The adsorption of oxygen on a stepped platinum single crystal surface

The adsorption of oxygen on the Pt(S)-[12(111) × (111) surface has been studied by Auger electron spectroscopy, low energy electron diffraction and thermal desorption spectroscopy. Two types of adsorbed oxygen have been identified by thermal desorption spectroscopy and low energy electron diffraction: (a) atoms adsorbed on step sites; (b) atoms adsorbed on terrace sites. The kinetics of adsorption into these two states can be modeled by considering sequential filling of the two adsorbed atomic states from a mobile adsorbed molecular precursor state. Adsorption on the step sites occurs more rapidly than adsorption onto the terraces. The sticking coefficient for oxygen adsorption is initially 0.4 on the step sites and drops when the step sites are saturated. The heat of desorption from the step site (45 ± 4 kcal/mole) is about 15% larger than the heat of desorption from the terraces.     

Study of Perylenetetracarboxylic Acid Dimethylimide Films by Cyclic Thermal Desorption and Scanning Probe Microscopy

Some results of studying the direct-current (DC) conductivity of perylenetetracarboxylic acid dimethylimide films by cyclic oxygen thermal desorption are presented. The microscopic parameters of hopping electron transport over localized impurity and intrinsic states were determined. The bandgap width and the sign of major current carriers were determined by scanning probe microscopy methods (atomic force microscopy, scanning probe spectroscopy, and photoassisted Kelvin probe force microscopy). The possibility of the application of photoassisted scanning tunneling microscopy for the nanoscale phase analysis of photoconductive films is discussed.     

Chemical reactions of water molecules on Ru(0 0 0 1) induced by selective excitation of vibrational modes

Tunneling electrons in a scanning tunneling microscope were used to excite specific vibrational quantum states of adsorbed water and hydroxyl molecules on a Ru(0 0 0 1) surface. The excited molecules relaxed by transfer of energy to lower energy modes, resulting in diffusion, dissociation, desorption, and surface-tip transfer processes. Diffusion of H₂O molecules could be induced by excitation of the O-H stretch vibration mode at 445 meV. Isolated molecules required excitation of one single quantum while molecules bonded to a C atom required at least two quanta. Dissociation of single H₂O molecules into H and OH required electron energies of 1 eV or higher while dissociation of OH required at least 2 eV electrons. In contrast, water molecules forming part of a cluster could be dissociated with electron energies of 0.5 eV.     

Size effects in photoexcitation of triplet states of ammonium tetraphenylborate

This paper reports on the results of complex investigations of photoexcited states of ammonium tetraphenylborate, which are characterized by self-sensitized luminescence. It has been established that the excitation by UV light at 77 K leads to the formation of stable triplet states due to the capture of electrons on electron traps. The EPR and luminescence excitation spectra exhibit the formation of a set of triplet states with different distances between electrons and holes. The performed investigations give grounds to affirm that, in bulk samples, cations in the structure of ammonium tetraphenylborate are electron traps. When the size of the ammonium tetraphenylborate sample is changed to 6 and 3 nm, the capture of excited electrons on sorbed oxygen molecules becomes dominant. In this case, the appearance of the spectrum of O₂⁻ anion radicals has been detected by the EPR method. The proposed interpretation of the observed effects has been confirmed by the thermoluminescence data on the recombination of electron-hole pairs, which correlate with a change in the intensity of the EPR spectra during annealing.     

Oxygen adsorption on the kinked Pt(321) surface

Oxygen adsorption and desorption were characterized on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy and thermal desorption spectroscopy. Molecular oxygen adsorbs mainly as a peroxo-like species at 100K with a heat of desorption of about 22 k^J/mol. Some of the molecular oxygen also adsorbs dissociatively at 100K. Atomic oxygen is adsorbed in three states. One state is due to adsorption on the terraces and another state is due to adsorption along the rough step sites. The heat of desorption of both of these states approximately equal and decreases from 290 k^J/mol to 195kJ/mol with increasing coverage. Atomic oxygen is also observed to adsorb in another state which is interpreted as adsorption at an on-top site.     

