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Time-resolved optical spectroscopies reveal multielectron transfer from the biexcitonic state of a CdS quantum dot to an adsorbed tetracationic compound cyclobis(4,4'-(1,4-phenylene) bipyridin-1-ium-1,4-phenylene-bis(methylene)) (ExBox(4+)) to form both the ExBox(3+•) and the doubly reduced ExBox(2(+•)) states from a single laser pulse. Electron transfer in the single-exciton regime occurs in 1 ps. At higher excitation powers the second electron transfer takes ∼5 ps, which leads to a mixture of redox states of the acceptor ligand. The doubly reduced ExBox(2(+•)) state has a lifetime of ∼10 ns, while CdS(+•):ExBox(3+•) recombines with multiple time constants, the longest of which is ∼300 μs. The long-lived charge separation and ability to accumulate multiple charges on ExBox(4+) demonstrate the potential of the CdS:ExBox(4+) complex to serve as a platform for two-electron photocatalysis.

Photo-oxidation of formaldehyde is important because it is an atmospheric pollutant and a possible intermediate along the pathway for CO2 reduction to methanol. By using temperature-programmed reaction spectroscopy as the main tool, we demonstrate that the efficiency for photo-oxidation of formaldehyde to adsorbed formate on rutile TiO2(110) is strongly dependent on the degree of reduction of the surface and near-surface region. The most efficient photoreaction occurs when O adatoms are present on titania that is highly bulk reduced. These data suggest that electron–hole pairs created deep in the bulk do not play a significant role in the photo-oxidation of formaldehyde. Exposure of the material to O2 heals most bridging oxygen vacancies, creates O adatoms in the Ti five-coordinate rows, and quenches charge at the surface due to the presence of interstitial defects in the near-surface region. Under these conditions, the surface is oxidized but the bulk remains reduced. The major product of formaldehyde ph...

This paper describes the surface composition-dependent binding of the dichloride salt of methyl viologen (MV2+) to CdS quantum dots (QDs) enriched, to various degrees, with either Cd or S at the surface. The degree of enrichment is controlled synthetically and by postsynthetic dilution of the QDs in their solvent, THF. NMR shows the Cd-enriched QDs to contain a relatively dense (2.8 ligands/nm2) surface layer of oleic acid, in the form of Cd-oleate, and S-enriched QDs to contain relatively sparse (1.0 ligands/nm2) surface density of native ligands containing both oleic acid and octadecene. Electron transfer-mediated photoluminescence quenching of the QDs by MV2+ serves as a probe for the binding affinity of MV2+ for the surfaces of the QDs. Diluting Cd-enriched QDs removes Cd-oleate from the surface, exposing the stoichiometric CdS surface beneath and increasing the quenching efficiency of MV2+, whereas diluting S-enriched QD does not change their surface chemistry or the efficiency with which they are quenched by MV2+. The photoluminescence quenching data for all of the surface chemistries we studied fit well to a Langmuir model that accounts for binding of MV2+ through two reaction mechanisms: (i) direct adsorption of MV2+ to exposed stoichiometric CdS surfaces (with an equilibrium adsorption constant of 1.5×10(5) M(-1)), and (ii) adsorption of MV2+ to stoichiometric CdS surfaces upon displacement of weakly bound Cd-oleate complexes (with an equilibrium displacement constant of 3.5×10(3) M(-1)). Ab initio calculations of the binding energy for adsorption of the dichloride salt of MV2+ on Cd- and S-terminated surfaces reveal a substantial preference of MV2+ for S-terminated lattices due to alignment of the positively charged nitrogens on MV2+ with the negatively charged sulfur. These findings suggest a strategy to maximize the adsorption of redox-active molecules in electron transfer-active geometries through synthetic and postsynthetic manipulation of the inorganic surface.

Defect sites in titania have a substantial effect on many thermal and photochemical reactions. Three common types of defects are titanium interstitials, oxygen vacancies and oxygen adatoms—all of which can react with organic molecules adsorbed on the surface. We look broadly at thermal reduction and photochemical oxidation reactions of oxygenates. In particular, we focus on the reductive coupling of benzaldehyde, the photo-oxidation of butyrophenone, the photostability of benzoic acid, and the photo-oxidative coupling of methanol to methyl formate. Methods used include temperature programmed reaction spectroscopy, scanning tunneling microscopy, and density functional theory.

