ALBERT

Notes: IR absorption lines of CO on ultrathin epitaxial iron films are both enhanced and asymmetric. In this letter we show new experimental results which demonstrate a correlation of the asymmetry of the CO-stretching line to electronic properties of the underlying metal film. The new finding indicates the important role of metal film morphology for nonadiabatic effects. Such effects are strongest slightly above the percolation threshold as our results show. © 2000 American Institute of Physics.

Notes: Glasses with a composition xLi2O⋅(1−x)B2O3 were investigated by low-frequency Raman scattering in the composition range x=0–0.28. The evolution of the quasielastic line, the boson peak, the Debye frequency, and some other glass parameters with the composition was analyzed. The frequency of the boson peak ωb shifts with changing x by a factor of 3 and the width of the quasielastic spectrum at room temperature is always equal to ∼0.24ωb. The Grüneisen parameter of the glasses is estimated on the basis of the light scattering data for the boson peak frequency within the frames of the anharmonic model of the fast relaxation and using the sound velocity data—for the Debye frequency. The anharmonic properties are compared with the fragility of these glassformers; it is shown that the fragility increases with anharmonicity. It is shown also that the width of the glass transition region correlates with the anharmonic properties. © 2000 American Institute of Physics.

Notes: Electron transfer reactions at semiconductor/liquid interfaces are studied using the Fermi Golden rule and a free electron model for the semiconductor and the redox molecule. Bardeen's method is adapted to calculate the coupling matrix element between the molecular and semiconductor electronic states where the effective electron mass in the semiconductor need not equal the actual electron mass. The calculated maximum electron transfer rate constants are compared with the experimental results as well as with the theoretical results obtained in Part I using tight-binding calculations. The results, which are analytic for an s-electron in the redox agent and reduced to a quadrature for pz- and dz2-electrons, add to the insight of the earlier calculations. © 2000 American Institute of Physics.

Notes: Measurements of initial adsorption probabilities, S0, as well as the coverage dependence of the adsorption probability, S(aitch-thetaCO), of CO on Zn–ZnO [ZnO(0001)] and O–ZnO [ZnO(0001¯)] are presented. The samples have been characterized by He atom scattering, He atom reflectivity measurements, LEED, and XPS. Samples with different densities of defects were examined, either by investigating different samples with identical surface termination (for O–ZnO) or by inducing defects by ion sputtering at low temperatures (for Zn–ZnO). The influence of kinetic energy and impact angle (for Zn–ZnO) as well as adsorption temperature on the adsorption dynamics have been studied. For both polar surfaces the shape of the coverage dependent adsorption probability curves are consistent with a precursor mediated adsorption mechanism. Adsorbate assisted adsorption dominates the adsorption dynamics for high impact energies and low adsorption temperatures, especially for Zn–ZnO. The He atom reflectivity measurements point to the influence of an intrinsic precursor state. In contrast to the Zn–ZnO surface, for O–ZnO a weak thermal activation of the CO adsorption was observed. Total energy scaling is obeyed for Zn–ZnO. The heat of adsorption for CO on both polar faces varies between 7 kcal/mol (low coverage) and 5 kcal/mol (high coverage). A comparison of He atom reflectivity with S(aitch-thetaCO) curves demonstrates that CO initially populates defect sites on both surfaces. For O–ZnO an increase in S0 with decreasing density of defects was observed, whereas for the Zn-terminated surface S0 was independent of the defect density within the range of parameters studied. © 2000 American Institute of Physics.

