H. Weiss

Fourier-transform infrared spectra of CO adsorbed on NaCl(100): structural changes at low temperatures

The monolayer of CO adsorbed on NaCl(100) forms a p(1×1) structure with CO aligned normal to the surface at temperatures above 35 K. This has been revealed from polarized infrared spectra, measured with a high-resolution Fourier-transform spectrometer. At temperatures above 35 K at monolayer coverage a single sharp absorption is observed with p-polarized light only, the wavenumber being 2155.21 cm−1 at 40 K. Below 35 K a second absorption at ∼2149 cm−1 is observed with p- and even stronger with s-polarized radiation. The p(1×1) structure described above is not in concert with these spectra. Monolayer spectra of isotopic mixtures of 12C16O/13C16O adsorbed on NaCl(100) at 19 K showed a decrease of the splitting with decreasing partial coverage. The data are most readily explained by the assumption of a correlation field splitting. The spectra of submonolayers show a temperature dependence similar to that of the full monolayer. The frequency shift and the observation of two bands suggest island formation.

Site symmetry and correlation field splitting in the adsorbate CO2NaCl(100)

Abstract Site symmetry and correlation field splitting in the s- and p-polarized infrared spectra of the v2 bending vibration of monolayer CO2 adsorbed on NaCl(100) were detected. Also the asymmetric v3 stretching vibration of the adsorbate was observed and shows a splitting due to the correlation field. The monolayer coverage was established by measurement of adsorption isotherms. These demonstrate a two-dimensional first order phase transition in CO2NaCl(100).

Low-energy electron induced restructuring of water monolayers on NaCl(100)

The influence of electron irradiation on the controversially discussed monolayer structure of H(2)O on NaCl(100) is investigated with helium atom diffraction before and after a low-damage low-energy electron diffraction (LEED) experiment. The ordered (1x1) structure observed initially with He atoms is found to be transformed to a stable c(4x2) structure after a 90 eV electron dosage of only 10(15) electrons cm(-2) or about 2 incident electrons per adsorbate molecule. Based on previously reported structure models for the two phases, the transition is attributed to a reorientation, and a possible compression of the water film induced by the electrons.

FTIR-spectroscopy as a highly sensitive technique to study adsorption and desorption on ionic film and single crystal surfaces

Infrared absorption by CO and CO2 adsorbed on NaCl (film) and NaCl(100) single crystal surfaces has been measured under ultrahigh vacuum conditions as a function of temperature and pressure, using grating and FTIR transmission-absorption spectroscopy. The absorption line of a monolayer of CO on NaCl(100) due to the stretching vibration is extremely narrow (<2 cm−1) and onlyp-polarized light is absorbed at oblique incidence. Resonant desorption in CO-NaCl (film) has been detected. Thev3 absorption of a monolayer of CO2 on NaCl(100) appears as a doublet.

Structure of CO2 adsorbed on the KCl(100) surface.

The structure and dynamics of the adsorbate CO(2)/KCl(100) from a diluted phase to a saturated monolayer have been investigated with He atom scattering (HAS), low-energy electron diffraction (LEED), and polarization dependent infrared spectroscopy (PIRS). Two adsorbate phases with different CO(2) coverage have been found. The low-coverage phase is disordered at temperatures near 80 K and becomes at least partially ordered at lower temperatures, characterized by a (2√2×√2)R45° diffraction pattern. The saturated 2D phase has a high long-range order and exhibits (6√2×√2)R45° symmetry. Its isosteric heat of adsorption is 26 ± 4 kJ mol(-1). According to PIRS, the molecules are oriented nearly parallel to the surface, the average tilt angle in the saturated monolayer phase is 10° with respect to the surface plane. For both phases, structure models are proposed by means of potential calculations. For the saturated monolayer phase, a striped herringbone structure with 12 inequivalent molecules is deduced. The simulation of infrared spectra based on the proposed structures and the vibrational exciton approach gives reasonable agreement between experimental and simulated infrared spectra.

The monolayer structure of CO2 adsorbed on the NaCl(100) single crystal surface: A tensor low-energy electron diffraction analysis

For the first time a detailed experimental analysis of all structural parameters of the p(2×1) monolayer phase of CO2 physisorbed at 75 K on a NaCl(100) single crystal substrate has been performed. The system was investigated by means of low-energy electron diffraction (LEED) at primary beam currents in the nA range. Intensities of six beams, including two of fractional order, were recorded at normal incidence in the energy range between 70 and 250 eV. These served as experimental data set for a TLEED-based I(V) analysis, in which the observed glide-plane symmetry as well as the linearity of the physisorbed molecules were fully considered. The substrate was represented by (a) an ideally bulk-terminated surface and (b) the relaxed surface which was reported before for the clean NaCl(100) surface. For both substrate models satisfactory agreement between experimental and theoretical I(V) curves was attained with Pendry R factors RP of 0.23 and 0.22, respectively, and similar molecular best-fit structures. Th...

