J.E. Black

RADAR OBSERVATIONS OF BIRD MIGRATION OVER THE GREAT LAKES

Abstract The Great Lakes and nearby agricultural midwestern United States together represent a geographical challenge to migratory land birds during flight and stopover. We explored large-scale migratory responses of land birds encountering the Great Lakes as revealed by weather surveillance radars (WSR-88D) and two smaller specialized radars. Those responses reveal comprehensive landscape- or regional-scale migratory patterns that would otherwise have been difficult to infer. Analysis of radar echoes showed birds crossed the Great Lakes in large numbers, although we also found evidence of birds avoiding lake crossing in some locations. Around dawn, birds over water in numerous locations frequently exhibited an increase in migratory height (dawn ascent) and often an accompanying reorientation toward nearest land if they were within ∼28 km of shore. Those behavioral responses to the Great Lakes influence the resulting distribution of birds stopping over in the intervening terrestrial landscapes.

The vibration of atoms at high miller index surfaces: Face centred cubic metals

Abstract Using a nearest neighbour model of interatomic forces we have obtained surface mode requencies for the face centred cubic (100), (110), (111), (210), …, (332) surfaces. In addition mode frequencies for the (511), (711), (911) and (553) surfaces were obtained so effects of increasing terrace size could be further studied. Frequencies and surface eigenvectors are presented for nickel. Since a nearest neighbour model is used a simple conversion factor allows a determination of the frequencies for other metals for which the nearest neighbour model is appropriate.

A structural study of PdCu(100) surface alloys

Abstract The structures formed by one-half and one monolayer (ML) of Pd evaporated onto Cu(100) at 300 K were studied by low energy electron diffraction (LEED), medium energy ion scattering (MEIS), thermal desorption spectroscopy (TDS), and embedded atom method (EAM) calculations. In the half monolayer case, the LEED I(E) curves are consistent with the established c(2 × 2) surface alloy model. The MEIS data, however, suggest that a fraction of the Pd ( ∼ 1 4 ) is in “second layer” sites, in agreement with previous LEIS, TDS and XPS forward scattering measurements. The EAM simulations support the formation of alloy islands, providing a mechanism for the covering of some Pd atoms. As the deposition proceeds, however, this island formation is indicated to occur preferentially over clean copper. In the one monolayer case, a p(2 × 2)-p4g LEED pattern is observed. Analysis of the I(E) curves suggests that this arises from (100) Pd packed above the c(2 × 2) alloy. EAM calculations confirm the stability of this model. Evidence from MEIS and TDS, however, shows that the one monolayer surface as prepared in this work is inhomogeneous. c(2 × 2) and Cu rich surface domains exist in addition to those having the p4g Pd c (2 × 2) PdCu structure.

Surface disordering, roughening and premelting of Ag(110)

We have examined the structure and the dynamics of Ag(110) from 300 to 1100 K using reliable, many-body interaction potentials and molecular dynamics simulations. It is seen that the surface in-plane vibrations are larger and more anharmonic than the out-of-plane vibration, and lead to the disordering of the surface through the formation of vacancies and adatoms. The results indicate that the surface begins to disorder around 750 K, it roughens around 930 K and premelts at about 1000 K. Results are compared with existing experimental data.

The lattice vibrations of nearly incommensurate overlayers Ag on Ni(100) and Cu(100)

Abstract A chemisorbed monolayer may form and N × M overlayer, where N or M (or both) may be large. We formulate an approximate description of the lattice vibrations in such structures that includes coupling between the overlayer vibrations, and the substrate phonons. The analysis is a lattice dynamical analogue of Brillouin-Wigner perturbation theory; the scheme is accurate, and readily employed to generate surface spectral densities. A key feature is the ability to analyze the response characteristics for wave vectors in the surface Brillouin zone of the substrate. This greatly simplifies the task of interpreting the output of the numerical computational schemes. We also discuss experimental data on the surface vibrations of a monolayer of Ag on the Ni(100) and the Cu(100) surface, which are closely approximated by a (2 × 8) and (2 × 10) structure, respectively. We achieve a quantitative fit with a simple model, and discuss the physical origin of the interaction between adsorbate and substrate phonons, in regions of the substrate surface Brillouin zone where the adsorbate vibration frequencies lie well below those of the substrate phonons of the same wave vector.

