals, PW91 and RPBE, have been adopted. Based on the DFT/GGA calculated potential energies, an analytical 6D PES has been constructed using the corrugation reducing procedure. A very accurate representation of the DFT/GGA data has been achieved, with an average error in the interpolation of about 3 meV and a maximum error not larger than about 30 meV. The top site is found to be the most reactive site for both functionals used, but PW91 predicts a higher reactivity than RPBE, with lower-energy and earlier-located dissociation barriers. The energetic corrugation displayed by the RPBE PES is larger than the PW91 PES while the geometric corrugation is smaller. The differences between the two PESs increase as the distance of the molecular center of mass to the surface decreases. A direct comparison with experimental investigations on H(2)/Ru(0001) could shed light on the suitability of these XC potentials often used in DFT calculations.

Adsorption↗
PubMed2004

Reactive scattering of H2 from Cu(100): six-dimensional quantum dynamics results for reaction and scattering obtained with a new, accurately fitted potential-energy surface.

Six-dimensional quantum dynamical calculations are reported for the dissociative chemisorption of (v=0, 1, j=0) H(2) on Cu(100), and for rovibrationally inelastic scattering of (v=1, j=1) H(2) from Cu(100). The dynamics results were obtained using a new potential-energy surface (PES5), which was based on density-functional calculations using a slab representation of the adsorbate-substrate system and a generalized gradient approximation to the exchange-correlation energy. A very accurate method (the corrugation reducing procedure) was used to represent the density-functional theory data in a global potential-energy surface. With the new, more accurately fitted PES5, the agreement between the dynamics results and experimental results for reaction and rovibrationally elastic scattering is not as good as was obtained with a previous potential-energy surface (PES4), which was based on a subset of the density-functional theory data not yet including the results for the low-symmetry Cu sites. Preliminary density-functional theory results suggest that the agreement between theory and experiment will improve over that obtained with PES5 if the density-functional calculations are repeated using a larger basis set and using more copper layers than employed in PES4 and PES5.

Journal Article↗
PubMed2004

Comparison of methods for finding saddle points without knowledge of the final states.

Within the harmonic approximation to transition state theory, the biggest challenge involved in finding the mechanism or rate of transitions is the location of the relevant saddle points on the multidimensional potential energy surface. The saddle point search is particularly challenging when the final state of the transition is not specified. In this article we report on a comparison of several methods for locating saddle points under these conditions and compare, in particular, the well-established rational function optimization (RFO) methods using either exact or approximate Hessians with the more recently proposed minimum mode following methods where only the minimum eigenvalue mode is found, either by the dimer or the Lanczos method. A test problem involving transitions in a seven-atom Pt island on a Pt(111) surface using a simple Morse pairwise potential function is used and the number of degrees of freedom varied by varying the number of movable atoms. In the full system, 175 atoms can move so 525 degrees of freedom need to be optimized to find the saddle points. For testing purposes, we have also restricted the number of movable atoms to 7 and 1. Our results indicate that if attempting to make a map of all relevant saddle points for a large system (as would be necessary when simulating the long time scale evolution of a thermal system) the minimum mode following methods are preferred. The minimum mode following methods are also more efficient when searching for the lowest saddle points in a large system, and if the force can be obtained cheaply. However, if only the lowest saddle points are sought and the calculation of the force is expensive but a good approximation for the Hessian at the starting position of the search can be obtained at low cost, then the RFO approaches employing an approximate Hessian represent the preferred choice. For small and medium sized systems where the force is expensive to calculate, the RFO approaches employing an approximate Hessian is also the more efficient, but when the force and Hessian can be obtained cheaply and only the lowest saddle points are sought the RFO approach using an exact Hessian is the better choice. These conclusions have been reached based on a comparison of the total computational effort needed to find the saddle points and the number of saddle points found for each of the methods. The RFO methods do not perform very well with respect to the latter aspect, but starting the searches further away from the initial minimum or using the hybrid RFO version presented here improves this behavior considerably in most cases.

