Absence of CO dissociation on Mo(112).

We revisit the problem of CO adsorption and thermal dissociation on the Mo(112) surface by means of density-functional calculations of binding energies, local densities of states, and CO vibrational frequencies for various configurations of equilibrated adlayers. The bridge-on-row adsorption sites on the Mo(112) surface are found to be the most favorable and CO molecules will occupy less stable in-furrow sites only after the completing of the first monolayer. At low coverages, CO molecules are tilted by approximately 40 degrees with respect to the normal to the surface (the beta state), but with increasing coverage, due to lateral interactions, attain an upright orientation with the carbon end down (the alpha state). The tilting of CO results in a significant elongation of the C-O bond (to 1.20 A) and, consequently, the C-O stretching vibration frequency decreases to 1159 cm(-1). Nonetheless, the beta state cannot be attributed to the precursor to CO dissociation, because the estimated potential barrier for the dissociation (approximately 2.8 eV) substantially exceeds the chemisorption energy (2.1 eV), which makes the thermally induced CO dissociation on Mo improbable. With estimated chemisorption energies, Monte Carlo simulations have shown that the two-peak shape of TPD spectra can be explained without involving the CO dissociation. We predict also that the lack of dissociation can be detected in photoemission studies for CO on Mo(112) by the presence of the -23 and -7 eV peaks, characteristic of chemisorbed CO, and absence of the -18 and -5 eV peaks characteristic of adsorbed O atoms.

Activated water desorption from poly(methylvinylidene cyanide).

We have investigated water desorption from the polymer poly(methylvinylidene cyanide). The angle resolved thermal desorption spectra show large deviations from the cosn theta distribution for water desorption from poly(methylvinylidene cyanide) indicative of an activated desorption process. The Arrhenius plots obtained from Polanyi-Wigner analysis of the thermal desorption data suggest that a two-state model of desorption applies, while theory suggests that lattice strain in the polymer plays a key role in the thermal desorption of water.

Water absorption and desorption from the dipole ordered polymer poly(methylvinylidene cyanide).

From thermal desorption studies, we find evidence that absorbed water in the bulk of poly(methylvinylidene cyanide) is more weakly bound than is the case for copolymer films of poly(vinylidenefluoride-trifluoroethylene). Ultraviolet laser enhanced thermal desorption of absorbed water exhibits little light polarization dependence for poly(methylvinylidene cyanide) in contrast to absorbed water in copolymer films of poly(vinylidenefluoride-trifluoroethylene). The implications of these differences are discussed.

H/D isotopic exchange in water interactions with the ferroelectric copolymer: Poly(vinylidene fluoride-trifluoroethylene) (70%:30%).

For both water and heavy water adsorption and absorption on crystalline poly(vinylidene fluoride with trifluoroethylene (30%)), P(VDF-TrFE 70:30), two distinctly different adsorption sites have been identified by thermal desorption spectroscopy. One adsorbed water species resembles ice and there is also an absorbed water species that interacts more strongly with the polymer thin film, and in addition, there is a polymer surface (polymer to ice interface) water species. We find that there is H/D exchange between the water or heavy water molecules and the ferroelectric polymer (largely -(CH2-CF2)-), particularly at the polymer surface.

Atomic wires on furrowed transition metal surfaces.

At low coverages, alkali, alkaline earth, and rare earth layers adsorbed on furrowed transition metal surfaces--such as the Mo(112) surface--demonstrate a quite unusual behavior. In contrast to well-known formation of one-dimensional wires lying in furrows of reconstructed surfaces of semiconductors and noble metals, these metals on the W(112), Mo(112), and Re(1010) surfaces tend to form commensurate linear structures built of monoatomic chains directed across the furrows. Monte-Carlo simulations of the increasing disorder of the chains with increasing temperature provides a transparent presentation of typical fluctuations in such linear structures. Such studies provide insight into the parameters of indirect interaction that are the basis for the formation of the atomic wires. Increasing coverage leads to a one-dimensional compression of the layers along the furrows, which results in dramatic changes in the surface electronic structure as revealed by photoemission ultraviolet photoelectron spectroscopy and electron energy loss (EELS) spectroscopies. Recent low energy electron diffraction, UPS, and electron energy loss spectroscopy data suggest a nonmetal to metal transition in the adsorbed alkaline earth layers when the coverage exceeds 1/2 monolayer and the film structure becomes incoherent with the substrate in direction along the furrows. The NMT in adsorbed layers is discussed in the framework of recent calculations of evolution of the band structure followed from increasing density of the films.

A density functional theory study of the electron-phonon coupling at the Mo(112) surface.

The electron-phonon (e-p) coupling for the surface bands of Mo(112) has been studied by density functional theory (DFT). There are significant contributions from the surface phonon modes to the low frequency part of the phonon density of states and Eliashberg function, which results in enhanced e-p coupling at the surface and, consequently, an increase of the mass enhancement factor and the imaginary part of the self-energy. We suggest that the enhanced e-p coupling, together with anharmonic effects, scattering off impurities, as well as decay channels through adjacent bulk bands, leads to the increase of the linewidth of the electronic surface bands crossing the Fermi level observed in high resolution photoemission spectra of the Mo(112) surface.

Hydrogen-induced mitigation of O on Ru(1010): a density-functional study.

The reaction of hydrogen with oxygen adsorbed on an Ru(1010) surface has been studied by density-functional calculations and kinetic Monte Carlo simulations. In agreement with experiment, it has been found that molecular hydrogen does not react with adsorbed O. In contrast, the hydrogenation of oxygen by an atomic H beam occurs spontaneously and results in the formation of adsorbed OH molecules. Subsequent impinging H-atoms can either initiate the formation of water, which readily desorbs at room temperature thus removing the O from the surface, or lead to formation and desorption of H2. It is the latter channel that hinders, at 300 K, a complete removal of O from Ru capping layers on Si/Mo mirrors for extreme ultraviolet radiation. The estimated height of the barrier for the Langmuir-Hinshelwood reaction between adsorbed H and OH, 0.92 eV, and related position of the H2O peak in model desorption spectra (approximately 320 K) are consistent with recent experiments. The H2 desorption peak appears at higher temperature, approximately 350 K, so that in the range from 320 to 330 K adsorbed hydrogen atoms will react predominantly with OH. Hence, the present simulations predict that an efficient removal of the chemisorbed O from Ru capping layers can be achieved by heating the surface to 320-330 K in a molecular hydrogen atmosphere.