Hot Oxygen Atoms Research Articles

Elucidation of the nature of active oxygen in the reaction of low-temperature oxidation of CO on single crystal surfaces platinum and palladium

The temperature-programmed reaction (TPR) method, high-resolution electron energy loss spectroscopy (HREELS), and molecular beam method were used to elucidate the role surface reconstruction, subsurface oxygen (Osubs), and COads concentration play in the low-temperature oxidation of CO on the Pt(100), Pt(410), Pd(111), and Pd(110) surfaces. The possibility of the formation of so-called hot oxygen atoms, which arise at the surface at the instant of dissociation of O2ads molecules and can react with COads at low temperatures (∼150 K) to form CO2, was examined. It was revealed that, when present in high concentration, COads initiates the phase transition of the Pt(100)-(hex) reconstructed surface into the (1 × 1) non-reconstructed one and blocks fourfold hollow sites of oxygen adsorption (Pt4-Oads), thereby initiating the formation of weakly bound oxygen (Pt2-Oads), active in CO oxidation. For the Pt(410), Pd(111), and Pd(110) surfaces, the reactivity of Oads with respect to CO was demonstrated to be dependent on the surface coverage of COads. The 18Oads isotope label was used to determine the nature of active oxygen reacting with CO at ∼150–200 K. It was examined why a COads layer produces a strong effect on the reactivity of atomic oxygen. The experimental results were confirmed by theoretical calculations based on the minimization of the Gibbs energy of the adsorption layer. According to these calculations, the COads layer causes a decrease in the apparent activation energy E act of the reaction due to changes in the type of coordination and in the energy of binding of Oads atoms to the surface.

Stochastic models of hot planetary and satellite coronas: A hot oxygen corona of Mars

The processes of the kinetics and transport of hot oxygen atoms in the transition (between thermosphere and exosphere) region of the upper atmosphere of Mars are studied. The reaction of dissociative recombination of the main ionospheric ion O 2 + with thermal electrons in the ionosphere of Mars is considered as a source of hot oxygen atoms. The distribution of suprathermal oxygen atoms by kinetic energy is calculated. It is shown that the exosphere is populated by a considerable number of suprathermal oxygen atoms with kinetic energies just below the escape energy of 2 eV; that is, a hot oxygen corona of Mars is formed.

Stochastic models of hot planetary and satellite coronas: Atomic oxygen in Europa’s corona

This paper analyzes the formation, kinetics, and transport of hot oxygen atoms in the atmosphere of the Jovian satellite Europa. Atmospheric sources of suprathermal oxygen atoms are assumed to be represented by the processes of dissociation of molecular oxygen, which is the main component of the atmosphere, by solar UV radiation and electron fluxes from the inner magnetosphere of Jupiter, as well as by the reaction of dissociative recombination of the main ionospheric ion O2+ which thermal electrons. It is shown that dissociation in Europa’s near-surface atmosphere is balanced by the processes of the loss of atomic oxygen due to the effective escape of suprathermal oxygen atoms into the inner magnetosphere of Jupiter along the orbit of Europa and due to ionization by magnetospheric electrons and catalytic recombination of oxygen atoms on the icy surface of the satellite. It thus follows that atomic oxygen is only a small admixture to the main atmospheric component—molecular oxygen—in the near-surface part of the atmosphere. However, the outer exospheric layers of Europa’s atmosphere are populated mostly by suprathermal oxygen atoms. The near-surface molecular envelope of Europa is therefore surrounded by a tenuous extended corona of hot atomic oxygen.

Spatially Anisotropic Etching of Graphite by Hyperthermal Atomic Oxygen

The spatially anisotropic kinetics involved in the chemical reaction between highly ordered pyrolytic graphite (HOPG) and a beam containing hyperthermal (approximately 8 km s(-1)) O((3)P) atomic oxygen and molecular oxygen yields unique surface morphologies. Upon exposure at moderate sample temperatures (298-423 K), numerous multilayer circular pits embedded in the reacted areas have been observed with the use of atomic force microscopy and scanning tunneling microscopy. These pits have diameters spanning nanometers to micrometers and depths from a few to tens of nanometers. The most striking characteristic of these pits is the convex curvature of the pit bottoms, where the highest point on the pit bottom is at the center and the lowest point occurs around the peripheral edge. Such structure arises by the interplay between kinetics of pit nucleation, the spatially anisotropic kinetics involved in the lateral and downward reactivity of HOPG, and the fluence of atomic oxygen. These kinetics, which are also influenced by the high reactivity of the translationally hot impinging oxygen atoms, govern the overall morphological evolution of the surface.