Journal Article↗
PubMed2004

Adsorption and diffusion on a stepped surface: atomic hydrogen on Pt(211).

We present density functional theory calculations for atomic hydrogen interacting with a stepped surface, the Pt(211) surface. The calculations have been performed at the generalized gradient approximation level, using a slab representation of the surface. This is the state-of-the-art method for calculating the interaction of atoms or molecules with metal surfaces, nevertheless only few studies have used it to study atoms or molecules interacting with stepped surfaces, and none, to the best of our knowledge, have considered hydrogen interacting with stepped platinum surfaces. Our goal has been to initiate a systematic study of this topic. We have calculated the full three-dimensional potential energy surface (PES) for the H/Pt(211) system together with the vibrational band structure and vibrational eigenfunctions of H. A deep global minimum of the PES is found for bridge-bonded hydrogen on the step edge, in agreement with experimental results for the similar H/Pt(533) system. All the local vibrational excitations at the global minimum have been identified, and this will serve as a helpful guide to the interpretation of future experiments on this (or similar) system(s). Furthermore, from the calculated PES and vibrational band structure, we identify a number of consequences for the interpretation or modelling of diffusion experiments studying the coverage and directional dependence of atomic hydrogen diffusion on stepped platinum surfaces.

Journal Article↗
PubMed2000

Quantum dynamics of the dissociation of H2 on Cu(100): dependence of the site-reactivity on initial rovibrational state.

We perform six-dimensional (6D) quantum wavepacket calculations for H2 dissociatively adsorbing on Cu(100) from a variety of rovibrational initial states. The calculations are performed on a new potential energy surface (PES), the construction of which is also detailed. Reaction probabilities are in good agreement with experimental findings. Using a new flux analysis method, we calculate the reaction probability density as a function of surface site and collision energy, for a variety of initial states. This approach is used to study the effects of rotation and vibration on reaction at specific surface sites. The results are explained in terms of characteristics of the PES and intrinsically dynamic effects. An important observation is that, even at low collision energies, reaction does not necessarily proceed predominantly in the region of the minimum potential barrier, but can occur almost exclusively at a site with a higher barrier. This suggests that experimental control of initial conditions could be used to selectively induce reaction at particular surface sites. Our predictions for site-reactivity could be tested using contemporary experimental methods: The calculations predict that, for reacting molecules, there will be a dependence of the quadrupole alignment of j on the incident vibrational state, v. This is a direct result of PES topography in the vicinity of the preferred reaction sites of v = 0 and v = 1 molecules. Invoking detailed balance, evidence for this difference in preferred reaction site of v = 0 and 1 molecules could be obtained through associative desorption experiments.

Journal Article↗
PubMed1991

Platelet-derived growth factor blocks the increase in intracellular free Ca2+ caused by calcium ionophores and a volatile anesthetic agent in Swiss 3T3 fibroblasts without altering toxicity.

Platelet-derived growth factor (PDGF) produced an almost complete block of the increase in intracellular free Ca2+ concentration ([Ca2+]i) in Swiss 3T3 fibroblasts caused by the Ca2(+)-selective ionophores 4-bromo-A23187 and ionomycin, and by the volatile anesthetic agent halothane. The effect of PDGF was similar to the decreased [Ca2+]i response to Ca2(+)-ionophores produced by phorbol 12-myristate 13-acetate, an activator of protein kinase C. There was no effect of PDGF or PMA on the acute or delayed toxicity of the Ca2(+)-ionophores to Swiss 3T3 cells, suggesting that the increase in [Ca2+]i is not the direct cause of toxicity of these agents.

Aequorin↗
PubMed1990

Inhibition of growth factor-dependent inositol phosphate Ca2+ signaling by antitumor ether lipid analogues.

