J. E. Demuth

The vibrations and structure of pyridine chemisorbed on Ag(111): the occurrence of a compressional phase transformation

Abstract High-resolution electron energy loss and UV photoemission spectroscopies have been used to study the chemisorption of pyridine on clean Ag(111) at T ≈ 140 K . Pyridine is weakly chemisorbed and undergoes a compressional phase transfomation from a Π-bonded species to a more weakly bound, nitrogen-lone-pair bonded species The molecular orientations of both chemisorbed phases are determined.

Observation of a methoxy species on Ni(111) by high-resolution electron energy-loss spectroscopy☆

Abstract The chemisorption and decomposition of methanol on Ni(111) has been studied by high-resolution electron energy-loss spectroscopy. We isolate and identify a methoxy species (CH3O) which forms as a quasi-stable surface intermediate during the thermal decomposition of chemisorbed methanol. The methoxy species is bonded with the oxygen end nearest to the surface and the methyl group inclined at an oblique angle to the surface.

Adsorption of oxygen on a Pd(111) surface studied by high resolution electron energy loss spectroscopy (EELS)

Oxygen adsorption on a Pd(111) surface has been studied in the temperature range from 30 K to 300 K by high resolution electron energy loss spectroscopy (EELS) and by low energy electron diffraction (LEED). At 30 K oxygen adsorption leads to two roughly equally populated molecular adsorption states characterized by vibrational losses at 850 and 1035 cm−1. These losses are assigned to the O-O stretching frequency of molecularly adsorbed oxygen in a peroxo-like state and a superoxo-like state, respectively. After saturation of these chemisorbed molecular states, a state of physisorbed oxygen with its vibrational frequency close to the gas phase value of 1556 cm−1 is populated. Upon warming the sample above 80 K, an additional loss feature at 650 cm−1 develops which is assigned to a second peroxo-like molecular species. Between 80 and 180 K an interconversion of the different molecular species takes place in which the molecular states are sequentially populated in the order of decreasing vibrational frequency νO-O as the sample temperature is raised. The dissociation process is completed at T ≈ 200 K leaving a layer of atomic oxygen on the surface which is characterized by a vibrational loss at 480 cm−1 and by a 2 × 2 pattern in LEED. The results demonstrate that the dissociative chemisorption of oxygen on Pd(111) does not proceed in a single step but through a sequence of several well defined molecular precursor states.

Electronic and geometric structure of Si(111)-(7 × 7) and Si(001) surfaces

Abstract The atomic origins of the intrinsic surface states of the Si(111)-(7 × 7) and Si(001) surfaces have been identified using the recently developed method of current imaging tunneling spectroscopy (CITS). On Si(111)-(7 × 7) three filled and two empty surface states are found and directly identified with atomic features of the dimer-adatom-stacking fault model. On Si(001) one filled and one empty state are observed and identified with atomic features of a dimer model. The STM images of Si(001) are shown to be dominated by the surface electronic structure rather than geometric structure.

Enhanced raman scattering from carbon layers on silver

Abstract The broad bands at 1350 and 1550 cm −1 observed in Raman scattering from Ag in electrochemical cells have been observed on Ag surfaces in tunneling structures and in ultrahigh vacuum. Both the shape and intensity of this scattering is explained by the presence of amorphous carbon on the Ag surfaces.

Chemisorption of C2H2 on Pd(111) and Pt(111): formation of a thermally activated olefinic surface complex

Abstract Ultraviolet photoemission spectroscopy studies indicate that acetylene chemisorbs on Pd(111) and Pt(111) as π-bonded acetylene for T ≲ 180 K but upon warming (or room temperature exposure) forms a new surface species. We identify the electronic structure of this new phase as being characteristic of an olefinic C 2 H 2 surface species - the result of rehybridization of the initially chemisorbed molecule. These results are discussed relative to previous work on Ni(111).

Surface Electronic Structure of Si(111)-(7 × 7) Resolved in Real Space

We have obtained the first energy-resolved real-space images of the filled and empty surface states of the Si(111)-(7 × 7) surface, with 3-A lateral resolution. This ability to resolve spatially these surface states with a scanning tunneling microscope depends upon a new method to acquire and separate geometric and electronic information. Our results not only are in good agreement with previous spectroscopic studies but also directly reveal the atomic location and geometric origin of the Si(111)- (7 × 7) surface states.

Low temperature oxidation of silicon (111) 7 × 7 surfaces

Abstract The coverage dependent interaction of molecular oxygen with the Si (111) 7 × 7 surface at 20 K has been studied with high resolution electron energy loss spectroscopy, ultraviolet photoelectron spectroscopy, and low energy electron diffraction. These results provide a new view of the initial stages of oxidation of silicon. In addition to physisorbed molecular oxygen, two other oxide species occur at monolayer coverages: diatomic-like and bulk-like forms of silicon monoxide. Formation of the diatomic-like monoxide begins at the lowest coverages while the appearance of the bulk-like monoxide is delayed. The diatomic silicon monoxide is stable to a temperature between 475 and 575 K, and the bulk-like monoxide is stable to approximately 950 K. This latter oxide completely desorbs at approximately 975 K. The sticking coefficient for decomposition of molecular oxygen was found to be much greater at 20 K than at room temperature. This effect can be rationalized if it is assumed that a mobile precursor state, differing from those of metal surfaces, has a major role in the oxidation reaction. We also present evidence that the detailed vibrational spectra frequently observed for oxygen absorbed on Si (111) is complicated by the presence of a very small amount of hydroxyl contamination.

Electronically excited states of chemisorbed molecules

The electronic excitations of CO, NO, and pyridine chemisorbed on a variety of surfaces have been studied with low energy (10–20 eV) electron energy loss spectroscopy. By comparing the observed electronic transitions of CO on Ni(100), Cu, and Si(111) surfaces, the ∼6 and ∼8.5 eV transitions of chemisorbed CO are reassigned as singlet and triplet coupled 5σ→2π* excitations, respectively. By simple arguments we show that screening (both image and charge transfer) and spin quenching by a metallic substrate should not significantly affect the transition energies of neutral adsorbate excitations. This behavior is discussed in relation to photoemission measurements and, in particular, the limitations of using photoemission data to estimate electronic excitation energies of adsorbed molecules. The dependence of electronic spectra on adsorbate bonding configurations are considered for NO and pyridine. Vibrational spectra of NO on Ni(100) reveal changes in bonding configuration as a function of NO surface covera...

Chemisorbed pyridine on Ni(001): A high resolution electron energy loss study of vibrational and electronic excitations

High resolution angle‐resolved electron energy loss spectroscopy has been applied to the study of the vibrational and electronic losses of pyridine chemisorbed on Ni(001). The evaluation of the contributions of in‐plane vs out‐of‐plane vibrational modes of the chemisorbed molecule in the dipole scattering regime shows that orientational phase transitions occur as a function of coverage and temperature. In particular, the molecular plane is parallel to the surface at low coverages at ∼170 K corresponding to a π‐bonded species; at higher coverages the molecular plane tilts vertically with respect to the surface which is indicative of a bonding interaction through the N lone pair. Heating to room temperature or room temperature exposures produce an approximately vertically oriented species with some rotation about the N end. This species might be assigned to a chemisorbed α‐pyridyl species. The different types of bonding at the surface are manifested in the characteristics of the electronic loss spectra take...