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

Jean-Paul Booth (1) , Olivier Guaitella (1) , Abhyuday Chatterjee (1) , Cyril Drag (1) , V. Guerra (2) , Dmitry Lopaev (3) , Sergey Zyryanov (3) , Tatyana Rakhimova (3) , Dmitry Voloshin (3) , Y. Mankelevich (3)

1 LPP - Laboratoire de Physique des Plasmas
2 Instituto Superior Técnico
3 SINP - D.V. Skobeltsyn Institute of Nuclear Physics

The recombination of O (3P) atoms on the surface of a Pyrex tube containing a DC glow discharge in pure O2 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, γ, 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 γ 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 γ is well-correlated with the gas temperature, T g, (determined from the rotational structure of the O2 (b1Σg , v = 0) → O2(X3Σg −, 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 γ 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 γ 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 ER 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 γ 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).

Domaines: Physique des plasmas [physics.plasm-ph]

Jean-Paul Booth, Olivier Guaitella, Abhyuday Chatterjee, Cyril Drag, V. Guerra, et al.. Oxygen (³P) atom recombination on a Pyrex surface in an O₂ plasma. Plasma Sources Science and Technology, 2019, 28 (5), pp.055005. ⟨10.1088/1361-6595/ab13e8⟩. ⟨hal-02272108⟩