Lyman-α driven molecule formation on SiO2 surfaces-connection to astrochemistry on dust grains in the interstellar medium.

As a model for silicate dust grains in the interstellar medium, we have used high area amorphous SiO(2) as a surface on which to carry out Lyman-α (10.2 eV) photodecomposition of adsorbed N(2)O at 71 K and at a coverage of ∼0.3 monolayer. The N(2)O molecules are adsorbed by hydrogen bonding to surface Si-OH groups. Transmission IR spectroscopy measurements permit the observation of the consumption of adsorbed N(2)O and the production of various photoproducts. It is observed that in comparison to N(2)O consumption, the relative rate of formation of the products NO(2) and N(2)O(4) made by combination reactions is enhanced significantly on the SiO(2) surface. Reactions between photogenerated radicals themselves or between radicals and parent N(2)O on the SiO(2) surface exceed the relative rates observed in the gas phase by factors of up to ∼20. As the complexity of the combination product increases, its relative production rate, compared to the gas phase, increases due to the involvement of multiple surface-combination elementary steps. It is proposed that the enhancement of combination reactions on the SiO(2) surface is due to the surface's ability to absorb excess energy evolved during the chemical-bond-forming events on the surface. This principle is probably significant on grain surfaces supporting photochemical processes of astrochemical interest, and indeed is expected. The cross section for adsorbed N(2)O photodecomposition on the porous SiO(2) surface is about 7 × 10(-20) cm(2) and the quantum yield for the adsorbed molecule decomposition is about 0.006, compared to a quantum yield of 1.46 in the gas phase. This decrease in photon efficiency is attributed to absorption and scattering of Lyman-α radiation by the SiO(2) particles.

The role of gold adatoms and stereochemistry in self-assembly of methylthiolate on Au(111).

On the basis of high resolution STM images and DFT modeling, we have resolved low- and high-coverage structures of methylthiolate (CH(3)S) self-assembled on the Au(111) surface. The key new finding is that the building block of all these structures has the same stoichiometry of two thiolate species joined by a gold adatom. The self-arrangement of the methylthiolate-adatom complexes on the surface depends critically on their stereochemical properties. Variations of the latter can produce local ordering of adatom complexes with either (3 x 4) or (3 x 4 square root(3)) periodicity. A possible structural connection between the (3 x 4 square root(3)) structure and commonly observed (square root(3) x square root(3))R30 degrees phase in methylthiolate self-assembled monolayers is developed by taking into account the reduction in the long-range order and stereochemical isomerization at high coverage. We also suggest how the observed self-arrangements of methylthiolate may be related to the c(4 x 2) phase of its longer homologues.

Direct observation of chemical bond dynamics on surfaces.

The dynamics of chemisorbed species as they swing to-and-fro on their adsorption sites may be directly observed with electron-stimulated desorption. The observation of the thermal disorder in adsorbate chemical bond directions, through studies of the thermal excitation of librational modes, allows one to visualize the potential energy surfaces controlling the structure and dynamics of adsorbates on single crystal metal and semiconductor surfaces. This information may be useful in understanding surface diffusion as well as the spatial aspects of surface chemical reactions.

Role of conformation in the electronic properties of chemisorbed pyridine on Cu(110): an STM/STS study.

Pyridine was chemisorbed on Cu(110) at 10 K and observed using STM at 5 K as dosed and after annealing to temperatures between 20 and 300 K. At very low coverage, two molecular species with different apparent heights are observed to coexist. The higher species is assigned to a pyridine molecule bonded with its symmetry axis perpendicular to the surface plane, while the lower species is assigned to a pyridine molecule that is tilted down toward the surface plane. At low coverage, the tilted pyridine species predominates on the surface, but as the total surface coverage of pyridine increases, the molecules stand up until the upright geometry becomes favored. Measurements of the STS of the two species show different molecular resonances derived from the lowest unoccupied pyridine pi* orbitals. The tilted pyridine species has a peak in the unoccupied local density of states at 2.6 +/- 0.1 eV, whereas the upright pyridine species has a peak at 2.3 +/- 0.1 eV.

Studies on amino acid incorporation in isolated toad bladder epithelial cells. Seasonal changes in protein synthesis.