Methyl formate is produced from the photo-oxidation of methanol on preoxidized TiO(2)(110). We demonstrate that two consecutive photo-oxidation steps lead to methyl formate using mass spectrometry and scanning tunneling microscopy. The first step in methanol oxidation is formation of methoxy by the thermal dissociation of the O-H bond to yield adsorbed CH(3)O and water. Formaldehyde is produced via hole-mediated oxidation of adsorbed methoxy in the first photochemical step. Next, transient HCO is made photochemically from formaldehyde. The HCO couples with residual methoxy on the surface to yield methyl formate. Exposure of the titania surface to O(2) is required for these photo-oxidation steps in order to heal surface and near-surface defects that can serve as hole traps. Notably, residual O adatoms are not required for photochemical production of methyl formate or formaldehyde. All O adatoms react thermally with methanol to form methoxy and gaseous water at rt, leaving a surface devoid of O adatoms. The mechanism provides insight into the photochemistry of TiO(2) and suggests general synthetic pathways that are the result of the ability to activate both alkoxides and aldehydes using photons.

Carboxylic acid moieties are used to anchor organic dyes to TiO2 in many applications; therefore, their structure, distribution on the surface, and thermal and photochemical stability are extremely important. Herein, we investigate the thermal and photochemistry of benzoic acid monolayers on TiO2. Benzoic acid deprotonates to form benzoate monomers on the TiO2 surface, and thermally decomposes to benzene and CO. Titanium interstitials migrate to the surface and react with oxygen atoms from the benzoate molecules to form new TiOx islands. A combination of X-ray photoelectron spectroscopy and scanning tunneling microscopy shows that the benzoate layer on TiO2 is stable under UV illumination for several hours. The stability of benzoate under illumination is in contrast to the photochemical instability of other small carboxylic acid containing molecules on TiO2 and demonstrates the importance of substituents on the stability of the molecular layer.

We demonstrate the one-dimensional confinement of weakly bound butyrophenone molecules between strongly bound complexes formed via reaction with oxygen on TiO(2)(110). Butyrophenone weakly bound to Ti rows through the carbonyl oxygen diffuses freely in one dimension along the rows even at 55 K, persisting for many minutes before hopping out of the 1-D well. Quantitative analysis yields an estimate of the migration barrier of 0.11 eV and a frequency factor of 6.5 × 10(9) Hz. These studies demonstrate that weakly bound organic molecules can be confined on a surface by creating molecular barriers, potentially altering their assembly.

We report the first visualization of a reactive intermediate formed from coupling two molecules on a surface-a diolate formed from benzaldehyde coupling on TiO(2)(110). The diolate, imaged using scanning tunneling microscopy (STM), is reduced to gaseous stilbene upon heating to ∼400 K, leaving behind two oxygen atoms that react with reduced Ti interstitials that migrate to the surface, contrary to the popular expectation that strong bonds in oxygenated molecules react only with oxygen vacancies at the surface. Our work further provides both experimental and theoretical evidence that Ti interstitials drive the formation of diolate intermediates. Initially mobile monomers migrate together to form paired features, identified as diolates that bond over two adjacent five-coordiante Ti atoms on the surface. Our work is of broad importance because it demonstrates the possibility of imaging the distribution and bonding configurations of reactant species on a molecular scale, which is a critical part of understanding surface reactions and the development of surface morphology during the course of reaction.