Notes: The coadsorption of CO and butane on a Pt(533) stepped surface has been investigated using reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). The adsorption of butane on Pt(533) with CO preadsorbed on step-atop sites reveals that butane can force CO to tilt with a minimum angle of 42° away from the surface normal and displace CO from step-atop to step-bridge sites. The energy required for this tilting should be less than 20.5 kJ/mol. The coverage at which the compressed butane phase occurred was found to be the same at which this phase occurs on bare Pt(533). Together with the observed tilting and displacement of CO, this suggests that at low coverages butane adsorbs on the terraces, rotated 60° away from the step edge. The second monolayer phase then consists of tilted butane molecules having two hydrogen atoms in direct contact with the surface, situated near the step edge. The presence of butane also results in a downward shift of the CO stretch frequency, caused by electron donation in the 2π* antibonding CO orbital. When butane is preadsorbed at a submonolayer coverage exposure to CO leads to displacement of butane into a compressed phase and even into a multilayer phase. This effect becomes smaller as the initial butane coverage is increased to the multilayer regime. © 2000 American Institute of Physics.

Notes: The atomic structure and diffusion at the solid–liquid heterophase interface are investigated by using Molecular Dynamics. The system studied is made of crystalline copper with surface terminations (100) and (111) and liquid aluminum, both modeled via adapted n-body potentials from the literature and cross interactions obtained by fitting the mixing enthalpy of the two species to experimental values. It is shown that at the interface the liquid forms layers with spacing such that the local average density equals that of the bulk liquid. The interfacial liquid is layered whatever the surface orientation is even if the solid is reduced to a single crystalline or amorphous layer, in agreement with density functional theory. Layering is however suppressed at the interface between the liquid and a bulk amorphous solid with a rough surface termination. Surprisingly, diffusion in the interfacial layers proceeds via vacancies, which also accommodate the density misfit between solid (Cu) and liquid (Al). These results are further discussed in the frame of existing experimental and theoretical works. © 2000 American Institute of Physics.

Notes: Penning ionization electron spectroscopy was applied to ultrathin pentacene films [monolayer (0.3 nm thick) to dozens of layers] prepared by vapor deposition under different conditions. Remarkable differences were found among the Penning ionization electron spectra (PIES). The local electron distribution of each molecular orbital (MO) protruding from the film surface was probed and the relation between the MO shape and the molecular orientation was investigated. Deposition onto a metal substrate without a crystallographical surface yields a crystalline film at room temperature. The molecules are oriented with the long axes almost perpendicular to the substrate and make the σ bands of the PIES by far stronger than the π bands. In the pure π region, the π9 and π7 MOs having large distribution at the long-axis end provide more intense bands than other π MOs. On the metal substrate held at 213 K, molecules form an amorphous film with the long axes inclined a little on average. The π and σ bands exhibit comparable intensities and no specific band is enhanced. When 1 monolayer equivalence (MLE) of pentacene is deposited onto a graphite substrate at 123 K, a monolayer of flat-lying molecules is obtained. The π MOs provide more enhanced bands than the σ MOs but the π9 and π7 MOs with little distribution around the C–H bonds are harder to detect than other MOs in the pure π region. Furthermore, the growth of each film was investigated using Penning spectroscopy and ultraviolet photoelectron spectroscopy in combination. Spectral dependence upon amount of deposition revealed three modes of film growth, which correspond to the three molecular aggregations. The crystalline "film" cannot cover the substrate to ca. 30 MLE because molecules landed on the substrate move around and gather to form crystallites which grow three-dimensionally. But, the crystallite formation is inhibited on the cooled metal substrate owing to the low mobility of molecules. The rough surface is completely covered at 3–5 MLE and the molecules are accumulated randomly but uniformly in thickness with further deposition. On the graphite substrate, every new monolayer of flat-lying molecules is formed at 123 K and piled up in succession to form a layered film. With increasing number of layers, however, the surface molecules become inclined little by little. Finally, at 60 MLE they are tilted to the same extent as in an amorphous film. The structures and growth modes were found consistent with the stability or sublimation properties of these and related films as well as with the relaxation shifts reflected in the positions of the first PIES bands. It was also indicated that the aggregation of the outermost molecules is considerably different between the amorphous and layered film of 60 MLE in spite of similar, somewhat-tilted orientation. That is, the molecules mutually overlay and sterically prevent the neighbors from desorbing in the former, whereas the molecules lack upper-side neighbors and are very liable to desorb in the latter. © 2000 American Institute of Physics.

