Effect of Bragg-Williams disorder on reconstructed polar surfaces of tetrahedrally coordinated compound semiconductors by optically simulated leed patterns

Laser generated optical diffraction patterns obtained from two-dimensional model gratings have been used to simulate the surface reconstructions observed by LEED with the polar surfaces of tetrahedrally coordinated compound semiconductors. The 2 × 2 and $\text{4}\text{3}\text{× 4}\text{3}\text{?30°}$ reconstructions conform ideally with the $\text{1}\text{4}$-monolayer criterion dictated by bonding ionicity [Nosker, Mark and Levine, Surface Science 19 (1970) 291] while the 3 × 3, $\text{3}\text{×}\text{3}\text{?30°}$ and $\text{19}\text{×}\text{19}\text{?23.4°}$ reconstructions do not. It is shown that the introduction of Bragg-Williams disorder that preserves long-range order into the latter structures does permit the achievement of conformity with the $\text{1}\text{4}$-monolayer criterion without altering the symmetry of the diffraction pattern. Specifically the intensities of the fractional order beams are reduced relative to those of the integral order beams. Thus all the observed reconstruction LEED patterns can be consistent with the $\text{1}\text{4}$-monolayer criterion, provided account is taken of the insensitivity of LEED to surface defect structure.     

The surface reconstructions of the (100) crystal faces of iridium,platinum and gold: I. Experimental observations and possible structural models

The structures of the reconstructed Ir(100), Pt(100) and Au(100) surfaces have been investigated. Low energy electron diffraction (LEED) patterns are analyzed and LEED intensity versus energy data are measured. A variety of structures is observed by LEED: Ir(100) exhibits a relatively simple (1 × 5) pattern; Pt(100) shows a series of closely related patterns, a typical representative of which has a $\text{(}\text{14}\text{1}\text{?1}\text{5}\text{)}$ structure; Au(100) usually exhibits a c(26 × 68) pattern, often inaccurately described in the literature as a (20 × 5) pattern. The reconstruction of Au(111) is also considered for comparison. Various plausible structural models are discussed, while laser simulation is used to lessen the number of these models. The analysis is completed in a companion paper where LEED intensity calculations are reported to determine the atomic locations.     

Sodium adsorption on aluminum (100) and (111) surfaces: ELEED,Auger, and contact potential measurements

The adsorption of Na on sputter-cleaned Al(100) and Al(111) single-crystal surfaces has been investigated under ultrahigh vacuum conditions. Sodium was deposited with an apertured ion source permitting quantitative dosage determination. Low energy electron diffraction patterns show well-ordered coherent structures corresponding to $\text{1}\text{2}$ monolayer on both surfaces, and $\text{1}\text{3}$ monolayer on Al(111). A hexagonal pattern corresponding to $\text{7}\text{8}$ monolayer occurs at a nonconforming dosage on Al(100), indicating a reduction in effective sticking coefficient. This result is verified by Auger intensity measurements, which show saturation beginning at $\text{1}\text{2}$ monolayer on both surfaces. Changes in contact potential with exposure were inferred from shifts in the low-energy cutoff of secondary electrons. The initial dipole moment is in good agreement with that on transition metal substrates, while the saturation value of the contact potential corresponds closely to the difference between reported work functions for bulk Al and bulk Na. A true minimum in the work function was not observed. Present results are compared with results obtained from transition metal substrates. Essential differences are attributed to the more metallic character of the bonding in the Al:Na system.     

Superlattices formed by electrodeposition of silver on iodine-pretreated Pt(111); studies by LEED,Auger spectroscopy and electrochemistry

Reported are studies by LEED and Auger spectroscopy of silver layers electrodeposited on well-characterized Pt(111) surfaces from aqueous solution. Prior to electrodeposition. the Pt(111) surface was treated with I₂ vapor to form the Pt(111) ($\text{7}\text{×}\text{7}$)R19.1°-I superlattice which protected the Pt and Ag surfaces from attack by the electrolyte and residual gases. Electrodeposition of silver occurred in four distinct ranges of electrode potential. Ordered layers having (3 × 3) and (18 × 18) (coincidence lattice) LEED patterns were formed at all coverages from the onset of deposition to the highest coverages studied, about twenty equivalent atomic layers. Formation of ordered Ag layers has therefore been demonstrated, at least for deposits of limited thickness. Auger spectra revealed that for deposits of a few atomic layers. The iodine layer remained attached to the surface during multiple cycles of electrodeposition and dissolution of silver from iodine-free solution. Each peak of the voltammetric current-potential scan produced a change in the LEED pattern.     