This paper describes the use of cadmium sulfide quantum dots (CdS QDs) as visible-light photocatalysts for the reduction of nitrobenzene to aniline through six sequential photoinduced, proton-coupled electron transfers. At pH 3.6-4.3, the internal quantum yield of photons-to-reducing electrons is 37.1% over 54 h of illumination, with no apparent decrease in catalyst activity. Monitoring of the QD exciton by transient absorption reveals that, for each step in the catalytic cycle, the sacrificial reductant, 3-mercaptopropionic acid, scavenges the excitonic hole in ∼5 ps to form QD(•-); electron transfer to nitrobenzene or the intermediates nitrosobenzene and phenylhydroxylamine then occurs on the nanosecond time scale. The rate constants for the single-electron transfer reactions are correlated with the driving forces for the corresponding proton-coupled electron transfers. This result suggests, but does not prove, that electron transfer, not proton transfer, is rate-limiting for these reactions. Nuclear magnetic resonance analysis of the QD-molecule systems shows that the photoproduct aniline, left unprotonated, serves as a poison for the QD catalyst by adsorbing to its surface. Performing the reaction at an acidic pH not only encourages aniline to desorb but also increases the probability of protonated intermediates; the latter effect probably ensures that recruitment of protons is not rate-limiting.

This paper describes a low-temperature scanning tunneling microscopy (STM) study of a simple thioether, dibutyl sulfide, on a Cu{111} surface. The literature is full of data about thiol-based monolayers; however, relatively little is known about thioether self-assembly. Thioethers are more resilient to oxidation than thiols and offer the potential for control over nanoscale assembly in two dimensions parallel to the surface. Therefore, robust assembly schemes derived from thioethers may offer a new class of self-assembled systems with novel and useful properties. At a medium surface coverage and a temperature of 78 K, dibutyl sulfide grows in small, highly ordered islands in which the ordering is driven by both the molecule-surface dative bonds and intermolecular van der Waals bonding. Annealing to around 120 K allows diffusion and reordering of the molecules and the formation of large, very well ordered domains with little or no defects. We show high-resolution images of the molecular arrays and propose a model for their packing structure. These data suggest the potential use of thioethers for a variety of self-assembly applications that require control over molecular spacing parallel to the surface. We also show how the STM tip can be used to manipulate individual molecules within the ordered structures and that the arrays can act as a nanoscale abacus. The range of motion of the manipulated molecules inside a regular array reflects the potential imposed upon them by their neighbors.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently concluded a set of engineering flights for Observatory performance evaluation. These in-flight opportunities are viewed as a first comprehensive assessment of the Observatory's performance and are used to guide future development activities, as well as to identify additional Observatory upgrades. Pointing stability was evaluated, including the image motion due to rigid-body and flexible-body telescope modes as well as possible aero-optical image motion. We report on recent improvements in pointing stability by using an active mass damper system installed on the telescope. Measurements and characterization of the shear layer and cavity seeing, as well as image quality evaluation as a function of wavelength have also been performed. Additional tests targeted basic Observatory capabilities and requirements, including pointing accuracy, chopper evaluation and imager sensitivity. This paper reports on the data collected during these flights and presents current SOFIA Observatory performance and characterization., Comment: 13 pages, 13 figure, and 2 tables; accepted by ApJS

The Stratospheric Observatory for Infrared Astronomy (SOFIA) has recently concluded a set of engineering flights for Observatory performance evaluation. These in-flight opportunities have been viewed as a first comprehensive assessment of the Observatory's performance and will be used to address the development activity that is planned for 2012, as well as to identify additional Observatory upgrades. A series of 8 SOFIA Characterization And Integration (SCAI) flights have been conducted from June to December 2011. The HIPO science instrument in conjunction with the DSI Super Fast Diagnostic Camera (SFDC) have been used to evaluate pointing stability, including the image motion due to rigid-body and flexible-body telescope modes as well as possible aero-optical image motion. We report on recent improvements in pointing stability by using an Active Mass Damper system installed on Telescope Assembly. Measurements and characterization of the shear layer and cavity seeing, as well as image quality evaluation as a function of wavelength have been performed using the HIPO+FLITECAM Science Instrument configuration (FLIPO). A number of additional tests and measurements have targeted basic Observatory capabilities and requirements including, but not limited to, pointing accuracy, chopper evaluation and imager sensitivity. SCAI activities included in-flight partial Science Instrument commissioning prior to the use of the instruments as measuring engines. This paper reports on the data collected during the SCAI flights and presents current SOFIA Observatory performance and characterization.

