CO Molecular Axis Research Articles

Multiorbital effects on CO ionization by intense short laser pulses

Within time-dependent Hartree–Fock theory, we study the CO ionization for six different photon wavelengths running from 200 to 700 nm. The parallel and perpendicular alignments of the CO molecular axis with the polarization direction of the laser electric field are considered for two different laser intensities, namely and W cm−2. At the laser intensity W cm−2, the ionization picture is characterized by the dominance of the highest occupied molecular orbital, while at laser intensity W cm−2 the inner valence molecular orbitals significantly contribute to the CO ionization. The present computations also reveal that orbital switching effects due to peak intensity variation may occur at some shorter wavelengths.

Effect of the Surface Oxidation of Molybdenum Boride on the Adsorption and Interaction of Carbon Oxide and Oxygen Molecules

Surface analysis conducted under ultrahigh vacuum conditions shows that the oxidation of a B‒Mo(110) alloy with a submonolayer concentration of boron atoms dramatically improves the efficiency of conversion of co-adsorbed carbon monoxide and oxygen molecules to carbon dioxide. It is established that this effect is associated with differences between the properties of the adsorbed particles before and after molybdenum boride oxidation, particularly an increase in the slope of the CO molecular axis to the plane of the adsorbent surface and the weakening of the chemisorptive bonds of oxygen with the substrate on the surface of the MoxByOz ternary compound. It is concluded that this substrate can be considered a model system and an alternative to the currently available catalysts for CO oxidation based on noble metals.

Dynamical corrugation: simulations of the sticking of CO on Cu(1 0 0)

Molecular dynamics (MD) simulations have been carried out of the non-dissociative sticking probability of carbon monoxide on the clean (1 0 0) face of a copper crystal, as a function of incident translational energy, E tr, and incident angle with respect to the surface normal, θ. The computed sticking probabilities do not obey the standard functional dependence on E tr cos nθ . Rather, in order to fit this functional form, the exponent n must vary from the usual positive values at low incident energies to negative values at high energies, with a crossover from positive to negative at 0.5 eV. A negative n reflects a higher sticking probability at normal incidence than at oblique incidence for the same incident energy. This suggests that the effective corrugation is energy dependent and very pronounced at energies above 0.5 eV, in spite of the fact that Cu(1 0 0) is a smooth crystal face and its interaction with intact CO is chemically simple. Similar behavior has been observed experimentally for related systems, CO on Ni(1 0 0) and NO on Pt(1 1 1). In order to identify the source of this apparent strong corrugation, we have carried out MD simulations in which the moment of inertia of the CO molecule was artificially altered. We show that deeper penetration into the surface at higher energies is a negligible effect. The major contribution to the large, energy-dependent corrugation is a dynamic one; at low incident energies the CO molecular axis has time to continuously reorient into energetically favorable directions, whereas at high energy this orientational “steering” is much less complete. This, in conjunction with the strong directionality of the CO–Cu(1 0 0) binding, produces a large effective corrugation at high incident energy. We conclude that this source of large effective corrugation will apply generally to molecule–surface systems for which the orientational and lateral degrees of freedom are strongly coupled.

NEXAFS study of CO adsorption on ZnO(0001̄)–O and ZnO(0001̄)–O/Cu

C K-edge near edge X-ray absorption fine structure (NEXAFS) measurements have been used to investigate CO adsorption at 130 K on ZnO(0001̄)–O and on the same surface after a 2D submonolayer Cu film has been deposited. CO adsorption on clean ZnO(0001̄)–O gives rise to two π ∗ resonances in the NEXAFS spectra at 287.7±0.2 eV and 290.4±0.2 eV. These are assigned to molecular CO, which is metastable under our experimental conditions, and CO 2− 3. Only one intense π ∗ resonance, at 287.7±0.2 eV, is observed for CO adsorption on ZnO(0001̄)–O after submonolayer Cu deposition. This is also attributed to adsorbed molecular CO. The polarisation dependence of this resonance indicates that the CO molecular axis is tilted by 17±10° away from the surface normal.