Thermospheric distribution of fast O(1D) atoms

[1] Detailed calculations are carried out of the sources of energetic metastable O(1D) atoms in the atmosphere at altitudes between 80 km and 200 km, and the corresponding energy distribution functions are derived, taking account of energy transfer and quenching in collisions of the metastable atoms with the ambient atmospheric gas constituents. The energy relaxation of metastable oxygen atoms produced by O2 and O3 photolysis and O2+ dissociative recombination is determined by solving the time-dependent Boltzmann equation. The O(1D) thermalization and quenching times are obtained as functions of the altitude. The steady state distributions of metastable O(1D) are computed and used for the determination of the parameters characterizing the nonthermal O(1D) atoms. The nonthermal atoms comprise 4–6% of the distribution, and their effective temperatures are larger by 25–46% than the local temperatures of the ambient gas. The role of hot metastable oxygen atoms in the production of vibrationally excited OH molecules is analyzed.

An auroral source of hot oxygen in the geocorona

The high‐energy proton‐hydrogen (H+/H) beam associated with proton auroral precipitation transfers momentum in elastic and inelastic collisions with ambient thermal atomic oxygen in the high latitude thermosphere. This process provides a localized novel source of hot oxygen atoms in addition to exothermic photochemistry, charge exchange and momentum transfer from O+ ion precipitation and charge exchange with accelerated ionospheric O+ ions. We suggest that this source contributes to the population of the hot oxygen geocorona and to the flux of escaping oxygen atoms. For an incident proton energy flux of 1 mW m−2 and a mean energy Emean ≈ 5 keV, we calculate a density of hot oxygen atoms with energy above 1 eV of 2.0 × 103 cm−3 and a mean kinetic energy of about 3.5 eV at 700 km. The total upward flux of hot oxygen atoms with energies higher 1 eV is estimated as 3.5 × 108 cm−2 s−1.

Stochastic models of hot planetary and satellite coronas: a photochemical source of hot Oxygen in the upper atmosphere of Mars

The processes of the kinetics and transport of hot oxygen atoms in the upper atmosphere of Mars are studied. A reaction of dissociative recombination of the main ionospheric ion O 2 + with thermal electrons is considered as a photochemical source of suprathermal oxygen atoms. Oxygen atoms are formed in the dissociative recombination reaction with an excess of kinetic energy of about 0.4–4 eV and lose that energy in elastic and inelastic collisions with the ambient thermal atmospheric gas. The altitude distributions of the concentrations of neutral and ionized components, as well as their temperatures, were taken from Krasnopolsky (2002). Unlike the models published earlier, detailed calculations of the formation, collisional kinetics, and transport of suprathermal oxygen atoms in the thermosphere-exosphere transition region of the upper atmosphere of Mars have been made for the first time. For this, we used a stochastic model of the formation of a hot planetary corona (Shematovich, 2004). It has been shown that the considered photochemical source of suprathermal oxygen leads to the formation of the hot corona and to higher nonthermal losses of oxygen from the upper atmosphere of Mars due to escape fluxes. The detailed energy spectra of the fluxes of suprathermal atomic oxygen were calculated for the thermosphere-exosphere transition region of the Martian atmosphere.

Hot carbon densities in the exosphere of Venus

The results of calculations of hot carbon densities in the exosphere of Venus are presented. The calculation is a two‐step process. First a two‐stream transport code is used to solve for the distribution function at the exobase, and then these results are used in a Liouville equation solution above the exobase. It is found that generally, photodissociation of carbon monoxide is the largest source of hot carbon atoms in the upper atmosphere of Venus, larger than dissociative recombination of CO+ and significantly larger than the creation of hot carbon through collisions with hot oxygen atoms. It is also found that the high solar activity densities are about 4 times larger than those for the low solar activity case. The results of these calculations are compared with the densities calculated by Paxton [1983].