The literature contains many studies of thiol-based, self-assembled monolayers (RSH); however, thioethers (RSR) have barely begun to be explored, despite having the potential advantages of being more resistant to oxidation and allowing for the control of self-assembly parallel to the surface. This paper describes a low-temperature scanning tunneling microscopy investigation of dimethyl sulfide on Cu{111}. Previous work on the adsorption of dibutyl sulfide on Cu{111} revealed that intermolecular van der Waals interactions directed the parallel ordering of dibutyl sulfide molecules in linear rows. Upon annealing to 120 K, small dibutyl sulfide domains reordered into very large, ordered domains free of defects. The current study reveals the effect of the shorter alkyl chain length of dimethyl sulfide on both the rate of diffusion and the packing structure of the molecule. At a medium surface coverage and at 78 K, it was found that dimethyl sulfide is mobile and forms large, ordered islands without the 120 K annealing that was required for dibutyl sulfide to arrange. Also, the molecular packing structure evolves from quadrupole-quadrupole interactions and results in a perpendicular arrangement of neighboring molecules instead of the parallel arrangement observed for dibutyl sulfide. We show high-resolution images of the dimethyl sulfide islands in which submolecular features are revealed. These high-resolution data allow us to propose a structural model for the adsorption site of each dimethyl sulfide molecule within the ordered structures. These results demonstrate that the length of the alkyl side chain is an important factor in determining how thioethers self-assemble on metal surfaces.

We describe how the presence of styrene, a weakly adsorbed molecule, dramatically restructures the Au{111} surface at temperatures as low as 80 K. The restructuring manifests itself both in mobility of step-edge atoms, as well as changes in the position of the herringbone reconstruction over time. These effects are explained in terms of the preferential adsorption sites of styrene allowing it to assist in atom detachment from step edges, as well as lowering of the energetic barrier for movement of the herringbone reconstruction. This work has important consequences for studies in which Au is used as a support for or as an electrical contact to molecules.

This paper describes the development and testing of a collapsible drogue parachute design, which was required for the testing of NASA’s X-37 Approach and Landing Test Vehicle (ALTV) from the B-52 H aircraft. The ALTV, which has a relatively low drop weight of 7000 lbs. and an L/D in excess of 4.4 at the time of release from the B-52, also has completely autonomous control authority during the test flights, which, in the event of a software failure, could result in re-contact of the ALTV and the B-52 after release. To eliminate the possibility of re-contact, several mitigation strategies were investigated. The strategy selected involved deploying a drogue parachute from the ALTV prior to release from the B-52, and then releasing the drogue from the ALTV a few seconds after the ALTV separates from the B-52. However, due to the high altitude at which the drogue is released, the predominant wind direction, and the close proximity of an air corridor of a major commercial airport, the drogue was required to collapse after release to minimize its drift potential. Since the drogue parachute required the use of a swivel, the collapse mechanism chosen was a silicone band attached at the drogue skirt. While the chute was attached to the ALTV, the high aerodynamic loading would cause the band to stretch to the skirt diameter, causing only negligible drag loss, but would retract when the drogue was released, causing the drogue to collapse and fall rapidly to the ground. Ground testing demonstrated that the design worked, but the program was cancelled prior to flight testing with the B-52.

By studying styrene, a simple hydrocarbon with a weak dipole moment, we have investigated ferroelectric ordering and ferroelectric transitions at the single-molecule level. Low-temperature scanning tunneling microscopy imaging of individual styrene molecules reveals their internal structure and hence the orientation of their dipole moment parallel to the Au{111} surface. At near monolayer coverages, both local ferroelectric ordering of the molecules and long-range antiferroelectric ordering of the domains are observed, and the dipole−dipole interaction energies are quantified. A piezoelectric transition from ferroelectric to paraelectric ordering is observed upon further increase of the coverage. The effect of the STM tip in randomizing ordered domains is also discussed. This work demonstrates that important ferroelectric properties such as spontaneous polarization, long-range ordering, and piezoelectricity can be achieved in nanoscale domains of a weakly polar molecule on a metal surface.