A tensor LEED analysis of the Rh(110)-p2mg(2 × 1)-2CO structure

The structural geometries of both the clean and CO covered Rh(110) surfaces have been determined by a LEED intensity analysis. The clean surface exhibits a top layer contraction of 0.09 ± 0.02 Å and a second layer expansion of 0.03 ± 0.03 Å relative to the bulk interlayer spacing. Upon the adsorption of 1 ML of CO the molecules are arranged in a zig-zag fashion along the [ 110] direction to produce the observed glide plane symmetry in the LEED pattern. The clean surface relaxations are removed and each CO bonds near a short bridge site with the CO molecular axis oriented approximately 24° from the surface normal. This bonding geometry results in a RhC bond length of 1.97 ± 0.09 Å and a CO bond length of 1.13 ± 0.09 Å.

Ligand binding to heme proteins: III. FTIR studies of His-E7 and Val-E11 mutants of carbonmonoxymyoglobin

Fouier-transform infrared (FTIR) difference spectra of several His-E7 and Val-E11 mutants of sperm whale carbonmonoxymyoglobin were obtained by photodissociation at cryogenic temperatures. The IR absorption of the CO ligand shows characteristic features for each of the mutants, both in the ligand-bound (A) state and in the photodissociated (B) state. For most of the mutants, a single A substate band is observed, which points to the crucial role of the His-E7 residue in determining the A substrate spectrum of the bound CO in the native structure. The fact that some of the mutants show more than one stretch band of the bound CO indicates that the appearance of multiple A substates is not exclusively connected to the presence of His-E7. In all but one mutant, multiple stretch bands of the CO in the photodissociated state are observed; these B substates are thought to arise from discrete positions and/or orientations of the photodissociated ligand in the heme pocket. The red shifts of the B bands with respect to the free-gas frequency indicate weak binding in the heme pocket. The observation of similar red shifts in microperoxidase (MP-8), where there is no residue on the distal side, suggests that the photodissociated ligand is still associated with the heme iron. Photoselection experiments were performed to determine the orientation of the bound ligand with respect to the heme normal by photolyzing small fractions of the sample with linearly polarized light at 540 nm. The resulting linear dichroism in the CO stretch spectrum yielded angles alpha > 20 degrees between the CO molecular axis and the heme normal for all of the mutants. We conclude that the off-axis position of the CO ligand in the native structure does not arise from steric constraints imposed by the distal histidine. There is no clear correlation between the size of the distal residue and the alpha of the CO ligand.

On the electronic structure of the coadsorbate system CO + O (2×1)/ Pd(111): A precursor for CO 2 formation

We report results of an angle resolved photoelectron spectroscopy (ARUPS) study using synchrotron radiation from the BESSY storage ring of the CO + O (2×1)/ Pd(111) coadsorbate system in comparison with the CO(√3 × √3) R30°/ Pd(111) adsorbate. The band structures of the CO induced 5σ 1π and 4σ levels for both structures are observed and favourably compared with the proposed structure models as well as tight-binding model calculations. Light polarization dependent measurements as well as the angle dependence of the 4σ shape resonance indicate that the CO molecular axis in the pure CO adsorbate is perpendicular to the Pd(111) surface, while it appears slightly (<10°) inclined with respect to the surface normal in the coadsorbate. We show by comparison with other pure CO adsorbates using the measured E versus k ‖ dispersions that the 4σ wave function in the coadsorbate is very similar to the pure adsorbate. It is shown that this is different with respect to other, namely CO + K, coadsorbates.

Bremsstrahlung isochromat spectroscopic study of the system CO/Ni(110): Strong intermolecular interaction

Adsorption of CO on Ni(110) at monolayer coverage results in a (2 × 1) p2mg overlayer structure with the CO molecular axis tilted by 15°–20° with respect to the surface normal. The low symmetry of the overlayer leads to splittings of all CO-derived levels at the center of the Brillouin zone. As a result of the high coverage and the concomitant strong lateral interaction between the CO molecules the level splittings are as large as 2 eV for both occupied and empty CO-derived levels. The same order of magnitude has been estimated from tight binding calculations for a free unsupported CO layer in p2mg structure.