Effect of Ballistic-Type Hot Atom Adsorption Mechanism on thePhase Diagram of Monomer–Dimer CO–O2 Surface Catalytic Reaction: AMonte Carlo Simulation

We investigate the effect of the ballistic mechanism on the phasediagram using a square surface. While using this mechanism whenever anO2 molecule hits a randomly vacant selected site, the moleculebreaks up into atomic form and then executes a ballistic flight. Thepaths of the two oxygen atoms are taken exactly to be opposite to eachother, i.e. anti-parallel, and the ranges of the atoms are taken to beequal, i.e. they may fly up to 1 or 1.414 or 2 of the atomic spacingfrom the site of impact. Four cases have been studied on the basis ofthe range of hot atoms. The range of the hot oxygen atoms executing aballistic flight might be up to the first nearest neighbourhood (1atomic spacing from the site of impact), the second nearestneighbourhood (1.414 atomic spacing from the site of impact), the thirdnearest neighbourhood (2 atomic spacing from the site of impact),known as cases a, b, and c, respectively, while for case d the range ofthe oxygen atoms executing the ballistic flight might be up to 1 atomicspacing or 1.414 atomic spacing or 2 atomic spacing from the site ofimpact. The steady reactive window is observed and the continuoustransition disappears. As soon as the CO partial pressure departs fromzero, the production of CO2 is observed, which clearly verifiesthe experimental observation.

NRLMSISE‐00 empirical model of the atmosphere: Statistical comparisons and scientific issues

The new NRLMSISE‐00 empirical atmospheric model extends from the ground to the exobase and is a major upgrade of the MSISE‐90 model in the thermosphere. The new model and the associated NRLMSIS database now include the following data: (1) total mass density from satellite accelerometers and from orbit determination (including the Jacchia and Barlier data sets), (2) temperature from incoherent scatter radar covering 1981–1997, and (3) molecular oxygen number density, [O2], from solar ultraviolet occultation aboard the Solar Maximum Mission. A new component, “anomalous oxygen,” allows for appreciable O+ and hot atomic oxygen contributions to the total mass density at high altitudes and applies primarily to drag estimation above 500 km. Extensive tables compare our entire database to the NRLMSISE‐00, MSISE‐90, and Jacchia‐70 models for different altitude bands and levels of geomagnetic activity. We also explore scientific issues related to the new data sets in the NRLMSIS database. Especially noteworthy is the solar activity dependence of the Jacchia data, with which we study a large O+ contribution to the total mass density under the combination of summer, low solar activity, high latitude, and high altitude. Under these conditions, except at very low solar activity, the Jacchia data and the Jacchia‐70 model indeed show a significantly higher total mass density than does MSISE‐90. However, under the corresponding winter conditions, the MSIS‐class models represent a noticeable improvement relative to Jacchia‐70 over a wide range of F10.7. Considering the two regimes together, NRLMSISE‐00 achieves an improvement over both MSISE‐90 and Jacchia‐70 by incorporating advantages of each.

Hot carbon densities in the exosphere of Mars

Theoretical results of hot carbon densities in the exosphere of Mars are presented. The calculation is a two‐step process: First a two‐stream transport code is used to solve for the distribution function at the exobase, and then these results are used in a Liouville equation solution above the exobase. It is found that photodissociation of carbon monoxide is the largest source of hot carbon atoms in the upper atmosphere of Mars, larger than dissociative recombination of CO+ and much larger than the creation of hot carbon through collisions with hot oxygen atoms. It is also found that the high solar activity densities are about an order of magnitude larger than those for the low solar activity case.

A NON-SPHERICAL MODEL FOR THE HOT OXYGEN CORONA OF MARS

We have constructed a non-spherical model for the hot oxygen corona of Mars by including the effects of planetary rotation and diurnal variation of the Martian ionosphere. Exospheric oxygen densities are calculated by integrating ensemble of ballistic and escaping oxygen atoms from the exobase over the entire planet. The hot oxygen atoms are produced by dissociative recombination of , the major ion in the Martian ionosphere. The densities of hot oxygen atoms at the exobase are estimated from electron densities which have been measured to vary with solar zenith angle. Our model shows that the density difference of hot oxygen atoms between noon and terminator is about two orders of magnitude near the exobase, but reduces abruptly around altitudes of 2000 km due to lateral transport. The diurnal variation of hot oxygen densities remains significant up to the altitude of 10000 km. The diurnal variation of the hot oxygen corona should thus be considered when the upcoming Nozomi measurements are analyzed. The non-spherical model of the hot oxy-gen corona may contribute to building sophisticate solar wind interaction models and thus result in more accurate escaping rate of oxygens from Mars.

