Showing papers on "Chemisorption published in 1984"

The electromagnetic theory of surface enhanced spectroscopy

Abstract: Surface enhanced spectroscopy (SES) was born in 1 974 with the measure­ ment of Raman spectra of molecules adsorbed on a roughened electrode surface (la-e). Given the smallness of the Raman cross section, the detection of a Raman signal should have generated some excitement. This did not happen, but was delayed until 1977 when Jeanmaire & van Duyne (2) and Albrecht & Creighton (3) showed that the rough silver surface enhances the Raman cross section by a factor ranging between 104 and 106. This started a great outpouring (4a-i) of theoretical and experimental work, whose central theme is to understand how the presence of a solid surface modifies the spectroscopic and photochemical properties of a molecule located nearby. We divide the effects of the surface in two categories: electromagnetic, which can be described by solving Maxwell's equations, and chemical, which belong to quantum chemistry. The enhancement of the local laser field due to the polarization of the surface is an example of an elec­ tromagnetic effect. The appearance of a new excited state caused by chemisorption, leading to an enhancement of the Raman cross section through resonance Raman scattering (which would not be expected on the basis of the gas phase properties of the molecule), is an example of a chemical effect. While there is no doubt that chemisorption modifies the optical response of adsorbed molecules, the magnitude of the modification is still a subject of controversy. We don't know whether we should expect large modifications for all molecules, or for a small class (e.g. those with 11: orbitals); or whether

Probing valence states with photoemission and inverse photoemission

Abstract: Photoemission and inverse photoemission (or bremsstrahlung spectroscopy) are shown to be complementary techniques for probing occupied and unoccupied electronic states, respectively. The momentum of delocalized valence states can be measured as well as their energy if low electron (photon) energies in the 10–40 eV range are used. Thus, energy band dispersions are obtained for bulk, surface, and adsorbate states which cannot be determined by other techniques. A status report of inverse photoemission is given and illustrated by results for bulk states in ferromagnets and silicides, for broken bond states on silicon surfaces, and for unoccupied molecular adsorbate orbitals.

The dissociation of H2 on the Ni(100) surface

Abstract: The dissociation of H2 on the (100) surface of Ni is investigated using a cluster model. The mechanism for dissociation of H2 directly above a Ni atom has little to no barrier and involves the Ni 3d electrons; elimination of the Ni 3d interaction with the H2 increases the barrier to more than 50 kcal/mol. The dissociation at the bridge site, treated without the Ni 3d interaction, leads to a barrier of about 30 kcal/mol, leading to the conclusion that the dissociation of H2 at any site on a Ni(100) surface requires strong 3d participation. The results are quantitatively different if the Ni 4p orbitals are not included. The effects of cluster size on the results are also discussed.

A high-resolution electron energy loss spectroscopy study of the surface structure of benzene adsorbed on the rhodium(111) crystal face

Abstract: Benzene adsorption on the Rh(111) crystal surface has been studied by HREELS (high-resolution electron energy loss spectroscopy), LEED (electron diffraction), and TPD (temperature programmed desorption). The vibrational spectra indicate that benzene adsorbs molecularly at 300 K and is ..pi..-bonded to the surface with the ring plane parallel to the surface plane. Recent dynamic LEED calculations together with the angle-dependent HREELS studies reported here establish a C/sub 3v/(sigma/sub d/) bonding symmetry for the c(2..sqrt..3 x 4)rect-C/sub 6/H/sub 6/ structure. Several other ordered benzene overlayers can be formed between 300 and 400 K depending on the benzene coverage. No large changes occur in the chemisorption bonding mode or geometry coincident with the two-dimensional ordering phase transitions in this temperature range. The vibrational spectra show that two molecular adsorption sites can be populated. Benzene adsorption is only partially reversible; less than 20% of the adsorbed benzene desorbs molecularly upon heating. The remaining benzene irreversibly decomposes, evolving hydrogen and leaving a carbon-covered surface. The TPD and HREELS data on Rh(111) and other single-crystal surfaces show correlations between the metal-benzene bond strength, the work function of the clean surface, and the frequency shifts of some of the molecular benzene vibrational modes. 44 references, 10 figures,more » 4 tables.« less

