Some fundamental aspects of elementary gas-surface interactions

Cacciatore, M.

doi:10.1351/pac199971101809

Cited by 5 publications

(8 citation statements)

References 21 publications

(26 reference statements)

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“…This occurs at surface locations where the atom can come close enough to a surface atom (or group of atoms) to undergo a strong "electron-exchange" interaction, at a distance comparable with the distance between surface atoms. This is known as atom chemisorption and the active surface sites are denoted as chemisorption sites [1,5,[14][15][16][17][18][19][21][22][23]. On clean metallic surfaces an atom can chemisorb almost anywhere, since free electrons from the metal are accessible.…”

Section: Surface Kinetics Model For Recombination At Pressures Above mentioning

confidence: 99%

“…The distributions are shown in the insert. The dashed lines correspond to the simple Arrhenius expression (16) with E act 0.13 eV (1500 K), A=0.09 for T w = +5C and A=0.044 for T w = +50C.…”

Section: Model Parametermentioning

confidence: 99%

“…Despite the wide variety of experimental conditions, a commonly-accepted view of the atom recombination processes has emerged. Atom surface recombination is generally understood to occur via two mechanisms, depending on whether the recombination probability depends on the atom concentration to first order (Eley-Rideal, ER mechanism) or second order (Langmuir-Hinshelwood, LH mechanism) [1,3,5,8,9,[13][14][15][16]. In the ER mechanism, an atom arriving at the surface directly recombines with an absorbed (either chemisorbed or physisorbed) atom.…”

Section: Introductionmentioning

confidence: 99%

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Oxygen (³P) atom recombination on a Pyrex surface in an O₂ plasma

Booth

Guaitella

Chatterjee

et al. 2019

Plasma Sources Sci. Technol.

258

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The recombination of O (3 P) atoms on the surface of a Pyrex tube containing a DC glow discharge in pure O 2 was studied over a wide range of pressure (0.2-10 Torr) and discharge current (10-40 mA) for two fixed surface temperatures (+50°C and +5°C). The recombination probability, , was deduced from the observed atom loss rate (dominated by surface recombination) determined by time-resolved optical emission actinometry in partially-modulated (amplitude ~15-17%) discharges. The value of  increased with discharge current at all pressures studied. As a function of pressure it passes through a minimum at ~0.75 Torr. At pressures above this minimum  is well-correlated with the gas temperature, T g , (determined from the rotational structure of the O 2 (b 1  g + ,v=0)  O 2 (X 3  g-,v=0) emission spectrum) which increases with pressure and current. The temperature of the atoms incident at the surface was deduced from a model, calibrated by measurements of the spatially-averaged gas temperature and validated by radial temperature profile measurements. The value of  follows an Arrhenius law depending on the incident atom temperature, with an activation energy in the range 0.13-0.16 eV. At the higher surface temperature the activation energy is the same, but the pre-exponential factor is smaller. Under conditions where the O flux to the surface is low  falls below this Arrhenius law. These results are well explained by an Eley-Rideal (ER) mechanism with incident O atoms recombining with both chemisorbed and more weakly bonded physisorbed atoms on the surface, with the kinetic energy of the incident atoms providing the energy to overcome the activation energy barrier. A phenomenological Eley-Rideal model is proposed that explains both the decrease in recombination probability with surface temperature as well as the deviations from the Arrhenius law when the O flux is low. At pressures below 0.75 Torr  increases significantly, and also increases strongly with the discharge current. We attribute this effect to incident ions and fast neutrals arriving with sufficient energy to clean or chemically modify the surface, generating new adsorption sites. Discharge modeling confirms that at pressures below ~0.3 Torr a noticeable fraction of the ions arriving at the surface have adequate kinetic energy to break surface chemical bonds (> 3-5 eV).

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Section: Surface Kinetics Model For Recombination At Pressures Above mentioning

confidence: 99%

Section: Model Parametermentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Oxygen (³P) atom recombination on a Pyrex surface in an O₂ plasma

Booth

Guaitella

Chatterjee

et al. 2019

Plasma Sources Sci. Technol.