Amino acid incorporation in isolated toad bladder epithelial cells was investigated by examining the rate of radiolabel incorporation into trichloroacetic acid-insoluble material. The results demonstrated that during the summer months, a large fraction of the radiolabel incorporated was not inhibited by cycloheximide and did not seem to correspond to incorporation of the radioactive amino acid into elongating peptides. A similar effect was not observed in other organs of the toad (e.g. liver, spleen). During the winter months, however, virtually all the acid-insoluble radiolabel appears to correspond to incorporation of the amino acid into elongating peptides. Ouabain, an inhibitor of sodium transport, inhibited amino acid incorporation into acid-insoluble material by affecting the acid-soluble pool and this effect appeared to be independent of the effect of ouabain on sidium transport. The isolated epithelial cells appeared to derive the energy for protein synthesis almost entirely from glycolysis and in this regard they resemble certain tissue culture cells.

Light-induced charge separation in anatase TiO2 particles.

Ultraviolet light-induced electron-hole pair excitations in anatase TiO(2) powders were studied by a combination of electron paramagnetic resonance and infrared spectroscopy measurements. During continuous UV irradiation in the mW.cm(-2) range, photogenerated electrons are either trapped at localized sites, giving paramagnetic Ti(3+) centers, or remain in the conduction band as EPR silent species which may be observed by their IR absorption. Using low temperatures (90 K) to reduce the rate of the electron-hole recombination processes, trapped electrons and conduction band electrons exhibit lifetimes of hours. The EPR-detected holes produced by photoexcitation are O(-) species, produced from lattice O(2-) ions. It is found that under high vacuum conditions, the major fraction of photoexcited electrons remains in the conduction band. At 298 K, all stable hole and electron states are lost from TiO(2). Defect sites produced by oxygen removal during annealing of anatase TiO(2) are found to produce a Ti(3+) EPR spectrum identical to that of trapped electrons, which originate from photoexcitation of oxidized TiO(2). Efficient electron scavenging by adsorbed O(2) at 140 K is found to produce two long-lived O(2)(-) surface species associated with different cation surface sites. Reduced TiO(2), produced by annealing in vacuum, has been shown to be less efficient in hole trapping than oxidized TiO(2).

Preparation and purity measurement of extremely pure helium gas.

A procedure is described for the production, storage, and handling of very pure helium gas and for the measurement of its purity in the part per 10(11) range.

Color vision with rapid-onset acceleration.

INTRODUCTION: Only sporadic information exists concerning perceived color shifts at increased G-loads. The purpose of this study was to investigate whether or not color vision is affected by rapid onset high G7-loads up to +9 Gz, and specifically whether perception of hue changes. METHODS: There were 10 male subjects, 9 with normal color vision and 1 with red-green protanomaly, all accustomed to Gz-loads in a human centrifuge. Each subject was tested on a total of 60 Gz-exposures with 10 s periods at +3, +5, +7, and +9 Gz in the centrifuge on three different days. G-onset rate was 6 G x s(-1). The subjects wore an anti-G suit and performed straining maneuvers if necessary to maintain vision. Five square color stimuli of medium saturation (yellow, red, blue, green, and gray) were projected one at a time on a screen in front of the subject, who gave his hue response orally. RESULTS: In 96.6% of exposures to various Gz-loads, the subjects responded by correctly naming colors. (The statistical analyses of the results were done for the subjects with normal color vision, with the protanomalous subject excluded.) Hue shifts occurred at the higher +Gz-levels, including 7.7% of the +9 Gz exposures. Yellow was the hue most frequently perceived as changed. Hue shifts were reported for yellow in 11% and 16% of the +7 and +9 Gz exposures, respectively. Hue shifts at +9 Gz occurred as frequently as blackout and G-LOC together. However, statistical analyses showed no significant effects for +Gz-load. CONCLUSIONS: Absolute identification of the color stimuli of medium saturation was stable and was not significantly affected by the rapid onset +Gz-loads up to and including +9 Gz.

Protection to +12 Gz.