The response of one-degree-of-freedom systems with cubic and quartic non-linearities to a harmonic excitation

The method of multiple scales is used to analyze the response of single-degree-of freedom systems with cubic and quartic non-linearities to a harmonic excitation. Two first-order ordinary differential equations describing the evolution of the amplitude and the phase are derived for superharmonic resonances of order two and four, subharmonic resonances of order one-half and one-fourth, and the supersubharmonic resonances of order $\text{3}\text{2}$ and $\text{2}\text{3}$. In all cases, the steady state solutions and their stability are determined and representative numerical results are included.     

He atom diffraction from a silicide precipitate on a Pt(100) surface

We have observed the formation of an ordered silicide at the surface of a nominally pure Pt(100) crystal. The silicide appears after thermally activated migration of a trace Si impurity (<10^?4 at%) and subsequent cooling. The silicide is about one monolayer thick and forms a large area unit mesh, $\text{(6}\text{2}\text{×6}\text{2}\text{)R45°}$, which is observed with both He diffraction and LEED. A thermal attenuation analysis of the specular and a diffracted He beam yields a consistent value for the attractive depth of the He-surface potential D=13 meV. From a simple classical analysis of the He diffraction pattern we have obtained an estimate for the corrugation of the He-surface potential, $\text{ξ=0.07}\text{A}\text{?}$. The well depth D is shown to be consistent with the wavelength dependence of the angular distribution of the diffraction pattern.     

Photoelectron diffraction effects in XPS angular distributions from GaAs(110) and Ge(110) single crystals

Angular distribution measurements of XPS intensities have been made for various spectral lines from GaAs(110) and Ge(110) single-crystal surfaces. Observed angular distribution curves (ADC's) showed steep intensity variations and sharp peaks due to X-ray photoelectron diffraction (XPED) phenomena. The effects of the type of transition process (photoelectron or Auger), electron kinetic energy and crystal structure on the XPED patterns were examined. Considerably different ADC patterns were observed for high-energy photoelectrons and Auger electrons and for low-energy photoelectrons. ADC's for Ga 3d, As 3d and Ge 3d showed almost the same patterns for scans of the type [110] → [100] → [1$\text{1}$0], but they showed substantially different patterns for [110] → [11$\text{1}$] → [00$\text{1}$] scans. These features correspond well with the structural characteristics of GaAs and Ge crystals. A discussion of the applicability of XPS angular distribution measurements to the geometric analysis of crystal surfaces is presented.     

Nonlinear first sound response of liquid helium at 1, 3 and 10 MHz

The first sound response of liquid helium at 1, 3 and 10 MHz is studied using piezoelectric ceramic transducers. Above specific threshold displacement amplitudes, subharmonic frequencies ($\text{?}{}_0\text{2}$, $\text{?}{}_0\text{3}$,…) and ultraharmonic frequencies ($\text{3?}{}_0\text{2}$, $\text{5?}{}_0\text{2}$,…) appear in the response spectrum. The magnitude of the threshold displacement amplitudes for the production of subharmonics suggests that vapor microbubble oscillation appears not to be the primary generation mechanism.     

Superlattice structures of the intercalation complex NbS2·(pyridine)12

Satellite spots were observed in NbS₂·(pyridine)_{$\text{1}\text{2}$} at room temperature by means of electron diffraction. The superlattice is composed of rectangular unit cell of 2√3a × 13a in the plane perpendicular to c-axis and appears with three-fold rotational overlapping in the diffraction pattern. In NMR measurements no evidence was observed of any chalcogenide lattice distortions. Hence, it is suggested that the superlattice has no relation with a charge-density-wave. Origins of the superlattice are discussed on a view point of pyridine molecule arrangement.     

