influence of the chemical nature of the surface on the reactivity of Oads

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Radiofrequency capacitive discharge

A capacitive rf discharge is ignited in the same discharge tube. Two copper ring electrodes are placed on the tube outer surface and driven symmetrically by a 13.56 MHz generator (Sairem 300W) through a specially designed push-pull matching network. The discharge is operated in either pulsed or continuous mode. Typically, the rf discharge is operated in the same pressure and gas flow conditions as the dc one. The length of visually homogeneous rf discharge column is 40 cm and it occupies entirely the interchangeable section of the discharge tube. The total length of rf plasma column is 50 cm, i.e. the same as in the case of the dc discharge. The use of a rf discharge in addition to the dc discharge is motivated by several reasons:
 When surface cleaning by plasma is performed it is desirable to use relatively high power coupled to the discharge in order to get reproducible results with reasonable pretreatment times. Increase of the dc discharge power would inevitably lead to the erosion of the electrodes and pollution of the studied surfaces.
 In the experiments on molecules oxidation on plasma pretreated surfaces it was found that some molecules react on the metallic dc electrodes. In this case a dedicated discharge tube with rf excitation using external electrodes only has to be used.
 The length of the rf discharge column can be easily changed by moving the ring electrodes along the discharge tube. Therefore the studied surfaces may be exposed directly to the discharge or to the flowing afterglow. This allows discrimination between the roles of ions and neutral radicals in surface reactivity.
Our discharge system is not adapted for detailed electric diagnostics. Grounded surfaces in the vicinity of the discharge column introduced uncontrolled stray capacitance and the determination of the rf current flowing in the plasma is not possible. Therefore, the discharge is characterized by the power absorbed in the plasma. Incident and reflected rf powers are measured by an in-line power meter placed between the rf generator and the matchbox. In order to measure the power absorbed by plasma, losses in the matching circuit should be taken into account. This was done using the subtraction method [69]. First, incident ( ) and reflected ( ) powers as well as the voltage ( ) on the electrodes are measured without plasma (at a pressure too high for the discharge ignition). Without the discharge the difference – represents the losses on the active resistance of the matcher and electrical contacts and therefore it is proportional to . When the discharge is ignited, power dissipation in the plasma is calculated.

Advantages of laser absorption measurements

Combination of high sensitivity, high time resolution (down to few s) and possibility of fast acquisition of a sequence of absorption spectra without signal accumulation make tunable laser absorption spectroscopy a unique tool for kinetic studies. It is the only gas sensing technique allowing time resolved in-situ measurements in a single plasma pulse.

Pitfalls of laser absorption measurements

When absorption spectroscopy with diode lasers is employed, individual roto-vibrational levels of absorbing molecules are probed (ni). In order to deduce the total concentration N, the vibrational and rotational distributions of absorbing molecules have to be taken into account.
In thermal equilibrium, these distributions are always Boltzmanm with the same temperature equal to the kinetic temperature of the gas (Tg). In non-equilibrium conditions which are typical for low temperature plasmas the following ordering of the characteristic temperatures is usually established Tg = Trot ≤ Tvibr. Even at low pressures (~1 mbar) fast rotationaltranslational relaxation leads to the equilibrium between rotational and translational degrees of freedom. Formation of non-equilibrium vibrational distributions is observed when vibrations are efficiently excited and the vibrational-translational relaxation is slow. In order to perform correct measurements during plasma ON phase, the knowledge of the gas temperature is required. An additional complication arises from the fact that temperature gradients are established between the axis of the discharge and the reactor walls. At constant gas pressure, temperature gradients result in the gradients of the neutral gas density what makes accurate absorption measurements a very difficult task.
In the present study in order to get rid of the undesirable distortions we combine the pulsed discharge technique with time resolved laser absorption measurements. In Figure 2. 4 a schematic of a typical behaviour of the laser absorption signal in the presence of gas cooling and vibrational relaxation effects is shown. For correct interpretation of absorption measurements, in every particular case the analysis of the characteristic times of different processes (chemical reactions, gas cooling, vibrational relaxation) should be performed.

Three-channel quantum cascade laser spectrometer

Quantum cascade laser (QCL) is a new type of semiconductor lasers that was theoretically predicted in 1970s in USSR [73] and implemented in 1994 in Bell Laboratories [74]. The laser active region of QCL comprises a periodic series of thin layers of different materials (a so-called super-lattice). A single electron moving through the QCL active region emits a photon every time it travels the distance equal to the period of the super-lattice. Therefore, photons are emitted in a cascade-like manner what gave the name to this type of lasers. Compared to classical TDLAS quantum cascade laser absorption spectroscopy (QCLAS) has a number of advantages:
 QCLs do not require cryogenic cooling and can operate at (-30..+30 )ºC with thermoelectric cooling. This reduces significantly the size and the cost of the laser system. Compact size permits combination of several lasers in one system for simultaneous multi species detection.
 Due to the cascade photon emission mechanism the quantum efficiency and hence the output power of QCLs is much higher. This allows utilisation of fast thermoelectrically cooled detectors.
 QCLs emit one single mode and therefore can be used without any mode selectors.
 The pulse repetition frequency of QCLs may reach few hundred kilohertz allowing for s time resolution.