The ionosphere of Ganymede

We consider the distribution of plasma density in the atmosphere of Ganymede and present electron density profiles inferred from data of the Plasma Wave instrument of Galileo. To study the question of ionospheric plasma, we present a zero-dimensional local rate equation model of the source and loss functions and the atomic and molecular processes we assume to be taking place. We conclude from the model that Ganymede has a bound ionosphere composed mainly of molecular oxygen ions in the polar regions and of atomic oxygen ions at low latitudes and that protons are absent everywhere. This implies that the plasma observed to be flowing out along the open flux tubes connected to the polar cap is composed of ions of atomic oxygen. We predict that Ganymede is surrounded by a corona of hot oxygen atoms. The model neutral atmosphere has a composition similar to that of the ionosphere and is exospheric everywhere. Our calculated neutral column density is consistent with values of Ganymede ultraviolet auroral brightness observed by means of the Hubble Space Telescope.

<i>Letter to the Editor</i>Effects of hot oxygen in the ionosphere: <i>TRANSCAR</i> simulations

Abstract. Recent studies of the ion energy balance in the mid-latitude ionosphere have led to the suggestion that hot neutral atomic oxygen may play a significant role; the presence of a population of hot O could explain some of the problems met in balancing the ion energy budget for Incoherent Scatter (IS) observations. The aim of the present study is to look at such effects by using numerical simulation. The TRANSCAR model is a time-dependent, 13-moment ionosphere model developed for high latitude studies. It was first adapted for mid-latitude conditions. In a first step the model was calibrated and cross-checked with St. Santin IS measurements for the winter case of 27 January 1972 around noon using, in particular, the MSIS neutral atmosphere model. This provides a reference diurnal variation of the ionosphere. The second step investigated the influence of a maxwellian population of hot neutral atomic oxygen introduced in addition to the standard neutral atmosphere. The paper describes the initial comparison between the model and St. Santin IS data, and then the effects induced by a hot atomic oxygen population.Key words. Ionosphere (ionosphere-atmosphere interactions; ion chemistry and composition; mid-latitude ionosphere)

Upper limits for the Martian exospheric number density during the Planet B/Nozomi mission

Dissociative recombination (DR) of ionospheric O 2 + ions is an important source of suprathermal atomic oxygen in the exosphere as previous studies about the Martian upper atmosphere have shown. Because of the weaker gravitational attraction a hot oxygen corona on Mars should be denser than that observed on Venus. Since the most important mechanism for the production of the hot oxygen atoms in the Martian exosphere is DR, we investigated the variability of this production mechanism depending of solar activity. The Japanese Nozomi spacecraft will have the possibility to detect with the neutral mass spectrometer (NMS) for the first time in-situ the theoretically predicted hot oxygen corona on Mars, if the corona number density above the cold background atmosphere is of the order of 10,000 cm −3 . Due to a problem in the propulsion system Nozomi failed its planned arrival rendevouzs with Mars in October 1999 and will, therefore, arrive at the red planet not before January 2004. Solar activity will reach its maximum in 2001, so the related production rate of hot oxygen atoms will be in the medium range during the new arrival date of Nozomi. We used the ionospheric profiles from the Viking mission for low solar activity conditions ( F 10.7≈70) and the Mariner 9 mission with a solar activity of about 120 for medium solar wind activity. The latter is comparable to the level we expect for the Mars arrival of Nozomi. The resulting influence of the hot oxygen corona number density distribution was calculated with a Monte Carlo technique. This technique is used to compute a hot particle density distribution function. We studied the atomic diffusion process in the Martian atmosphere by simulating the collision probability, particle direction and energy loss after collisions by generating random numbers. Compared to previous studies we have improved the Monte Carlo model by using more and smaller altitude steps and more detailed treatment of particles with a temporary downward motion. This has resulted in an increased amount of collisions and a shift to lower energies in the energy spectrum. Our results show that the hot oxygen component should begin to dominate above the cold background atmosphere at an altitude of about 500 km above the Martian surface. The NMS instrument on board of Nozomi should detect the hot oxygen component after its arrival at Mars in January 2004, at an altitude of about 600 km above the Martian surface. Since the solar activity will decrease during the mission the measurements during the first orbits will be the most significant ones. The first in-situ measurements of the hot oxygen number density would be very important for adjusting atmospheric escape models by separating ballistic, satellite and escape trajectories of the hot oxygen atoms, which are significant for studies of the evolution and solar wind interaction of the Martian atmosphere.