Structure and infrared absorption of the first layer C2H2 on the NaCl(100) single-crystal surface

The adsorbate structure of the first layer acetylene on the NaCl(100) single-crystal surface is investigated using polarization infrared spectroscopy in combination with low-energy electron diffraction (LEED) experiments, and potential calculations on the basis of pair potentials. In agreement with a previous study, a triplet of infrared absorptions in the region of the asymmetric stretch vibration of C(2)H(2) was observed and assigned to an adsorbate phase with (3 square root 2 x square root 2)R45 degrees translational symmetry determined in the LEED experiment. The polarization dependence of the infrared spectra is consistent with a parallel orientation of the molecules with respect to the surface. The number of molecules per unit cell is four to six as determined by photometric considerations. Total energy minimizations support a new structure model which contains five inequivalent molecules per unit cell in a herringbone arrangement. The application of a vibrational exciton approach demonstrates that this new structure model can reproduce the triplet spectrum observed in the infrared experiments.

Evaluation and X-Ray Induced Modification of the Cerium Oxidation State in Cerium Calixarene Complexes

The cerium oxidation state in novel calixarene-supported cerium(IV) β-diketonate complexes [p-tBu-calix[4](OMe)2(O)2]Ce(acac)2 (1) and [p-tBu-calix[4](OMe)2(O)2]Ce(hfac)2 (2), which are a new class of potential precursors for homogeneous oxidative transformations, has been determined using X-ray photoelectron spectroscopy (XPS). Cerium oxidation states between 3.6 and 3.65 were detected, distinctly different from their nominal value of +4. An X-ray induced photoreduction of these compounds was detected. Because of the observed stability of the X-ray modified oxidation state under ambient conditions this effect might be used for a long-standing fine tuning of the Ce oxidation state in cerium calixarenes.

Mechanical Alloying of Ti-24Al-11Nb (AT%) Using the Simoloyer (ZOZ-Horizontal Rotary Ball Mill)

The production of large quantities of contamination free mechanically alloyed powders from titanium based materials has proven to be major challenge. Feasibility of such a goal can be carried out, at laboratory level, by any milling device like the very common planetary all mill. In this case however, the possibility of of subsequent scaling up for larger production is hindered by the intrinsic limits of planetary ball mill design. On the contrary the horizontal Simoloyer can be experimented at laborator level using small volume chamber-units (0.25, 0.5, 2 l) and, for industrial produdion, using the large volume units (up to 400 l) based on the same conceptual design. Therefore, experiments have been conducted on blended elemental Ti-24Al-11Nb (at%) powder using a simoloyer with a small unit-chamber (0.5 l). Due to the inherent ductility of the powder, the material has the tendency to adhere to the grinding unit and the steel balls. Further, in order to avoid high contamination and to make the process realistic from an economical point of view, the milling time has to be reduced to a minimum. The above points identify a Critical Milling Behaviour (CMB) of the system under investigation that must be kept under control to achieve the wanted goal. It will be shown that by adopting a suitable milling and discharging procedure low contamination and good yield have been substantially achieved with respect to preliminary not unambiguous trials carried out at Idaho university.

Physisorption of Small Molecules on Single Crystal Alkali Halide Surfaces: CO and N2 Adsorbed on KCl(100)

Abstract The adsorption of CO and N2 on KCl(100) single crystal cleavage planes has been investigated by means of low energy electron diffraction (LEED) at primary currents in the nA range as well as polarization infrared spectroscopy (PIRS) in transmittance geometry. These isoelectronic adsorbates behave very similarly, and three different adsorption phases can be distinguished as a function of surface coverage for both of them. Initially a (1×1) structure is observed and assigned to a commensurate monolayer with one molecule per KCl ion pair. The CO infrared spectrum of this ‘phase I’ is characterized by a doublet absorption of the CO stretching vibration, which is discussed in the context of a correlation field splitting. The lack of superstructure diffraction peaks is attributed to a high degree of orientational disorder in this phase. From LEED adsorption isotherms the isosteric heat of adsorption of N2 has been determined to be 11±3 kJ mol−1. Upon increase in coverage by 50% ‘phase II’ is formed which exhibits a large number of additional diffraction spots. It is assigned to a bilayer in which the second layer is only half filled. Based on the LEED experiments a structure model is proposed in which the second layer is characterized by growth in rows along the [210] direction and a high degree of roughness in the perpendicular direction. This model can explain all experimentally observed main features. Finally upon further exposure formation of three-dimensional solid is observed, which proceeds via Stransky–Krastanov growth of crystalline 3D clusters with the structure of the low-temperature cubic α-phase. A simulation of the CO cluster infrared spectra within the dipole–dipole coupling approach can reproduce all major observed vibrational features.