Orientation Of Molecular Axis Research Articles

Role of projectile-motion-induced electronic transitions in the dissociation of fast H 2 at a Cu(111) surface

We report measurements of the final molecular axis orientation, released kinetic energy and incident beam translational energy dependence of dissociative scattering of 500–6000 eV H 2 and H + 2 from Cu(111) under glancing angles of incidence (normal energies 0.7−4 eV). We find some evidence that the scattering dynamics of those molecules initially in the ground X 1 Σ g + state may be influenced by the topography of the continuum manifold of PES correlating with the adiabatic ground state. However, we suggest that electronic transitions from the ground state manifold to a dissociative upper state manifold are ultimately responsible for the dissociative scattering events we detect.

Orientational study of low coverage CO and NO on Ni(111)

The molecular axis orientation of CO and NO molecules adsorbed at room temperature on Ni(111) at low coverage has been studied by the XPD technique. The CO and NO uptake versus exposure has been followed by XPS. Our results show that CO molecules are perpendicular to the surface, while NO molecules are strongly tilted or lying down.

Orientational study of low coverage CO and NO on Ni(111)

The molecular axis orientation of CO and NO molecules adsorbed at room temperature on Ni(111) at low coverage has been studied by the XPD technique. The CO and NO uptake versus exposure has been followed by XPS. Our results show that CO molecules are perpendicular to the surface, while NO molecules are strongly tilted or lying down.

Direct measurement of the energy release and final molecular axis orientation in dissociative molecular scattering at surfaces

We describe a technique for the direct measurement of the kinetic energy release and final molecular axis orientation in dissociative molecular scattering at surfaces. The energy released and the final orientation are derived in principle from coincident measurement of the laboratory-frame velocities of the dissociation fragments. The measurement principle is independent of either the charge state, translational or internal energy of the incident beam, and can be used to investigate dissociative scattering of both homonuclear and heteronuclear molecules. To illustrate the technique we present some results of an investigation of the dissociative scattering of H + 2 and H 2 from a Cu(111) surface under glancing incidence conditions.

X-ray photoelectron diffraction study of CO- and NO-saturated Ni(111)

We studied the molecular axis orientation of CO and NO adsorbed on Ni(111), at saturation and at room temperature, by X-ray photoelectron diffraction of the C, N and O 1s core level peaks. Although previous studies are indicative of strong interactions between adsorbed molecules, we do not observe any induced tilt of the molecular axis with respect to the surface normal.

Dissociative scattering of molecular beams from crystal surfaces: H2, N2 and NO from Cu(111) studied by translational spectroscopy

We have measured the kinetic energy release and final molecular axis orientation in dissociative molecular scattering of neutral H2, N2 and NO from Cu(111) under glancing incidence conditions. The final orientation distributions of H2 and N2 and NO are completely different. The results are insensitive to the crystal azimuth and thus to the geometrical corrugation of the crystal surface. We propose that dissociative scattering occurs via repeated transitions between potential-energy surfaces, and provides classical simulations of this process in the limits of strong and weak coupling. The influence on the results of both the strength of the coupling and the topography of the participating states is discussed.

Organic reactions in liquid-crystalline solvents. Regiochemical control of bimolecular pericyclic reactions by cholesteric and smectic liquid-crystalline solvents

The addition reactions of N-phenyl,N-para-biphenyl, and N-para,para'-terphenyl-maleimide with 7-dehydrocholesteryl acetate have been carried out in the isotropic and cholesteric liquid crystalline phases of a series of steroidal mesogens, and in the isotropic, smectic A, and smectic B phases of 4,4'-dialkylbiphenyl mesogens at temperatures between 180 and 240°C. In isotropic solvents, mixtures of four adducts are obtained, in relative yields that are essentially independent of the maleimide substituent. The tree major products (two ene-adducts and one Diels-Alder adduct) are formed via transition states in which the long molecular axes of the reactants are oriented perpendicular to one another, while the fourth (minor) product is an ene-adduct formed via a transition state with a parallel relative orientation of the reactants' long molecular axes