Bremsstrahlung isochromat spectroscopic study of the system CO/Ni(110): Strong intermolecular interaction

Abstract Adsorption of CO on Ni(110) at monolayer coverage results in a (2 × 1) p2mg overlayer structure with the CO molecular axis tilted by 15°–20° with respect to the surface normal. The low symmetry of the overlayer leads to splittings of all CO-derived levels at the center of the Brillouin zone. As a result of the high coverage and the concomitant strong lateral interaction between the CO molecules the level splittings are as large as 2 eV for both occupied and empty CO-derived levels. The same order of magnitude has been estimated from tight binding calculations for a free unsupported CO layer in p2mg structure.

Interactions of CO + K on Ru(001): Structure and bonding

Recent studies of CO + K on Ru(001) revealed an anomalously low CO streaching frequency of 1460 cm −1 for low CO and K coverages. Hoffmann and de Paola proposed a side-on bound molecule with the CO molecular axis parallel to the metal surface: Weimer and Umbach suggested that CO is bound perpendicular to the surface, as on clean Ru(001), but that the CO is sp 2 hybridized in the presence of K(ads). The main objectives of the present work were to search for configurational changes of adsorbed CO on K + Ru(001) and to compare these results with CO + O(ad). We used the ESDIAD (electron stimulated desorption ion angular distribution) method in combination with LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). The ESDIAD data for low coverages ( θ K × 0.1) of CO + K on Ru(001) show that the O + ESD signal from CO is suppressed by coadsorption with K; this can arise from a reorientation of the CO molecular axis, from perpendicular to inclined, or from increased reneutralization rates due to coadsorption with K. TDS provides evidence for strong interaction between CO and K, and a combination of LEED and ESD measurements suggest the formation of ordered species having K x (CO) 2 x stoichiometry.

Orientation of chemisorbed species from orthogonal-plane arups: Tilted CO on Pt{110} and upright CO on Pt{111}.

Angle-resolved photoemission spectra obtained with electron collection in the plane orthogonal to the incidence plane, using a He II photon source, are used to provide an accurate measure of the orientation of chemisorbed diatomic molecules. As previously established, CO in the Pt{111}c(4 × 2) structure is found to be chemisorbed in an upright configuration, whereas in the Pt{110}(2 × 1) p1g1 structure the CO molecular axis is tilted 26 ± 2° away from the surface normal, in a direction between [211] and [433].

Theoretical study of electronic structure for “inclined” CO chemisorbed on Pd (210) surface

Electronic structure for CO chemisorbed on Pd (210) surface has been calculated by the model potential Xα method which involves Bonifacic-Huzinaga's model potential and Slater's Xα potential. The results of the present calculations are consistent with the “inclined” CO model which was proposed by Madey et al. The results also show that the electronic structure of CO chemisorbed on Pd (210) surface resembles surprisingly those of CO on the other low-index planes where the CO molecular axis is supposed to be perpendicular to the surfaces.

A novel bonding geometry of CO on Cu(311) as determined by angle-resolved photoemission spectroscopy

Angle-resolved photoemission spectroscopy has revealed a previously unobserved bonding geometry for CO adsorbed on the Cu(311) surface. In this new bonding mode the CO molecular axis is parallel to the (311) plane and lies along the step direction.

A study of the sorption of CO on W and Ni single-crystal surfaces by ion scattering

Helium ion backscattering is used to study the sorption of CO on W(100), Ni(110), and Ni(111) surfaces. It is concluded that the CO molecular axis is oriented normal to the Ni(110) and Ni(111) surfaces. The molecular orientation of CO on W(100) is more complicated, depending on the temperature. No orientation for CO molecules can be obtained at room temperature while at higher temperatures, CO appears to be either oriented parallel to the surface or dissociated.

The chemisorption of CO on Cu(100) studied with angle resolved photoelectron spectroscopy

Using angle resolved UV photoelectron spectroscopy, coupled with the continuum of polarized light available with synchrotron radiation, we make an interpretation of the photoelectron spectrum of CO adsorbed on Cu(100). We point out that the bonding of CO on Cu as observed by photoemission is considerably different from the bonding of CO on Ni. These differences do not seem to be caused by bonding orientation differences, however, as the CO molecular axis is found to be very close to the surface normal, as was the case for CO on Ni(100). No evidence is found for a second phase of CO on Cu(100).