Structure and electronic properties of cleaved Si(111) upon Ge adsorption

The effect of ultrahigh vacuum deposition of Ge below and at monolayer coverage onto clean cleaved Si(111) surface held at room temperature is studied by low energy electron diffraction, Auger electron specroscopy and photoemission yield spectroscopy. A well ordered $\text{3}\text{×}\text{3}$ R 30° structure developes at $\text{1}\text{3}$ ML, where it replaces the 2 × 1 initial pattern; it persists at $\text{2}\text{3}$ ML before transforming into a 1 × 1 diagram which fades into increasing background at 1 ML and up. Si surface dangling bonds are replaced at $\text{1}\text{3}$ ML by states associated with Ge-Si bonds and Ge dangling bonds to which states due to Ge-Ge bonds added upon increasing coverage.     

Low temperature ordering of sodium overlayers on RU(001)

The adsorption of alkali metals on transition metals can produce several technologically important effects, but only limited results have been reported on the geometrical structure of such adlayers, especially for adsorption temperatures below 300 K. We have examined the adsorption of Na on Ru(001) as a function of coverage and temperature using LEED to determine the adlayer structure and thermal desorption spectroscopy to characterize binding kinetics and relative Na coverages. The only Na LEED pattern observed following adsorption at 300 K was that of $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)}$ structure which occurred near saturation of the first layer. However, Na adsorbed at 80 K produces a progression of distinct, ordered LEED patterns with increasing coverage which does not include the $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)}$ pattern. These patterns result from increasingly compressed, hexagonal arrangements of adsorbate atoms which are uniformly spaced due to mutually repulsive interactions. The order-disorder transition temperature for each structure was also determined by LEED and used to develop a 2D phase diagram for Na on Ru(001). Ordered structures were observed only when Na thermally induced motion was sufficiently limited and the repulsive Na-Na interaction could force the uniform spacing of Na atoms. Thus, low coverage structures only developed where Na mobility was limited by low temperature. High coverage structures were stable to much higher temperatures since motion was inhibited by the high Na density.     

Anomalous 13 422 diffraction spots from {111} flat gold crystallites: (111) surface reconstruction and moiré fringes between the surface and the bulk

This paper describes very flat {111} gold crystallites grown on graphite. The diffraction patterns of such crystallites show the “usual” $\text{1}\text{3}$ 422 anomalous spots. It is shown that these spots are in fact split in such a way that each $\text{1}\text{3}$ 422 spot is surrounded by an hexagonal array of additional spots, aligned in the 〈110〉 directions. When seen by TEM, the crystallites show 〈112〉 periodic contrasts which are not normal moiré fringes or misfit dislocations between gold and graphite. An interpretation following Yagi et al. is proposed which connects these 2 kinds of anomalies: they should result from the surface superstructure 23 × 1 of clean (111) gold face, described very recently by Melle and Menzel.     

Magnetic structure of BaCaFe4O8-analysis of neutron diffraction measurements

The compound BaCaFe₄O₈ crystallizes in the trigonal space group P$\text{31}$m with one formula unit per unit cell with lattice constants $\text{a = 5.4059}\text{A}$ and $\text{c = 7.7023}\text{A}$ Neutron diffraction measurements carried out on a powder sample over the temperature range 300–900 K showed that the compound undergoes a magnetic transition to an antiferromagnetic state at a Néel temperature T_N = 680 ± 5 K. Analysis of the room temperature neutron diffraction pattern gave a magnetic unit cell that has the same periodicty as the crystallographic one. An antiferromagnetic model is proposed with the iron spin magnetic moments parallel to the c-axis of the unit cell. The magnetic moment of the Fe³⁺ ion was found to be (4.5 ± 0.1)μ_B     

Oxydation de la face (111) d'un monocristal de chrome

Interaction of oxygen with (111) oriented chromium single crystal surface was studied by electron diffraction (LEED and RHEED). From the clean surface, oxygen adsorption induces a $\text{Cr}\text{(111)(}\text{3}\text{×}\text{3}\text{)}\text{R}\text{30°}$ -O structure. No change in the geometry of the LEED pattern occurs with additional oxygen exposure or heating, although the RHEED study denotes the nucleation of rhombohedral oxide on the surface. The orientation relationships with the substrate are determined and compared to those found in the case of (110) chromium surface.     