Direct measurement of the orientation dependence of dissociative scattering of H2+ from Cu(111)

We report a direct measurement of the molecular axis orientation dependence of dissociative neutralisation of 2.98 keV H2+ at a Cu(111) surface. The kinetic energy ϵ released and the final orientation θ of the molecular axis to the surface normal are derived from the measured laboratory-frame velocities of the neutral dissociation fragments. Above an incident beam perpendicular energy E⊥i ≈ 3.5 eV the ϵ energy distribution appears to consist of one structure peaking at or below ϵ ≈ 0.75 eV. As we reduce E⊥i, an additional broad structure peaking at ϵ ≈ 3 eV, θ ≈ 60° appears. The intensity ratio of these two structures remains approximately constant for E⊥i values below 1 eV. We propose that neutralisation occurs on the incident portion of the trajectory, primarily to the (surface modified) X1Σg+ state of H2, and furthermore, that the observed change of the spectrum with E⊥i may be related to reflection from (for E⊥i < 1 eV) and traversal over (for E⊥i > 1 eV) an activation barrier of height ∼ 1 eV on the H2/Cu potential energy surface.

Orientation and perpendicular energy dependence of dissociative scattering of hydrogen from Cu(111)

We report measurements of the final molecular axis orientation and kinetic energy released in the dissociative neutralization of 2.98 keV H 2 + incident at glancing angles to a Cu(111) surface. We observe two distinct structures which we correlate with scattering on two distinct regions of the potential hypersurface (PES) describing the interaction. The relative contribution of each of these structures to the total spectrum exhibits a threshold-like behaviour as a function of the perpendicular energy of the beam in the (nominal) range 1–2 eV. The perpendicular energy at which this threshold appears is dependent on the molecular axis orientation. We correlate this threshold with a barrier on the PES, and its dependence on orientation with an orientation dependent barrier height. The height variation is of order 0.9 eV, and the barrier is lowest for the surface-parallel axis orientation.

Molecular axis orientation and transitional path effects in the rotational structure of multiphoton transitions

The intensity distribution among the rotational branches in a multiphoton process is considered. It is shown that the excitation probability of a rotational line depends drastically on the transitional path involved as well as on the molecular axis orientation. Each transitional path derives a distinct rotational structure and the intensity distribution of the rotational branches in the excitation spectra reveals the dominant transitional path.

Orientation dependence of the hydrogen molecule's interaction with Rh(001).

First-principles calculational results, that conflict with conventional wisdom, are reported for the orientation dependence of the energy of H{sub 2}, a few bohrs above a Rh(001) surface. In geometric configurations near the saddle point where H-Rh bonds begin to form, the favored molecular axis orientation is along a {l angle}100{r angle} direction, corresponding to the attraction of each H atom to a twofold bridge site. This is true even though the final destinations of the H atoms are fourfold hollows, and even though the {l angle}100{r angle} orientation corresponds to relatively low symmetry, i.e., {ital C}{sub {ital s}} rather than {ital C}{sub 2{ital v}}.

Scattering of molecular hydrogen at fixed orientation

Ionization of one-electron heavy ion projectiles in collisions with molecular hydrogen is usually treated as collisions with two free hydrogen atoms. An approach is proposed here to solve this six-body problem within the framework of Born theory taking full consideration of the molecular properties of H 2, such as wave function and orientation of molecular axis. For C 5+ collisions on H 2, the obtained cross sections as a function of molecular orientation show interference patterns typical of two-center scattering. Orientation-averaged total cross sections for H 2 are found to be essentially the same as for 2H, in agreement with recent experiment.