BACKGROUND: The U.S. Air Force has developed +Gz-protective equipment that will provide most pilots protection to +9 Gz with minimal-to-no straining. This equipment includes a pressure breathing system called COMBAT EDGE (CE), which is currently operational, and the Advanced Technology Anti-G Suit (ATAGS), which is not yet operational. For future high-performance aircraft design it is important to know the upper limit of various protective equipment and techniques. METHODS: Six subjects were randomly exposed to a 12-cell matrix composed of +Gz and the following combinations of protective equipment at three seat-back angles (13 degrees, 30 degrees and 55 degrees from the vertical): 1) the standard CSU-13B/P anti-G suit (STD); 2) the STD suit with CE; 3) the ATAGS; and 4) the ATAGS with CE. Relaxed, followed by straining +Gz tolerance was determined using 15-s rapid onset runs to a maximum of +12 Gz. A comprehensive battery of baseline and post-exposure medical surveillance studies was performed to evaluate the medical consequences of these high +Gz exposures. RESULTS: All 6 subjects were able to achieve +12 Gz with various combinations of +Gz-protective equipment, seat-back angle, and various amounts of straining, from none to maximum. When the data were collapsed over all protective equipment there was a significant (p < 0.05) seat effect. Relaxed tolerance to ROR increased with seat-back angle from 13 degrees to 30 degrees to 55 degrees. There was also a significant protective equipment effect when the data were collapsed over all seat-back angles. CONCLUSIONS: These data confirm that effortless protection to +9 Gz is available using ATAGS/CE with the 13 degree and 30 degree seat-back angle (F-15, F-16 and F-22) and to +10.5 Gz with a 55 degree seat-back angle. Moreover, with ATAGS/CE, and a moderate degree of straining, +12 Gz is definitely achievable at 55 degrees, even with reduced anti-G suit pressure at 55 degrees. With additional straining +12 GC is also achievable at the 13 degree and 30 degree seat-back angles.

Localized deposition of coronene molecules on Si(001)-2x1 using a scanning tunneling microscope tip source.

The deposition of coronene molecules from scanning tunneling microscope (STM) tips onto a clean Si(001)-2x1 surface at 25 degrees C was investigated. The STM tips, contaminated with coronene, were found to deposit coronene molecules on the clean Si(001) surface, allowing patterns to be generated. Covalent Si-C chemical bonds, formed between the coronene molecules and the Si substrate, froze the flip-flop motion of the adjacent Si-Si dimers on the substrate. In most cases, the mode of coronene bonding to Si(001) is independent of whether deposition occurs from the gas phase or from the STM tip. Despite the covalent chemical bonds formed between the coronene molecule and the Si substrate, the STM tip can drag the coronene laterally on the Si substrate without inducing a chemical change in the molecule. Sharp spikes observed in the tunneling current during the coronene deposition reflect the abrupt decrease of the tip-substrate distance at the instant of transport of the molecule from tip to surface.

Local spectroscopy of image-potential-derived states: from single molecules to monolayers of benzene on Cu(111).

Stark-shifted image-potential states were measured with an STM tip for benzene adsorbed on a Cu(111) surface. A single benzene molecule locally shifts the position of the first image state toward the Fermi level by 0.2 eV relative to its position on the clean surface. The energetic position of this molecule-modified state shifts to lower energy with increasing coverage of benzene on the surface. This is attributed to local surface potential changes that are correlated with the lowering of the crystal work function due to adsorption of benzene.

The chemisorption of dibenzo[a,j]coronene on Si(001)-2 x 1.

Adsorption structures of the dibenzo[a,j]coronene (C(32)H(16)) molecule on the clean Si(001)-2 X 1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with electronic structure calculations. The dibenzo[a,j]coronene molecules were found to adsorb on three different sites: one major adsorption site and two minor adsorption sites. The formation of four to eight Si-C covalent bonds is responsible for the different surface bonding structures observed. Bond strain effects due to out-of-plane bending of the molecule play a significant role in governing the surface bond energies. The geometries of the three adsorption sites were established by comparison of the experimental and simulated STM images. By applying an electrical pulse, the molecule can be made to hop from one site to another site without breaking the dibenzo[a,j]coronene molecular structure.

Dynamic depth reversal stereograms.

A new stimulus was generated to create dynamic stereopsis. Binocular disparity was alternated between crossed and uncrossed conditions, while mean luminance remained constant, to produce a stereoscopic stimulus that appeared to move continuously in depth. These dynamic depth reversal stereograms provided an effective stimulus to elicit the visual evoked potential (VEP). Neural coding of stereopsis was reflected in the cortical response with VEP amplitudes changing as a function of binocular disparity and of image blur in the stereograms.