Cytotoxic ether lipid analogues have been studied for their ability to inhibit growth factor-dependent [Ca2+]i signaling in Swiss 3T3 fibroblasts. 1-Octadecyl-2-methyl-rac-glycero-3-phosphocholine (ET-18-OCH3) inhibited 45Ca2+ uptake and inositol(1,4,5)trisphosphate-induced 45Ca2+ release in saponin permeabilized cells with concentration producing 50% inhibition values of 55 and 360 microM, respectively. When cells were exposed to ET-18-OCH3 for 18 h before permeabilization there was selective inhibition of inositol(1,4,5)trisphosphate-induced 45Ca2+ release with a concentration producing 50% inhibition value of 20 microM, but no effect on 45Ca2+ uptake, or on 45Ca2+ release by arachidonic acid. The concentration of ET-18-OCH3 with continuous exposure to inhibit cell growth 50% was 19 microM. The ether lipid analogues 1-hexadecylthio-2-ethyl-rac-glycero-3- phosphocholine and 1-S-octadecyl-2-O-methylthiopropyl-3-N,N-dimethyl-gamma-hydroxy pro pyl ammonium iodide had effects similar to those of ET-18-OCH3 but the noncytotoxic analogue 1-alkyl-2-hydroxy-sn-glycero-3- phosphocholine was without effect. Exposure of cells to 10 microM ET-18-OCH3 produced 81% inhibition of platelet-derived growth factor-stimulated inositol phosphate formation and 66% inhibition of fluoroaluminate anion-stimulated inositol phosphate formation. Addition of ET-18-OCH3 to cells in medium with 10% fetal calf serum gave a transient increase in [Ca2+]i without causing an increase in resting [Ca2+]i, while the addition of ET-18-OCH3 to cells in medium without serum gave a sustained increase in resting [Ca2+]i. Cells exposed to 5 microM ET-18-OCH3 for 18 h showed no increase in resting [Ca2+]i but there was 95% inhibition of the [Ca2+]i response to platelet-derived growth factor, 63% inhibition of the response to bradykinin, and 55% inhibition of the response to vasopressin. The block by ether lipid analogues of inositol phosphate-mediated [Ca2+]i signaling suggests a mechanism for preventing the action of growth factors that could contribute to the inhibition of cell proliferation by the agents.

Animals↗
PubMed1990

Platelet-derived growth factor stimulates non-mitochondrial Ca2+ uptake and inhibits mitogen-induced Ca2+ signaling in Swiss 3T3 fibroblasts.

Changes in intracellular free Ca2+ concentration [( Ca2+]i) were used to study the interaction between mitogens in Swiss 3T3 fibroblasts. Platelet-derived growth factor (PDGF) produced an increase in [Ca2+]i and markedly decreased the increases in [Ca2+]i caused by subsequent addition of bradykinin and vasopressin. If the order of the additions was reversed the [Ca2+]i response to PDGF was not inhibited by bradykinin or vasopressin. Inhibition of protein kinase C by staurosporine or chronic treatment of the cells with phorbol 12-myristate 13-acetate prevented the inhibitory effect of PDGF on the [Ca2+]i response to vasopressin but not bradykinin. PDGF did not decrease the receptor binding of bradykinin and produced only a small decrease in the receptor binding of vasopressin. PDGF decreased the rise in [Ca2+]i caused by the Ca2+ ionophores 4-bromo-A23187 and ionomycin and by a membrane perturbing ether lipid, 1-octadecyl-2-methyl-rac-glycero-3-phosphocholine, both in the presence and absence of external Ca2+. There was no change in cell 45Ca2+ influx caused by PDGF, vasopressin, or bradykinin. 45Ca2+ efflux from cells exposed to PDGF and vasopressin mirrored the changes in [Ca2+]i caused by the agents, that is, PDGF added after vasopressin produced a further increase in 45Ca2+ efflux but vasopressin did not increase 45Ca2+ efflux after PDGF. PDGF but not vasopressin caused an increase in the uptake of 45Ca2+ into an inositol 1,4,5-trisphosphate-insensitive non-mitochondrial store in permeabilized cells. The results suggest that the decreased [Ca2+]i response to mitogens after PDGF represents an action of PDGF at a point beyond the release of intracellular Ca2+ and the influx of external Ca2+, caused by an increase in the rate of removal of cytoplasmic free Ca2+. This increased removal of cytoplasmic Ca2+ by PDGF is not due to the increased export of Ca2+ from the cell but results from increased Ca2+ uptake into non-mitochondrial stores.