Sequential desorption of dimers from square lattices: A novel mechanism for phase transitions

This work describes a novel mechanism for phase transitions during desorption, involving the formation of lattice size dependent intermediate states when there is enough adsorbate mobility. Monte Carlo simulations are performed to analyze the mechanism of the thermal desorption for adsorbed homonuclear dimers on two-dimensional square lattices. The lattice-gas model with nearest-neighbor repulsive interactions between particles is implemented to study the cases of mobile (with diffusion) and immobile desorption. The number of peaks for the immobile desorption spectra is related to the connectivity of the adsorbed species for both monomer and dimer molecules. However, for the case of mobile desorption, the spectra give information about the desorption mechanism, which differs significantly for monomers and dimers, particularly when the initial temperatures correspond to the critical region.     

Recombination luminescences resulting from the photoionization of N,N, N′, N′-tetramethyl-p-phenylenediamine in the low temperature matrices. II

The delayed luminescences and thermal glow curves from ultraviolet-irradiated rigid solutions of N, N, N′, N′-tetramethyl-p-phenylenediamine (TMPD) were investigated. The luminescence which comprises the flourescence and phosphorescence of TMPD was affected by the composition of the matrix and an added electron trap, that is, benzene, diphenyl, carbon dioxide, oxygen, etc. It was found that the glow curves for systems containing these electron-trapping reagents show two components, which are concluded to be due, respectively, to electrons trapped by the solvent and these reagents. The electrons trapped by the solvent are released at the lower temperature. These results are indicative of the capabilities of these added molecules to capture electrons in the rigid organic matrices. It has also been found that the glow curve for a concentrated solution of TMPD shows two maxima, suggesting that TMPD itself acts as an electron trap. Oxygen quenches the delayed luminescence and thermoluminescence strongly. The intensity ratios between fluorescence and phosphorescence were measured and correlated with the glow curves.     

Coadsorption de l'oxygène et du monoxyde de carbone sur des surfaces de rhénium

Temperature programmed desorption ($\text{2.65}\text{K}\text{sec}$) has been used to study carbon monoxide and mixed layers of carbon monoxide and oxygen on rhenium ribbons, strongly oriented parallel to the (0001) plane. Four binding states, populated in decreasing energy have been detected. Interpretation of the results on β states agrees qualitatively with King's model postulating dissociation of carbon monoxide molecules and a repulsive interaction energy between carbon and oxygen atoms. However, in the coadsorbed layers studies, it is shown that all the oxygen atoms do not play a part in the recombination process, during desorption, and that when oxygen is adsorbed after carbon monoxide, a displacement reaction occurs, due to apparent transfer from β states towards molecular α states. Optimization of the results on pure carbon monoxide layers leads to an interactional energy ω, equal to 3 $\text{kcal}\text{mole}$, and is only possible if is assumed that β states are formed on alternatively filled and empty rows.     

Electrical breakdown in solid dielectrics

The mechanism of electrical breakdown in solid dielectrics is analyzed using the results of our investigations performed in this direction over a period of several decades. It is shown that the electrical breakdown in solid dielectrics involves interrelated prebreakdown processes, such as high-voltage polarization, defect formation, electron impact excitation and electron impact ionization of luminescence centers and ions in the host crystal lattice, etc. The electrical breakdown is initiated by electric-field and thermal generation of defects in the crystal. In turn, the generation of defects leads to the formation of defect regions and channels that provide an assisted transfer of charge carriers. Electron currents flow (and electrons are accelerated by the electric field to energies sufficient to induce impact ionization) in these regions of the crystal with a lattice distorted by defects. In this respect, the known approaches to the elaboration of the breakdown theory for alkali halide and other dielectric crystals on the basis of analyzing the motion and acceleration of electrons in an ideal crystal structure have appeared to be incorrect.     

银氧相互作用对表面非线性光学效应的贡献

本文讨论氧吸附银表面的非线性光学效应,以及在超高真空环境下所作的实验研究。银氧之间的电子交换使得表面二次谐波信号发生变化。由分子氧在银表面吸附动力学过程的二次谐波测量可推测氧的满π轨道向银反馈的电子数多于银向氧π^*反键轨道输运的电子数,测量了氧在银表面的温度解离过程,且能用一简单的理论模型加以解释。 关键词：     

Oxygène adsorbé sur platine; résultats expérimentaux et modèle d'interprétation

Isotopic exchange and programmed thermal desorption have been used to study the problem of the nature of the adsorbed states of oxygen on platinum. On strongly oriented ribbons (100 face), five adsorption states can be identified. At low temperature (120 K), the adsorption is partially molecular. Above 300 K, oxygen adsorbs completely dissociatively. The multiplicity of desorption peaks cannot be explained by the existence of distinct planes or adsorption sites. A model with two types of lateral interactions has been applied to take account of the experimental results. It is in agreement with recent result on (100) vicinal surfaces.     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