Distributions of hot oxygen for Venus and Mars

Hot atomic oxygen velocity distributions and corresponding profiles of density and temperature have been calculated for the Venus and Mars exospheres by Monte Carlo simulation. The Venus results are realistically based on well‐established models of the Venus atmosphere and ionosphere that were derived from Pioneer Venus orbiter measurements near solar maximum. The confidence level is lower for Mars because the only viable data come from Viking entry measurements in daytime at solar minimum, and the global morphologies of the atmosphere and ionosphere are poorly understood. What is clear is that planetary differences arise because the exothermal velocities of hot oxygen created by dissociative recombination of O2+ are below satellite speeds on Venus and greater than escape on Mars. As a result, the distribution of velocities on Venus tends to be nearly isotropic, whereas it is grossly anisotropic on Mars, favoring the vertical with increasing distance from the planet. The calculated lower bound for the Martian oxygen escape rate indicates that the lifetime of CO2 in the Mars atmosphere is probably less than 10 Myr.

Dynamics of rare gas desorption from Pt(111) induced by collisions with photogenerated hot oxygen atoms

We present a classical trajectory study of the collision-induced desorption of rare gas (Rg=Ar, Xe) atoms from Pt(111) by ‘hot’ O atoms generated by photodissociating coadsorbed O 2. Empirical potential energy functions are used to describe the Rg/O/Pt(111) systems. The calculated angular and translational energy distributions of the desorbed Ar and Xe are in good agreement with experimental observations. It is found that the desorption dynamics involves two collisions. The first collision between O and Rg supplies the necessary kinetic energy for desorption, whereas the second collision between the departing Rg atom and a coadsorbed Rg atom results in a desorption angle of 25–35° from the surface normal for Ar and 35–45° for Xe. The light oxygen atom is found to chatter between the Xe atom and the surface during an O–Xe collision, enhancing the energy transfer.

Enhanced empirical models of the thermosphere

The Naval Research Laboratory (NRL) has embarked on a development program to upgrade empirical models of the neutral upper atmosphere (thermosphere and upper mesosphere) and to apply these models to scientific and engineering problems. The program focus has been the Mass Spectrometer — Incoherent Scatter Radar (MSIS) model of composition and temperature. The new NRLMSIS model, due for release in 2000, has ingested additional data sets, including, for the first time, the drag and accelerometer data of Jacchia and others. The formulation in the lower thermosphere now has improved flexibility, and a new species, “anomalous oxygen,” allows for appreciable O+ and hot atomic oxygen contributions to the total mass density at high altitudes. A new full disk proxy for the solar chromospheric EUV driver of thermospheric variability is available for studies in combination with the NRLMSIS model and database. This will determine the value of augmenting F10.7 in the model formulation — a longstanding issue. Whereas F10.7 correlates more closely with coronal EUV flux, chromospheric fluxes provide the primary thermospheric heating.

Hot-atom mechanism and reaction sites in photoinduced carbon monoxide oxidation on a stepped platinum (113) surface

Angular and velocity distributions of product CO2 were measured in CO oxidation photoinduced by 193 nm laser pulses of 16 ns duration on a stepped Pt(113) surface. At low oxygen and CO coverages, the CO2 desorption was collimated at ±23° from the surface normal in a plane along step edges. CO2 is formed by the collision between CO on an atop site and translationally hot oxygen atoms emitted along the axes of oxygen molecules that lie along the step edge, resulting in declined desorption due to momentum transfer from the hot atoms. With increasing oxygen coverage, new components collimating at the (111) terrace and (100) step normal became major, while an increase in the CO coverage enhanced the CO2 desorption collimated in the normal direction. It is likely that at high coverage the reaction sites extend to (111) and (100) microfacets and also to surface troughs, yielding the above three components. The translational temperature of product CO2 was often maximized at around angles where the CO2 desorption is collimated.

Solar cycle variability of hot oxygen atoms at Mars

The population of hot oxygen atoms in the Martian exosphere is reexamined using newly calculated hot O production rates for both low and high solar cycle conditions. The hot oxygen production rates are assumed to result from the dissociative recombination of O2+ ions. These calculations take into account the calculated vibrational distribution of O2+ and the new measured branching ratios. Furthermore, these calculations also consider the variation of the dissociative recombination cross section with the relative speed of the participating ions and electrons, the rotational energy of the O2+ ions, and the spread of the ion and electron velocities. These production rates were next used in a two‐stream model to obtain the energy dependent flux of the hot oxygen atoms as a function of altitude. Finally, the calculated flux at the exobase was input into an exosphere model, based on Liouville's theorem, to calculate the hot oxygen densities as a function of altitude in the exosphere and the resulting escape flux. It was found that hot oxygen densities vary significantly over the solar cycle; the calculated densities vary from about 2×103 to 6×103 cm−3 at an altitude of 1000 km. The escape flux also varies from about 3×106 to 9×106 cm−2s−1.