Alkaloids↗
PubMed1990

Transient increases of intracellular Ca2+ induced by volatile anesthetics in rat hepatocytes.

The affects of volatile anesthetics on mobilization of intracellular Ca2+ was monitored in primary cultures of rat hepatocytes using the fluorescent Ca2+ probe Fura-2. The use of Fura-2 was limited by several factors which complicated the quantitative analysis of the results, such as: (i) a high rate of dye leakage; (ii) changes in the redox state of the hepatocytes which interfered with the fluorescence produced by the dye at various excitation wavelengths; (iii) compartmentalization of the dye producing high local intracellular concentrations; and, of particular importance for this study, (iv) enhanced photobleaching of the dye in the presence of halothane. To aid in the interpretation of the Fura-2 data, the Ca2(+)-sensitive photoprotein aequorin was also used to monitor changes in [Ca2+]i. The aequorin and Fura-2 techniques qualitatively yielded the same result, that the volatile anesthetic agents halothane, enflurane, and isoflurane induce an immediate and transient increase of [Ca2+]i. The durations of these transients were approximately between 5 and 10 min and were not related to any evident acute cell toxicity. The [Ca2+]i increases induced by the volatile anesthetic agents were dose-dependent, with halothane the most potent. The exact mechanism governing these increases in [Ca2+]i induced by these anesthetics in rat hepatocytes is unknown, but is likely to involve effects on both the cell surface membrane and endoplasmic reticulum components of the signal transducing system.

Aequorin↗
PubMed1990

Suramin blocks intracellular Ca2+ release and growth factor-induced increases in cytoplasmic free Ca2+ concentration.

Suramin, a polysulfonated naphthylurea with antitumor activity, has been shown to be an inhibitor of the release of Ca2+ from non-mitochondrial stores induced by the putative intracellular second messengers inositol 1, 4, 5-trisphosphate and GTP in saponin permeabilized Swiss 3T3 fibroblasts. The IC50 for the effect of suramin was about 40 microM in both cases. Suramin did not block Ca2+ release induced by the Ca2+ ionophore 4-bromo A23187 or by the membrane perturbing agent halothane. Suramin, 7 x 10(-5) M, caused a 49% decrease in the elevation of intracellular free Ca2+ concentration ([Ca2+]i) caused by platelet derived growth factor (PDGF) in intact Swiss 3T3 fibroblasts but did not block the increases in [Ca2+]i caused by bradykinin or vasopressin. Suramin decreased PDGF binding to its receptor on intact Swiss 3T3 fibroblasts but had no effect on the binding of bradykinin and vasopressin. The results show that the effect of suramin in decreasing the [Ca2+]i response to growth factors may be mediated by a block of growth factor-receptor binding, but an effect on intracellular Ca2+ release cannot be ruled out.

Animals↗
PubMed1990

Accumulation of radiocaesium in fungi.