The surface chemistry of H2O on clean and oxygen-covered Cu(110)

Adsorption of water at 100 K. on clean and oxygen-covered Cu(110) has been studied using UPS, TDS, Δφ and LEED measurements. The results indicate that two-dimensional hydrogenbonded islands are formed on the clean surface. The long-range order in these islands is in registry with the substrate lattice and gives rise to a c(2×2) LEED pattern. Upon the formation of multilayer ice, the ordering disappears. The presence of oxygen on the surface disrupts the hydrogen bonding, and composite oxygen-water layers are formed. A model of the arrangement of oxygen atoms and water molecules is presented, based upon the LEED observations for these layers and an estimate of the relative oxygen and water coverages. The intensity variation of a thermal desorption peak at 290 K, attributed to adsorbed OH species, with oxygen coverage is in accordance with this model. For low oxygen coverages, the TDS and Δφ results indicate that small oxygen-water clusters with an enhanced ratio of water molecules per adsorbed oxygen atom are present.     

Molecular and atomic oxygen adsorption on the kinked Pt(321) surface

Oxygen adsorption and desorption were characterized on the kinked Pt(321) surface using high resolution electron energy loss spectroscopy, thermal desorption spectroscopy and Auger electron spectroscopy. Some dissociation of molecular oxygen occurs even at 100 K on the (321) surface indicating that the activation barrier for dissociation is smaller on the Pt(321) surface than on the Pt(111) surface. Molecular oxygen can be adsorbed at 100 K but only in the presence of some adsorbed atomic oxygen. The dominance of the v(OO) molecular oxygen stretching mode in the 810 to 880 cm^?1 range indicates that the molecular oxygen adsorbs as a peroxo-like species with the OO axis parallel or nearly parallel to the surface, as observed previously on the Pt(111) surface [Gland et al., Surface Sci. 95 (1980) 587]. The existence of at least two types of peroxo-like molecular oxygen is suggested by both the unusual breadth of the v(OO) stretching mode and breadth of the molecular oxygen desorption peak. Atomic oxygen is adsorbed more strongly on the rough step sites than on the smooth (111) terraces, as indicated by the increased thermal stability of atomic oxygen adsorbed along the rough step sites. The two forms of adsorbed atomic oxygen can be easily distinguished by vibrational spectroscopy since oxygen adsorbed along the rough step sites causes a v(PtO) stretching mode at 560 cm^?1, while the v(PtO) stretching mode for atomic oxygen adsorbed on the (111) terraces appears at 490 cm^?1, a value typical of the (111) surface. Two desorption peaks are observed during atomic oxygen recombination and desorption from the Pt(321) surface. These desorption peaks do not correlate with the presence of the two types of adsorbed atomic oxygen. Rather, the first order low temperature peak is a result of the fact that about three times more atomic oxygen can be adsorbed on the Pt(321) surface than on the Pt(111) surface (where only a second order peak is observed). The heat of desorption for atomic oxygen decreases from about 290kJ/mol (70 kcal/mol) to about 196 kJ/mol (47 kcal/mol) with increasing coverage. Preliminary results concerning adsorption of molecular oxygen from the gas phase in an excited state are also briefly discussed.     

Effect of phonon-induced desorption of oxygen molecules from the surface of a solid on adsorbate luminescence kinetics

A mathematical model to describe the pulse shape of delayed adsorbate fluorescence is proposed that accounts for desorption of oxygen molecules from the surface of a substrate induced by interaction with phonons. A numerical study of the temperature dependence of the probability of desorption of oxygen molecules from the substrate surface and of the luminescence intensity is carried out.     

Optical and electrophysical properties of nanocomposites based on PEDOT: PSS and gold/silver nanoparticles

The absorption spectra in the visible region and current-voltage characteristics in a wide range of electric fields have been investigated at the macroscopic level (planar structures) and at the microscopic level (using a conductive atomic force microscope) in films based on the electroactive polymer PEDOT: PSS and gold/silver nanoparticles (PEDOT: PSS + Au/AgNP). It has been shown that the behavior of the current-voltage characteristics of the nanocomposite films depends significantly on the electric field strength. It has been found that the introduction of gold nanoparticles into PEDOT: PSS in weak electric fields leads to an increase in the bulk conductance by almost two orders of magnitude (due to donor-acceptor interactions), a 50% decrease in the conduction activation energy, and an increase in the sensitivity to adsorbed oxygen. It has been demonstrated that electrical conduction of PEDOT: PSS + AuNP films is provided by hopping charge transfer both in the system of intrinsic localized states and in the system of impurity states of adsorbed oxygen. In strong electric fields, the current-voltage characteristics exhibit a different behavior in the forward and reverse scanning modes.     