A molecular dynamics study of the behaviour of xenon physisorbed on Pt(111): Coverages less than one monolayer

Abstract In recent He beam studies of Xe adsorbed on Pt(111) at less than monolayer coverages the xenon has been found to exist in incommensurate and commensurate phases both rotated by 30° with respect to the substrate. We present here molecular dynamics studies of small rafts of xenon particles on Pt(111). We find evidence that the corrugation of the xenon-platinum potential of interaction can lead to an incommensurate phase but that, using reasonable interaction potentials, the corrugation cannot lead to a commensurate phase which can exist over a range of temperatures. A variety of molecular dynamic studies of raft behaviour are presented.

Phonon spectra and mean square displacements on Cu(11n) vicinal surfaces

Abstract Surface phonon spectra on Cu(11 n ) vicinal surfaces, with odd number n = 1 to 19, are studied by using the continued fraction method and embedded atom potentials. Vibration frequencies for atoms at step edges are found to be softer along the surface normal and the step normal directions than those of terrace atoms and bulk atoms. Stiff frequencies along the surface normal direction are found for those atoms (with bulk nearest neighbors) located just beneath the step atoms. We find that surface relaxation induces 20–35% stiffening in the force constants of stepped surface atoms. The mean-square displacements (MSD) for atoms at vicinal surfaces are calculated at different temperatures. The surface MSD is found to be isotropic for the (100) surface and anisotropic for the (11 n ) surfaces.

Persistent and non-persistent uniaxial strain events observed in small rafts of xenon physisorbed on Pt(111): A molecular dynamics study

Abstract We have performed molecular dynamics studies of a 49 atom raft, or island, of xenon atoms adsorbed on a platinum (111) substrate. A number of different platinum-xenon corrugations and xenon-xenon interactions were considered. We were not able to find a XePt pair potential which would fit the experimentally observed corrugation and perpendicular vibration frequency of the xenon. We find that uniaxial strain events can occur in the raft along either the \ gGK or \ gGM directions. These events may persist for hundreds of picoseconds at low temperatures and penetrate deeply into the raft. At high temperatures the events are short-lived and typically do not penetrate deeply into the raft. The transition from persistent to non-persistent events can take place over a temperature range as small as 5 K. This transition from persistent to non-persistent events corresponds to the transition from incommensurate to nearly commensurate phases of the raft of xenon. Transition over a small range of temperatures is only found at high platinum-xenon corrugations. It closely resembles the transition seen by Kern et al. [Phys. Rev. Letters 56 (1986) 620] using high resolution He scattering on various coverages of Xe physisorbed on Pt(111).

Structure and dynamics of an Ag overlayer on Cu(100): a study using the embedded atom method

We present the results obtained from molecular dynamics simulations of an overlayer of Ag on Cu(100) using potentials from the embedded atom method. As in the case of an Ag overlayer on Ni(100), the overlayer slides back and forth on the substrate along the nearly incommensurate direction. The mean square vibrational amplitudes of the Ag atoms are also largest along this direction. The Goldstone mode has a frequency of 1.3cm−1 at 300 K. At higher temperatures the surface disorders and Ag and Cu atoms interdiffuse. The dispersion of the lowest frequency vertical mode is in good agreement with inelastic electron scattering data. We also present some explanations of the phonons at Λ and the structure factor of the system.

The vibrational spectrum of H2O on Si(100)

Abstract Recent EELS measurements for H 2 O adsorbed on the Si(100) reconstructed surface have been interpreted on the basis of dissociation of H 2 O into OH plus H. We attempt to construct a valence force potential which yields harmonic vibrational frequencies in agreement with the EELS experiments. There is some consideration of experimental intensities in the achievement of this agreement. The experimental data cannot be fitted without introducing interaction force constants into the potential energy expression, such as the force constant coupling the silicon-oxygen stretching motion and the silicon-oxygen-hydrogen bending motion.