The accumulation of radioactive Cs by fungi was studied by analysis of fruit bodies (n (total) = 205, n greater than or equal to 5 for 22 species) collected in 1988 in a Norwegian mountain area with high deposition of radiocaesium from the Chernobyl accident. To account for site variation, the radiocaesium content of soil and plants was determined for each sampling spot. The soil contained 5-600 kBq/m2 (median = 50 kbq/m2, 134Cs + 137Cs). The plant content ranged from 0.25 to 23 Bq/g dry weight (median = 3.1 Bq/g) and was positively correlated with radiocaesium concentration in the soil (r = 0.56) and negatively correlated with soil pH (r = -0.28). The ratio between radiocaesium content in fungi and that in plants at the same spot (F/P) differed among species: 25 species had F/P values between 30 and 270, 12 species had F/P values between 10 and 30, and the rest (16 species) had F/P values below 10 (only four samples had values below 1). The concentration of nonradioactive Cs in fruit bodies was positively correlated with their radiocaesium content. Certain species selectively accumulated one or several trace elements (V, Cd, Hg, Pb, Th).

Accidents↗
PubMed1989

High molecular weight dextran sulfate inhibits intracellular Ca2+ release and decreases growth factor-induced increases in intracellular free Ca2+ in Swiss 3T3 fibroblasts.

High molecular weight (500 kDa) dextran sulfate (DXS) inhibited the release of Ca2+ induced by myoinositol 1,4,5-trisphosphate from non-mitochondrial stores of saponin-permeabilized Swiss 3T3 fibroblasts with an IC50 of 20 micrograms/mL. Low molecular weight (5 kDa) DXS did not have this effect. DXS was more inhibitory than heparin, which in the same system had an IC50 of 62 micrograms/mL. DXS also produced a small inhibition of Ca2+ release by arachidonic acid and GTP but did not affect Ca2+ release by 4-bromo A23187 or halothane. The transient increase in intracellular free Ca2+ concentration ([Ca2+]i) in intact Swiss 3T3 cells caused by platelet-derived growth factor was completely inhibited by 100 micrograms/mL of DXS, but DXS had no effect on the [Ca2+]i increase caused by bradykinin or vasopressin. The specific binding of platelet-derived growth factor, but not of bradykinin or vasopressin, to Swiss 3T3 fibroblasts was decreased by DXS. The effect of DXS in decreasing growth-factor mediated increases in [Ca2+]i may be mediated by an effect on the binding of growth factor to its receptor. An effect of DXS on the intracellular release of Ca2+ by second messengers to decrease changes in [Ca2+]i, however, cannot be ruled out.

Arachidonic Acid↗
PubMed1988

Signal transduction during human natural killer cell activation: inositol phosphate generation and regulation by cyclic AMP.

NK cells mediate both direct cytotoxicity against a variety of tumor cells and indirect (FcR-dependent) cytotoxicity against antibody-coated targets. When cloned human NK cells (CD16+/CD3-) were exposed to NK-sensitive targets for 30 min, the level of inositol phosphates rose two to five times above background. The rise in inositol phosphates induced by NK-sensitive targets was paralleled by an increase in intracellular free calcium concentration ([Ca2+]i). A panel of tumor cells that were resistant to NK cell lysis did not stimulate significant levels of inositol phosphate production and did not induce an elevation of intracellular free calcium. Ligation of the FcR (CD16) with the mAb 3G8 also triggered phosphoinositide turnover. Kinetic experiments demonstrated that stimulation by either susceptible target cells or by FcR ligation led to rapid (less than 1 min) generation of the Ca2+-mobilizing second messenger, inositol trisphosphate, with slower accumulation of inositol bisphosphate and inositol monophosphate. Previous studies have demonstrated that activation of the cAMP-dependent second messenger pathway strongly inhibits NK cell-mediated cytotoxic functions. Treatment of NK effector cells with forskolin to elevate intracellular cAMP levels resulted in a concentration-dependent inhibition of phosphoinositide hydrolysis induced by both NK-sensitive targets and 3G8-mediated FcR ligation. These results suggest that phosphoinositide turnover represents a critical early event in the human NK cell cytolytic process. Moreover, the potent inhibitory effect of cAMP on NK cell cytotoxicity may be explained by the uncoupling of NK receptors from phospholipase C-mediated phosphoinositide hydrolysis.

Antibody-Dependent Cell Cytotoxicity↗