Monkey contrast threshold for aperiodic patterns.

A monkey's and a human subject's threshold responses were measured for single- and double-bar patterns of high spatial frequency. The observed values were compared to predicted values which were derived from each subject's contrast sensitivity function. A theoretical peak-to-trough threshold mechanism was assumed in the calculations. The threshold predictions, which are couched in the assumption that the spatial visual system is linear near its threshold, were found to be close to observed values. The results are interpreted to suggest that the monkey visual system analyzes spatial information in qualitatively and quantitatively similar ways to human, and that a linear theory may be used for analyses of primate vision.

Adsorption geometry of 4-picoline chemisorbed on the Cu(110) surface: a study of forces controlling molecular self-assembly.

The adsorption of 4-picoline (4-methylpyridine) on the Cu(110) surface has been studied with time-of-flight electron stimulated desorption ion angular distribution (TOF-ESDIAD) and other methods. Using deuterium labeling in the methyl group and hydrogen labeling on the aromatic ring, it has been possible to separately monitor by TOF-ESDIAD the C-D bond directions and the C-H bond directions in the adsorbed molecule. These triangulation measurements have led to a detailed understanding of the conformation of the adsorbed molecule relative to the Cu(110) crystal lattice, allowing one to witness changes in the molecular conformation as adsorbate-adsorbate interactional effects take place for increasing coverages. At low coverages, the molecule adsorbs by the N atom at an atop Cu site with the aromatic ring parallel to the <001> azimuth and with the molecular axis inclined 33 (+/- 5) degrees along the <001> azimuth. As rows of 4-picoline molecules form long range ordered chain structures oriented along the <112> azimuth, the aromatic ring twists 29 degrees about the inclined molecular axis as a result of forces between the adsorbate molecules. The initial tilting of the molecular axis at low coverage is likely due to the interaction of the positive-outward dipole with its image in the substrate. The ring twist may result from dipoleminus signdipole forces between the adsorbate molecules in the rows formed tending to form nested parallel pyridine rings. These studies are the first to apply the TOF-ESDIAD method for the measurement of the direction of chemical bonds at more than one molecular location within an adsorbed molecule and the new method is named electron stimulated desorption-molecular triangulation (ESD-MT). The results obtained give information of importance in understanding the factors which control conformational effects during the molecular self-assembly of complex adsorbed molecules on surfaces.

Perception of images moving at saccadic velocities during saccades and during fixation.

During saccadic eye movements, images of external objects move rapidly across the retina. Small, unpredictable displacements imposed upon a target moving at saccadic velocity were detected with equal accuracy when (1) the retinal image velocity was caused by an eye movement, or (2) when the same velocity was produced during fixation (simulated saccadic conditions). The results provide no evidence of a specific non-visual suppression of vision during saccades, nor of any other compensatory modification of afferent visual inflow which might contribute to our sense of directional stability during saccades.

Photooxidation of a mustard gas simulant over TiO2-SiO2 mixed-oxide photocatalyst: site poisoning by oxidation products and reactivation.

The photooxidation of 2-chloroethyl ethyl sulfide (2-CEES), a simulant for mustard gas, was studied using transmission IR spectroscopy on a mixed-oxide TiO2-SiO2 photocatalyst. Ultraviolet irradiation in the photon energy range from 2.1 to 5 eV was employed at a catalyst temperature of 200 K. Rapid photooxidation was observed by the loss of infrared intensity in the v(CHx) stretching region, and concomitant infrared features of adsorbed oxidation products were observed to develop. The oxidation products, captured on the photocatalyst at 200 K, were found to block 2-CEES readsorption. Upon heating the poisoned photocatalyst to about 300 K, infrared measurements indicate that the adsorbed CO2 oxidation product was desorbed. The capability for full readsorption of 2-CEES was achieved upon heating the poisoned photocatalyst to 397 K, and continued rapid photooxidation of the 2-CEES was then possible at about 1/3 the rate found for the fresh catalyst. Thus thermal treatment at 397 K of oxidation-product-poisoned TiO2-SiO2 material is able to partially restore the TiO2-SiO2 photooxidation activity.