258

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“…At low pressure the main source of the uncertainties is surface, namely, absence of exact knowledge about surface processes with O atoms. Despite that the mechanisms of O( 3 P) surface recombination are known and have been studied in whole [1][2][3][4][5][6][7][8][9][10], application of the mechanisms in each specific case requires exact knowledge of many parameters, such as, for example, desorption and activation energies, densities of active surface sites, steric reaction factors, etc [5][6][7][8][9][10][11][12][13][14]. Moreover, these surface characteristics are not constant, but have complex distributions since surface sites are not absolutely the same [3,15].…”

Section: Introductionmentioning

confidence: 99%

Volume and surface loss of O(³P) atoms in O₂ RF discharge in quartz tube at intermediate pressures (10–100 Torr)

Volynets¹,

Lopaev²,

Zyryanov³

et al. 2019

J. Phys. D: Appl. Phys.

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The O( 3 P) atom loss has been studied in O 2 RF plasma in the quartz tube in the intermediate pressure range (10-100 Torr) when the transition from the surface to volume loss is observed. Space-and time-resolved actinometry on Ar and Kr atoms was used to study O( 3 P) atom loss. The research has shown that such a transition actually takes place. However, it was revealed that the gas temperature plays a significant role in this. Gas temperature was measured spectroscopically using theIt was demonstrated that the gas temperature in the plasma volume was rather high (>1200 K) due to the high values of the specific input RF power which in turn led to a significant O( 3 P) loss rate decrease in the discharge volume. The atomic oxygen loss is limited by the O( 3 P) surface recombination as well as by the volume recombination in a thin layer near the wall. It leads to a low integral loss rate and provides a high oxygen dissociation degree. Analysis based on measured [O( 3 P)]/N, T gas , O 3 spatial profiles and surface loss model including recombination with chemisorbed and physisorbed atoms has revealed the importance of both surface and volume losses. High values of the specific input RF power also lead to increase of the gas temperature near the wall and the temperature of the internal tube surface. As a result, the O atom surface loss rate increases and the volume loss rate near the wall decreases, so overall the contributions of both O( 3 P) volume and surface recombination are comparable at pressures up to 100 Torr. The O( 3 P) loss kinetics at intermediate pressures appears to be a rather complex phenomenon and requires at least 1D or even 2D modeling for its correct description. Using global models under similar conditions may lead to dubious results in a detailed study of kinetic mechanisms and processes, but it can be useful for a simple analysis of experimental results.

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“…The energy released in the reaction can be shared among the degrees of freedom of the newly formed molecules (rotations, vibrations, translation and, possibly, electronic) and the degrees of freedom of the substrate (phonons and, possibly, electrons). Different systems of interest in aerospace applications have been investigated using Molecular Dynamics (MD) studies by means of classical or quasi-classical trajectories [72,73,74] and semiclassical methods [75,76,77,78,79,80,81,82], by deriving the probability of recombination and the ro-vibrational nascent distributions.…”

Section: Introductionmentioning

confidence: 99%

Atomic and molecular data for spacecraft re-entry plasmas

Celiberto

Armenise

Cacciatore

et al. 2016

Plasma Sources Sci. Technol.

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The modeling of atmospheric gas, interacting with the space vehicles in re-entry conditions in planetary exploration missions, requires large set of scattering data for all those elementary processes occurring in the system. A fundamental aspect of re-entry problems is represented by the strong nonequilibrium conditions met in the atmospheric plasma close to the surface of the thermal shield, where numerous interconnected relaxation processes determine the evolution of the gaseous system towards the equilibrium conditions. A central role is played by the vibrational exchanges of energy, so that collisional processes involving vibrationally excited molecules assume a particular importance. In the present paper, theoretical calculations of complete sets of vibrationally state-resolved cross sections and rate coefficients are reviewed, focusing on the relevant classes of collisional processes: resonant and non-resonant electronimpact excitation of molecules, atom-diatom and molecule-molecule collisions as well as gas-surface interaction. In particular, collisional processes involving atomic and molecular species, relevant to Earth (N 2 ,O 2 ,NO), Mars (CO 2 ,CO,N 2) and Jupiter (H 2 ,He) atmospheres are considered.

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Some fundamental aspects of elementary gas-surface interactions

Cited by 5 publications

References 21 publications

Oxygen (³P) atom recombination on a Pyrex surface in an O₂ plasma

Oxygen (³P) atom recombination on a Pyrex surface in an O₂ plasma

Volume and surface loss of O(³P) atoms in O₂ RF discharge in quartz tube at intermediate pressures (10–100 Torr)

Atomic and molecular data for spacecraft re-entry plasmas

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Some fundamental aspects of elementary gas-surface interactions

Cited by 5 publications

References 21 publications

Oxygen (3P) atom recombination on a Pyrex surface in an O2 plasma

Oxygen (3P) atom recombination on a Pyrex surface in an O2 plasma

Volume and surface loss of O(3P) atoms in O2 RF discharge in quartz tube at intermediate pressures (10–100 Torr)

Atomic and molecular data for spacecraft re-entry plasmas

Contact Info

Product

Resources

About

Oxygen (³P) atom recombination on a Pyrex surface in an O₂ plasma

Oxygen (³P) atom recombination on a Pyrex surface in an O₂ plasma

Volume and surface loss of O(³P) atoms in O₂ RF discharge in quartz tube at intermediate pressures (10–100 Torr)