Cosmic-ray-driven electron-induced reactions of halogenated molecules adsorbed on ice surfaces: Implications for atmospheric ozone depletion and global climate change

The cosmic-ray-driven electron-induced reaction of halogenated molecules adsorbed on ice surfaces has been proposed as a new mechanism for the formation of the polar ozone hole. Here, experimental findings of dissociative electron transfer reactions of halogenated molecules on ice surfaces in electron stimulated desorption, electron trapping and femtosecond time-resolved laser spectroscopic measurements are reviewed. This is followed by a review of the evidence from recent satellite observations of this new mechanism for the Antarctic ozone hole, and all other possible physical mechanisms are discussed. Moreover, new observations of the 11-year cyclic variations of both polar ozone loss and stratospheric cooling and the seasonal variations of CFCs and CH₄ in the polar stratosphere are presented, and quantitative predictions of the Antarctic ozone hole in the future are given. Finally, a new observation of the effects of CFCs and cosmic-ray-driven ozone depletion on global climate change is also presented and discussed.     

Size-Dependent Effects in EPR and Luminescence Spectra of NH<Subscript>4</Subscript>BPh<Subscript>4</Subscript> Excited States: from Bulk to Nanoparticles

Photoexcited states of NH₄BPh₄ (TPhBA) particles embedded in mesoporous frameworks of different pore sizes were studied by combined electron paramagnetic resonance (EPR), optical and photoluminescence techniques. A distribution of triplet states with short and long electron–hole distances was found. While EPR studies on TPhBA bulk sample suggested that the formation of electron–hole pairs upon the excitation was caused by electron capture on electron traps, the samples in mesoporous frameworks exhibit two ways of the pair formation. The first one is attributed to the capture on phenyl rings and another one is to be thought as the capture of electrons on adsorbed oxygen molecules. These results are also consistent with the thermoluminescence spectra and EPR studies of the photoexcited samples during annealing.     

The formation of new adsorption centers on platinized SiO2 surfaces

Kinetic and spectral characteristics of luminescence and excitation of luminescence of magnesium phthalocyanine (MgPc) molecules adsorbed on silicon dioxide (SiO₂) are studied. They are found to be affected by finely divided platinum (Pt) present at the surface and hydration. The deposition of a Pt catalyst on SiO₂ leads to the formation of new centers. Adsorption of MgPc molecules at these centers increases the lifetime of excited states of the former. Luminescence of charge transfer complexes and the protonized form of phthalocyanine is detected at the platinized surface of silicon dioxide.     

The surface chemistry of ethylene adsorbed on Mo(100), oxygen-covered Mo(100) and MoO2

Ethylene adsorbed on Mo(100) and oxygen-covered Mo(100) can thermally decompose to yield hydrogen and adsorbed carbon, desorb molecularly, self-hydrogenate to produce ethane or dissociate to form adsorbed C₁ species which can hydrogenate to form methane. Complete thermal decomposition of the ethylene is proposed to take place on the four-fold sites on Mo(100) since the hydrogen yield decreases linearly with oxygen coverage. The ethylene desorption activation energy increases with increasing oxygen coverage suggesting that ethylene bonds to Mo(100) predominantly by donation of π electrons to the molybdenum surface. The ethylene hydrogenation activation increases as a function of oxygen coverage in accord with this effect. The yield of methane also varies with oxygen coverage so that no methane desorption is detected for clean Mo(100) but the yield increases with oxygen coverage reaching a maximum at a coverage of ≈0.6 ML and decreasing at higher coverages. Photoelectron spectroscopy results suggest that adsorbed oxygen increases the dissociative probability of ethylene. In addition, experiments in which carbenes are grafted onto the surface by decomposing methylene iodide show that carbenes are stabilized by the addition of oxygen to the surface. These effects both explain the increase in methane yield as a function of increasing oxygen coverage. The decrease at higher coverage is likely due, at least in part, to the lack of hydrogen. The ethane yield also decreases at higher coverages due to a similar effect.