Analysis of a ZnO surface structure by a combination of leed and X-ray diffraction

The hexagonal LEED pattern obtained from a clean, reconstructed (0001)Zn face of a ZnO crystal could indicate either a (2 × 2) or a (√3 × √3) structure. We prove that we observed a (√3 × √3) pattern by identifying the line of h, h, $\text{2}$h, λ reflections from an X-ray diffraction photograph of the sample.     

A statistical model for oxygen adsorption on W (211)

A series of previous experimental investigations revealed that oxygen adsorption on a W (211) surface causes, at $\text{θ =}\text{1}\text{2}$, a 2 × 1 LEED pattern whose half order spots are streaked at lower coverages and become continuously weaker at $\text{θ >}\text{1}\text{2}\text{until at}\text{θ = 1}$ a 1 × 1 structure is formed. A model is proposed wherein the configuration of the adsorbed atoms with respect to each other is governed by a set of anisotropic interactions between neighbouring particles. These (three) interaction energies have magnitudes between kT and 2kT. The equilibrium arrangements for various coverages were simulated by means of the Monte Carlo technique, and the corresponding LEED patterns (angular profiles) were evaluated by using the kinematic approximation. Good agreement with experimental data is obtained for the variation of the relative intensity as well as of the breadth of the half order LEED spots, without the need for any further assumptions on the mechanism of the formation of the ordered adsorbed layers.     

Strange baryoniums: Their spectrum,decay and production

Using a quark-gluon model we study exhaustively the states with $\text{qq}\text{q}\text{q}$, where at least one q is a strange quark. The spectrum is extremely rich, but a few trajectories are singled out by their distinctive decay pattern and production mechanism. States of unusual colour content, formed from a diquark in colour-6 and an antidiquark in $\text{6}\text{-}\text{colour}$, are weakly coupled to $\text{B}\text{B}$ or two-meson channels through decay. Instead they prefer to cascade by emitting a π or a K if possible. Narrow $\text{Λ}\text{N}$ states among these are expected to be seen in Kp interactions. States with S = 2 will occur with $\text{K}{}^{\mathrm{?}}\text{Λ}\text{p}\text{or K}{}^{\mathrm{+?}}\text{gL}\text{p}$ final states, but not for example $\text{π}\text{Λ}\text{Λ}$. Some of these can be produced diffractively, with cross sections in the region of 1 μb. Those that require quantum-number exchange for production are expected to have considerably lower cross sections. Some can occur as cascade decay products.     

Distinct binding states in adsorption on a homogeneous surface: Te on W(100)

The adsorption of Te on a W(100) surface is studied by thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and work function change (Δ?) measurements. Three distinct binding states are observed in the first monolayer corresponding the coverages from 0 to $\text{1}\text{2}$ monolayers (ML), $\text{1}\text{2}$ to $\text{2}\text{3}$ ML and$\text{2}\text{3}$ to 1 ML. Within each state a coverage dependence of the desorption parameters is found. The three binding states are discussed in terms of heterogeneity induced by lateral interactions and in terms of inherently different adsorption sites.     

Threshold of the 2ωpe instability in a laser produced plasma

$\text{3}\text{2}$ω_L emission from various plane targets irradiated by a 20 J, 5 ns neodymium laser pulse has been investigated. The onset of $\text{3}\text{2}$ω_L emission and an increase in intensity by five orders of magnitude is observed at a laser intensity of ≌2×10¹³W cm^-2, the threshold intensity predicted for excitation of the 2ω_pe instability.     

The size of inelastic diffraction dissociation

It is suggested that the inelastic diffraction dissociation cross section is a constant fraction β of the unitary bound, i.e. $\text{σ}{}_{\mathrm{DIFF}}\text{(s) = β[}\text{1}\text{2}\text{σ}{}_{\mathrm{TOT}}\text{(s) ? σ}{}_{\mathrm{EL}}\text{(s)]}$. Available data from pp collisions for s ? 20 (GeV/c)² are consistent with our hypothesis with $\text{β ～}\text{1}\text{2}$. Consideration of a specific model for diffraction suggests that the value of $\text{β ～}\text{1}\text{2}$ is a consequence of there being no lower mass state than pp with its quantum numbers. Predictions for πp and Kp scattering are presented.