Liquid Crystal Elastomers with Piezoelectric Properties

During the last few years, liquid crystalline elastomers (LCEs) have been systematically produced by cross-linking liquid crystalline side-chain polymers. In these networks, a liquid crystalline molecule is fixed at each monomeric unit. LCEs exhibit a novel combination of properties. Due to liquid crystalline groups, they show anisotropic liquid crystalline properties similar to conventional liquid crystals (LCs); but due to the three-dimensional network-structure of the polymer chains, they show typical elastomer properties, such as rubber elasticity or shape stability. One exceptional property of this combination is demonstrated when a mechanical deformation to the LCE causes macroscopically uniform orientation of the long molecular axis of the LC units (the so-called “director”).This response of the LC-phase structure to an applied mechanical field is similar to the effect of electric or magnetic fields on low molecular weight liquid crystals (LMLC), as illustrated in Figure 1. Figure la shows an undeformed LCE. Because of the non-uniform orientation of the director, the sample scatters light strongly so the elastomer is translucent like frosted glass. On the other hand, applying a mechanical field the director becomes uniformly aligned and the sample is transparent (Figure 1b). Such a macroscopically ordered rubber exhibits optical properties very similar to single crystals. These propertie s of LCEs offer new prospects for technical application, e.g., in nonlinear and integrated optics.

Ethylene oxide adsorption on K-promoted Ni(111)

Ethylene oxide (Et-O) adsorption on clean and K-modified Ni(111) surfaces was studied with thermal desorption spectroscopy (TDS) and work function change measurements (Δø) in the temperature range of 100–1000 K. On clean Ni(111), in agreement with previous measurements, evidence is found for a molecular adsorption of Et-O. The binding energy towards the Ni(111) surfaces is weak. With TDS desorption maxima are detected at 120 K (Et-O ice layer), 150 K (Et-O from a second adsorption layer) and finally at 195–215 K from fractional Et-O monolayer coverages. On clean Ni(111), Et-O lowers the work function with a maximum change of Δø = −1.6 eV. The negative sign of Δø is consistent with a perpendicular orientation of the Et-O molecular axis with the negative end — the O atom — directed towards the substrate. On K-dosed Ni(111) the Et-O adsorption behavior is significantly changed for low K coverages ( θ(K) < 0.15) and drastically altered for higher coverages, θ(K) ⩾ 0.36. Up to θ(K) = 0.15 the Et-O adsorption is still molecular. Due to the lateral interaction between Et-O and K the effective Et-O dipole moment as judged from Δø measurements and the repulsive interaction between neighboring Et-O (TDS peak narrowing) decreases to zero. For higher K coverages, gq(K) ⩾ 0.36, Et-O is thermally stabilized to temperatures above 400 K. This behavior is believed to be due to a ring breaking of Et-O and the formation of a K-stabilized radical. Above 540 K, evolution of H 2 and other molecular fragments gives evidence for further decomposition processes.

Slowing down of molecular rotation at the smectic-A-->smectic-C transition of liquid crystals.

We report the first observation of the slowing down of molecular rotation around the long axis in the smectic-A phase of a ferroelectric liquid crystal. This effect, evidenced by degenerate four-wave mixing, is interpreted in terms of cooperative orientation of the transverse molecular axes, due to a coupling between the fluctuating ferroelectric order and the transient biaxial mode induced by the laser fields.

Raman studies of orientational disorder in crystals

The O.D.I.C. phase of 1.fluoroadamantane was studied by Raman light scattering in the temperature range T c = 222K, 300K. From the measurements of integrated intensities of the A 1 internal modes recorded with Eg and F 2g symmetries, it is shown that the <111> directions of the cubic lattice are the preferred orientation of the threefold molecular axis. In other hand the activation of reorientational process at T c is clearly shown from the temperature dependence of widths A 1 and E internal modes. Interesting features related to the molecular reorientations and librations have been derived from the band shape analysis of F 2g and Eg low frequency spectra. The broad inelastic band which appears in the two symmetries is explained as due to libration around <111> directions and the origin of the temperature dependence of its width is discussed. The quasi elastic component that appears essentially in the F 2g low frequency spectrum is related to the deformation of the molecular polarizability tensor during the molecular reorientation.