Unusual C-O Bond Weakening on a Clean Metal Surface: CO on Cr(110)

Abstract: Donnees sur un mode de chimisorption particulier mis en evidence par spectroscopie de perte d'energie electronique, diffraction d'electrons peu energetiques, spectroscopie Auger, etc

Hydride formation on the Si(100): H 2 O surface

Abstract: ${\mathrm{H}}_{2}$O chemisorption on Si(100) is studied with high-resolution infrared spectroscopy. Water molecules are found to dissociate upon room-temperature exposure forming Si-H and Si-OH. Mild annealing breaks up the O-H molecule resulting in oxide, monohydride, and dihydride species. The angle made by the effective dynamic dipole moment associated with the Si-H bond, with respect to the surface normal, is measured for the different hydrides formed.

Chemisorption of hydrogen on the Si(100) surface: Monohydride and dihydride phases

Abstract: We present a systematic study of the clean and chemisorbed-hydrogen-covered Si(100) surfaces. Experimentally, we used He i and He ii ultraviolet photoemission spectroscopy and were able to resolve important chemisorption features which had not been observed previously. We carried out electronic energy calculations using both empirical tight-binding and extended H\"uckel methods, from which we deduce a consistent interpretation of the spectra. It is found that the low-binding-energy surface states resulting from the asymmetric dimer-bond model are in good agreement with the experimental data, whereas one high-binding-energy surface state associated with the displacements of the subsurface atoms does not appear in the spectra. Hydrogen chemisorbing on the Si(100)-(2\ifmmode\times\else\texttimes\fi{}1) surface forms a monohydride phase, and removes most of the surface states, except for the dimer bond and some of the backbonding states. Based on the theoretical and experimental results it is suggested that at high coverage the Si---H bonds of the dihydride phase rotate and depart from the tetrahedral directions. The occurrence of this phase on the other surfaces of silicon, and its possible connections with the observed spectral features, are extensively discussed.

Potential energy surfaces of MH2 (M=Co, Fe, and Cu)

Abstract: In many important catalytic processes, the dissociation of the hydrogen molecule is a fundamental step, and the chemisorption of dihydrogen by metal surfaces has been studied. However, quantitative results from theoretical treatments are not very encouraging. Blomberg and Siegbahn (1983) have found that the negative binding energy for NiH2 obtained with the aid of the commonly used Hartree-Fock approximation differs greatly from the result found on the basis of a more sophisticated calculation, including electron correlation. Another study showed the great importance of d orbitals in the dissociation process. A study, similar to the one on NH2, has been conducted for three other metals, including cobalt, iron, and copper. The present investigation is concerned with the results of this study. The potential surfaces of CoH2, FeH2, and CuH2 are discussed, taking into account the lowest atomic states, the bent low spin complex, the bent high spin saddle points, and the linear states.

Stoichiometric adsorbate species interconversion processes in the chemisorbed layer. An infrared study of the carbon monoxide/palladium system

Abstract: On montre qu'a 80 K, l'adsorption d'especes a CO lie a l'extremite, sur du Pd sature avec des especes a CO ponte entraine la conversion stœchiometrique de 2 especes a CO-ponte en especes a CO-terminal pour chaque CO terminal adsorbe

The vibrational spectra of chemisorbed molecular clusters: H2O on Ru(001)

Abstract: Electron energy loss spectroscopy and thermal desorption mass spectroscopy have been used to correlate vibrational spectra of H2O on Ru(001) with ESDIAD patterns reported recently. We find that the ‘‘extended bilayer’’ is characterized by a sharp fundamental O–H vibration at 3500–3565 cm−1 which is assigned to nonhydrogen‐bonded OH bonds of molecularly adsorbed water. Hydrogen bonds within the well‐ordered bilayer give rise to features at ∼3290–3450 cm−1 which are of relatively weak intensity in electron scattering due to the orientations of these bonds. In addition, very small clusters exhibit a weak vibrational mode at ∼2935 cm−1 which possibly represents H2O molecules with one O–H bond pointing into the surface. Isolated H2O molecules at low temperature and low coverage exhibit a fundamental O–H vibration at 3600 cm−1. A reinterpretation of the thermal desorption spectra is suggested.