Low-coverage Phase Research Articles

The room temperature phase of propene on Rh(111): A caveat on thermal processing for the production of adsorbed phases at definite temperatures

Hydrocarbon phases from the thermal processing of low temperature adsorption of propene on Rh(111) and those from direct adsorption at particular temperatures were characterized by HREELS and were found to show differences. The phase from direct adsorption at room temperature contains C x H species and ethylidyne which shows a better bond breaking ability than the room temperature phase produced by thermal processing which contains ethylidyne, propylidyne and di-σ adsorbed propene. The use of the phase from direct adsorption at room temperature, especially the low coverage phase, in comparison with similarly prepared phases on Ru(001), Ir(111), Ni(111), Pd(111) and Pt(111) shows a correlation on bond breaking ability that agrees with Sinfelt's correlation of the hydrogenolysis activity of the group 7–10 metals. This suggests that the room temperature phase of an alkene on a surface can be used to predict the hydrogenolysis activity of that surface.

Unoccupied electron band structure of Na overlayers on Al(111)

Using the technique of inverse photoemission spectroscopy in the isochromat mode, we have determined the unoccupied part of the electron band structure of Na/Al(111) for two ordered overlayers: (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{}, ${\ensuremath{\Theta}}_{\mathrm{Na}=(1/3}$ monolayer (ML) and (2\ifmmode\times\else\texttimes\fi{}2), ${\ensuremath{\Theta}}_{\mathrm{NA}=1/2}$ ML. At normal incidence, we find peaks at energies 1.1 and 2.1 eV above the Fermi energy ${E}_{F}$ for the low-coverage phase and 1.8 and 2.7 eV for the high-coverage phase. These peaks disperse away from ${E}_{F}$ as a function of increasing parallel momentum of the incident electron. At larger angles a new state is observed in each phase which disperses towards ${E}_{F}$. We assign these peaks to unoccupied p and d levels of the Na/Al(111) overlayers and compare our measurements to a recent band-structure calculation of isolated alkali-metal monolayers.

The influence of orientation on the H-D exchange reactions in chemisorbed aromatics: Benzene and pyridine adsorbed on Pt{110}

High resolution electron energy loss spectroscopy has been used to determine the orientation of benzene and pyridine adsorbed on a Pt[110} surface and to follow the H-D surface exchange reactions in these molecules. Benzene is found to non-dissociatively adsorb at 300 K, with the ring plane parallel to the surface at all coverages. Pyridine is adsorbed in a similar configuration at low coverages, but at higher coverages the majority of the molecules are bound through the N atom and stand proud of the surface. Near-complete H-D exchange occurs in benzene at all coverages, but only for the low coverage phase of pyridine. It is proposed that the very incomplete exchange observed at saturation coverage is due to the inaccessibility of the 3,4,5 ring positions in σ-bonded pyridine.

Interactions of H 2 with the Ni(110) surface: EELS and LEED studies

High-resolution vibrational electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED) have been used to study the interactions of hydrogen with the Ni(110) surface in the temperature range from 100 to 300 K. H 2 is adsorbed dissociatively on the Ni(110) surface. At 100 K, the (2 × 1)-H and (1 × 2)-H structures are formed; the streaky (1 × 2)-H structure is formed by heating the H-covered surface to above ~ 220 K or by the exposure of the Ni(110) surface to H 2 at 300 K. For the (2 × 1)-H surface which is characterized by the vibrational losses at 79 and 130 meV, H atoms are adsorbed in the low-symmetry short-bridge sites of the bulk-like Ni(110) surface. Adsorbed sites of H atoms for the low-coverage phases (found by molecular beam diffraction and video-LEED) are similar to those for the (2 × 1)-H structure. For the (1 × 2)-H surface with losses at 76, 117 and 150 meV, H atoms are considered to be adsorbed in the low-symmetry short-bridge sites and high-symmetry short-bridge sites of the second layer Ni atoms. The streaky (1 × 2)-H surface with losses at ~ 80, 117 and 140 meV consists of the undistorted and distorted patches whose local structures are similar to those of the (2 × 1)-H and (1 × 2)-H surfaces, respectively. The (2 × 1) → (1 × 2) phase transition is considered to be the first-order transition.

High resolution infrared study of hydrogen chemisorbed on Si(100)

The vibrational spectrum associated with hydrogen chemisorbed on Si(100) was studied for the first time by means of Fourier transform infrared spectroscopy. The low coverage phase (monohydride) is characterized at room temperature by a broad line (Δ⋍17cm −1) at 2095cm −1 which sharpens upon mild annealing, indicating an ordering of the hydrogen monolayer. The high coverage phase (dihydride) features symmetric [ν 1=2099cm −1, Δ=5 cm −1] and antisymmetric [ν 3=2087 cm −1, Δ=5cm −1] stretching modes with dynamic dipole moments perpendicular and parallel to the surface, respectively. The effective charge, e*/e⋍0.01, is consistent with early theoretical predictions. The resulting dipolar interaction is too weak to account for the 12cm −1 splitting, suggesting a small chemical interaction.

Atomic underlayer formation during the reaction of Ti{0001} with nitrogen

Changes in the {0001} surface of Ti during exposure to nitrogen gas are monitored by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Evidence is provided for the formation of a Ti{0001}1 × 1-N phase followed by a Ti{001} 3 × 3−30°-N structure. A LEED intensity analysis of the Ti{0001}1 × 1-N phase reveals that the N atoms penetrate into the octahedral interstitial holes underneath the first layer of Ti atoms. This is the first ordered monoatomic underlayer found in the earliest stages of any solid-gas interaction. The surface structure bears close resemblance to that of {111} planes in bulk TiN. We find a Ti-N bond length of 2.095 Å to be compared with the value of 2.120 A in bulk TiN. The analysis of the Ti{0001}1 × 1-N structure indicates that Ti{0001} 3 × 3−30°-N is not a low-coverage phase. The importance of recognizing the existence of 1 × 1 phases prior to the formation of superstructures is emphasized, and some procedures for extracting the information from experimental observations are discussed.