Notes: The structure of water around methane during hydrate crystallization from aqueous solutions of methane is studied using neutron diffraction with isotopic substitution over the temperature range 18 °C to 4 °C, and at two pressures, 14.5 and 3.4 MPa. The carbon–oxygen pair correlation functions, derived from empirical potential structure refinement of the data, indicate that the hydration sphere around methane in the liquid changes dramatically only once hydrate has formed, with the water shell around methane being about 1 Å larger in diameter in the crystal than in the liquid. The methane coordination number in the liquid is around 16±1 water molecules during hydrate formation, which is significantly smaller than the value of 21±1 water molecules found for the case when hydrate is fully formed. Once hydrate starts to form, the hydration shell around methane becomes marginally less ordered compared to that in the solution above the hydrate formation temperature. This suggests that the hydration cage around methane in the liquid may be different from that when hydrate is forming and from that found in the hydrate crystal structure. Methane–methane radial distribution functions show that methane molecules can adopt a range of separations during hydrate formation, corresponding to the more distorted nature of the methane–water correlations. There is noticeable ordering of the methane molecules with a monolayer of water molecules between them once hydrate has formed. The dipole moments of the hydrating water molecules lie mostly tangential to the methane–water axis, both before, during, and after hydrate formation. © 2000 American Institute of Physics.

Notes: The A˜ 1B1–X˜ 1A1 electronic transition of germylene has been reinvestigated. A room temperature absorption spectrum of the central portion of the 000 band of GeH2 has been obtained using the technique of laser optogalvanic spectroscopy. A rotationally resolved spectrum of the 000 band of jet-cooled GeD2 has been recorded with a pulsed discharge source. Analysis of these spectra has yielded ground and excited state rotational constants for the 74GeH2, 72GeH2, 70GeH2, 76GeD2, 74GeD2, 72GeD2, and 70GeD2 isotopomers and approximate equilibrium structures of: r″(Ge–H)=1.5883(9) Å, θ″(H–Ge–H)=91.22(4)°, r′(Ge–H)=1.5471(6) Å, and θ′(H–Ge–H)=123.44(2)°. The ground state ν1 and ν2 vibrational frequencies have been determined from wavelength-resolved fluorescence spectra of jet-cooled GeH2 and GeD2. There is good evidence that GeH2 rotational levels with Ka′〉1 are so strongly predissociated that lifetime broadening makes them diffuse, severely restricting the information that can be obtained from absorption and laser-induced fluorescence experiments. © 2000 American Institute of Physics.

Notes: We report an accurate ab initio study of the effects of chirality on the intermolecular interactions between two small chiral molecules bound by a single hydrogen bond. The methods used are second-order Møller–Plesset theory (MP2), as well as density functional theory with the B3LYP functional. The differential interaction energy between two homochiral molecules, e.g., R⋅⋅⋅R′ and the analogous heterochiral molecules R⋅⋅⋅S′ measures the degree of chiral discrimination, termed the chirodiastaltic energy, ΔEchir. Formation of the O–H⋅⋅⋅O hydrogen bond between the chiral H-bond donor HOOH and the chiral H acceptor 2-methyl oxirane leads to four diastereomeric complexes. There are two distinct contributions to the chirodiastaltic energies, the diastereofacial contribution which controls the face or side of the acceptor to which the H bond is formed, and the diastereomeric contribution, which is the energy difference between two complexes formed by (M)- and (P)-HOOH to the same face. The largest chirodiastaltic energy is ΔEchir=0.46 kcal/mol (6% of the binding energy) between the syn-(M)- and syn-(P)-HOOH⋅2-methyl oxirane complexes. The chiral 2,3-dimethyloxirane acceptor is C2 symmetric and hence offers two identical faces. Here the chirodiastaltic energy is identical to the diastereomeric energy, and is calculated to be ΔEchir=0.36 kcal/mol or 4.5% of the binding energy. © 2000 American Institute of Physics.