The photofragmentation of butyrophenone yields benzoate and a propyl radical on oxidized TiO2(110). Oxygen dissociates in native oxygen vacancies to produce reactive oxygen adatoms which react with butyrophenone to create photoactive butyrophenone-O complexes that are sensitive to hole oxidation created upon UV illumination. The same O adatoms also trap one of the primary photoproducts, phenyl-CO, to produce benzoate. The reaction proceeds via a Norrish Type I like process involving α-CC cleavage on the surface, in contrast to the gas phase where a Norrish Type II pathway predominates. The mechanism is probed using mass spectrometry and, for the first time, scanning tunneling microscopy (STM). Our STM experiments show that there is a 1-to-1 correspondence between the immobile butyrophenone-O complex and formation of a benzoate on the surface. We also demonstrate that the benzoate species is in close proximity to the original butyrophenone complex, indicating that benzoate is produced on a time scale more rapid than diffusion of the photoproducts. While the photoproducts of butyrophenone decomposition are similar to ketone oxidation reported previously, butyrophenone reacts via a different starting ground state, based on STM and density functional theory studies. Specifically, butyrophenone does not produce a dioxyalkylene species, which has been proposed to be the photoactive state for other ketones. Based on a combination of STM experiments and density functional theory, we propose that a peroxy-like configuration where the oxygen adatom stabilizes the butyrophenone through its carbonyl oxygen is the surface intermediate that photodecomposes. These results demonstrate the importance of the excited state in determining the photochemistry of ketones on surfaces.

The development and use of Cognitech's measure package is described. The Measure program allows the user to enter the locations and dimensions of known points in an image, and through the established principles of photogrammetry, recover dimensions and positions of objects that are unknown. Several cases in which Measure was employed in the investigative process are discussed.

We investigate the role of defects—adatoms, vacancies, and steps—in the bonding and reaction of propene on Au(111) containing atomic oxygen, using density functional theory (DFT) calculations. The adsorption of propene is stronger on a surface containing defects compared to the flat, bulk-terminated surface, with the largest gain in binding (∼0.7 eV) on a surface with a 1/9 monolayer (ML) of Au adatoms. Charge-density difference plots reveal that the difference between defective surfaces and the bulk-terminated surface is a more pronounced depletion of electron density from the carbon–carbon π bond and a charge accumulation between the double bond and the gold atom to which the propene is bound. We calculate the energy barriers for two competing reactions that are important in determining the selectivity for propene oxidation. Allylic H abstraction by adsorbed O leads to combustion, whereas O addition to form an oxymetallacycle is the first step in propene epoxide formation. A comparison of the energetics of these two pathways on flat and defect-containing Au surfaces indicates that the reactivity depends on the nature and prevalence of surface defects. Both electronic and geometric factors, such as the path and the distance the oxygen must travel to meet the allylic hydrogen for abstraction, are important in explaining the reaction trends.

A fiber-optic temperature sensor has been developed for aerospace applications on the basis of the time rate of decay (TRD) principle, with a view to an operational temperature range of -60 to 350 C. This TRD system has completed qualification testing and will then undergo flight tests. Attention is presently given to the design and performance of four low temperature sensors that are subelements of the larger sensor system; in order to convert analog signals into over/under temperature indications, simple comparators are implemented in software.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Cost, accuracy and reliability requirements dictate the need to critically examine the fiber optic sensor approaches that are currently being pursued for making absolute measurements. Typical absolute measurements require only moderate sensitivity but high static accuracy over wide environmental conditions. In many applications, in situ calibration is not possible, yet high accuracy must be maintained with tens or hundreds of meters of fiber optic cable and multiple connectors separating the optical sensor head from the associated electronics. For many fiber optic sensor approaches, uncontrollable cable and connector loss variations limit achievable accuracy to a few percent. We present experimental data on cable and connector loss variations and review the resulting accuracy limitations for a variety of generic sensor approaches.

Two methods of reducing subaperture interferograms to produce full aperture aberration coefficients now exist: the Kwon-Thunen and the simultaneous fit method. The results of a numerical comparison show that for no flat-to-flat alignment the two work equally well and are equally sensitive to both higher-order noise and random noise. For ideal flat-to-flat alignment the Kwon-Thunen approach is less sensitive to noise by a factor of ~3. The simultaneous fit method requires fewer sample points and runs faster than the Kwon-Thunen method.