Chemisorption of dioxygen on the Ag(110) surface

We have explored the chemisorption of molecular O2 at the long bridge site on the Ag(110) surface using a 24 atom ‘‘supercell’’ cluster model for the Ag(110) surface. From first principles generalized valence bond and configuration interaction calculations, we find that for chemisorption at this site, orientation of the O2 molecular axis along a [11̄0] azimuth is greatly preferred. In this orientation, we find geometric and spectroscopic parameters that are in excellent agreement with available experimental measurements. The molecule is found to sit 0.94 Å above the surface, with an O–O bond distance of 1.55 Å, and vibrational frequencies of ν(O–O)=627 cm−1 and ν⊥=278 cm−1. The O2 π orbitals normal to the surface interact strongly with second layer Ag atoms, while π orbitals parallel to the surface interact with first layer Ag atoms. Charge transfer is extensive (≊1.0–1.5 electrons) at the equilibrium geometry. The O2 is found to bond to an excited state of the Ag24 cluster when in the preferred geometry, and as a result, is endothermic with respect to the cluster ground state. We estimate the true bond energy on an Ag(110) surface to be 11.1 kcal/mol.

Molecular Packing in Thermotropic Smectic Phases by Neutron and X-Ray Diffraction

Abstract X-ray and neutron diffraction measurements of the intensity of the 00/ Bragg reflections from the smectic layers in hydrogenous and partly deuterated isobutyl 4-(4′-phenyl-benzylideneamino)cinnamate are reported. A model calculation has been used to deduce from these intensities, the configuration and orientational order of the molecules. The orientations of the long molecular axes are well ordered with P2 > 0.9 in S E, S B and S A phases and there is no appreciable interdigitation of the molecules of adjacent layers. These results are compared with those obtained from incoherent quasielastic neutron scattering experiments.

Interaction of fast molecular ions with surfaces

The scattering of molecular ions from surfaces has in general the same aspects as atomic scattering: nuclear and electronic interactions. For the nuclear or elastic part we have with molecules the additional channels of vibrational and rotational excitation, which are very important for the scattering of molecules at thermal energies, and may contribute to the dissociation of fast molecules. For the electronic or “inelastic” part molecules provide an additional channel for charge exchange processes, i.e. neutralization into dissociative states known as dissociative attachment in gas phase interaction. Experimental and theoretical results of the interaction of diatomic molecules with clean and Cs covered surfaces show that further improvements of present day charge exchange theories are needed. The results include the influence of the molecular axis orientation on the neutralization in the case of H + 2 and the influence of work function changes on the molecular survival and neutralization in case of H + 2, N + 2 and O + 2.

Simulation studies of bilayers of N 2 adsorbed on graphite at 25 and 35 K

Molecular dynamics simulations of bilayers of model nitrogen molecules adsorbed on the graphite basal plane at 25 and 35 K are reported. Systems considered include commensurate bilayers and three uniaxially compressed films, namely, two with different compressions along the X-axis (perpendicular to the herringbone lattice glideline) and one with a compression along the Y-axis, relative to the commensurate spacing. For comparison, commensurate and X-compressed monolayer films were also simulated. Properties simulated include: center-of-mass (COM) molecule-surface densities as a function of separation distance: in-plane COM pair correlation functions; in-plane and out-of-plane distributions of the molecular axis orientations; average potential energies for N 2-surface and for N 2-N 2 interactions; and velocity time-correlation functions for translational and orientational motion perpendicular to and parallel to the surface. These results indicate that the N 2 layers at these temperatures are nearly harmonic oscillator solids, with considerable orientational freedom. Molecules tend to be coplanar with the surface in both layers. The in-plane orientations for the compressed films form a herringbone lattice, in both layers, with variable angles between the molecular axis and the crystal glidliines; the commensurate film does not show herringbone ordering for the potential model used in this work. It is concluded that both the orientational and translational the degrees of freedom strongly coupled, between layers as well as within the layer. It is also shown that the Y-compressed film is unstable and that both layers in the bilayer spontaneously rearrange into a (rotated) uniaxially X-compressed structure.