Table of contents :

I. Introduction
I.1 Plasma-surface interactions: historical overview
I.2 Context of the study
I.3 Organization of the thesis
1. Chapter I: Surface reactivity in N2/O2 plasmas
1.1 Interaction of radicals with surfaces: basic concepts
1.1.1 Adsorption
1.1.1.1 Physisorption
1.1.1.2 Chemisorption
1.1.2 Mechanisms of surface reactions
1.1.3 Surface reactions: thermodynamic viewpoint
1.2 Modelling of surface reactivity in plasmas
1.3 Mesoscopic modelling of atomic recombination on surfaces
1.3.1 Surface kinetics in N2/O2 mixtures
1.3.2 Role of plasma exposure
1.4 Surface reactions in N2/O2 plasmas: review of experimental studies
1.4.1 Recombination of O and N atoms on surfaces
1.4.2 Molecule production/conversion on surfaces
1.5 Research questions
1.6 Research strategy
1.7 Definitions and notations
2. Chapter II: Experimental setup and diagnostic techniques
2.1 Discharge setup
2.1.1 Reactor and gas system
2.1.2 Pulsed discharge systems
2.1.2.1 Direct current glow discharge
2.1.2.2 Radiofrequency capacitive discharge
2.2 Diagnostics employed
2.2.1 Tuneable diode laser absorption spectroscopy in mid-infrared range
2.2.1.1 Principles of laser absorption spectroscopy
2.2.1.2 Advantages of laser absorption measurements
2.2.1.3 Pitfalls of laser absorption measurements
2.2.1.4 Diode laser spectrometer
2.2.1.5 Three-channel quantum cascade laser spectrometer
2.2.2 Time resolved emission and absorption spectroscopy in UV-Vis range.
2.2.2.1 Optical emission spectroscopy for gas temperature determination
2.2.2.2 OES setup and data treatment.
2.2.2.3 Time resolved measurements of ozone concentration
2.2.3 TALIF measurements of atomic oxygen.
2.2.3.1 Principles and calibration of O TALIF measurements.
2.2.3.2 Laser setup
2.2.4 Mass spectrometric gas analysis
2.2.5 XPS surface diagnostics
2.2.6 Summary on used diagnostics
3. Chapter III: Adsorption and reactivity of N atoms on silica surface under plasma exposure
3.1 Introduction
3.1.1 Experimental procedures
3.2 Determination of the coverage of Nads: XPS study
3.2.1 Dynamics of N adsorption on SiO2
3.2.2 Species responsible for nitridation: ions or neutrals?
3.2.3 Reactivity of SiOxNy under plasma exposure
3.2.4 Conclusions on XPS study
3.3 Reactivity of Nads: Isotopic study
3.3.1 Experimental details
3.3.2 Characterization of the discharge: measurements of N2 dissociation degree
3.3.2.1 Dissociation of N2 in pulsed dc discharge
3.3.2.2 Dissociation in pulsed rf discharge
3.3.2.3 Estimation of atomic nitrogen exposure on the surface
3.3.3 14N adsorption on SiO2 under 28N2 plasma exposure
3.3.4 Reactivity of grafted 14N atoms under 30N2 plasma exposure
3.3.4.1 Evidence for a distribution of reactivity of Nads
3.3.4.2 Do Nads participate in surface recombination of N atoms?
3.3.5 Conclusions on the isotopic study
3.4 Recombination of Nads with O atoms
3.4.1 Experimental details
3.4.2 Kinetics of NO production on the surface
3.4.2.1 Proof of recombination mechanism Nads + O → NO
3.4.2.2 Kinetics of NOx in the probe discharge
3.4.2.3 Estimation of the coverage of Nads that produce NO
3.4.2.4 NO production on the surface under continuous O2 plasma exposure
3.4.3 Investigation of adsorption and reactivity of N using measurements of NO production on the surface
3.4.3.1 Kinetics of adsorption
3.4.3.2 Are Nads * active for recombination of N on the surface?
3.5 Discussion and conclusions
3.5.1 Summary and conclusions on used diagnostics
4. Chapter IV: Adsorption and reactivity of oxygen atoms on oxide surfaces under plasma exposure
4.1 Introduction
4.1.1 Experimental procedures
4.2 Chemisorption of O on silica-like surfaces: isotopic study
4.2.1 Experimental details
4.2.2 Do O atoms of the material participate in surface reactivity?
4.2.3 16O adsorption on the surface under 32O2 plasma exposure
4.2.4 Conclusions on the isotopic study
4.3 Adsorption of atomic oxygen on Pyrex and related reactivity towards NO
4.3.1 Experimental details
4.3.2 Evidence of NO oxidation by adsorbed O atoms
4.3.3 Determination of the surface coverage of adsorbed O atoms
4.3.4 Evidence of a distribution of reactivity adsorbed O atoms
4.3.5 Conclusions on the study of Oads reactivity on Pyrex
4.4 On the role of Oads for VOC oxidation
4.4.1 Context of the study: influence of the chemical nature of the surface on the reactivity of Oads
4.4.2 C2H2 oxidation by adsorbed oxygen atoms on TiO2
4.4.2.1 Experimental details
4.4.2.2 Kinetics of C2H2 destruction on pretreated TiO2 surface
4.4.2.3 Oxidation of adsorbed reaction intermediates
4.4.3 Discussion and conclusions on the study of Oads reactivity on TiO2
4.5 Investigation of ozone formation on surfaces
4.5.1 Experimental details
4.5.2 Ozone production in bare silica tube
4.5.3 Ozone production in the presence of high specific surface material
4.5.3.1 Influence of gas pressure and surface pretreatment
4.5.4 Modelling of ozone production in bare silica tube
4.5.5 Conclusions on the ozone production study
4.6 Conclusions
5. Chapter V: Study of surface vibrational relaxation of N2
5.1 Introduction
5.2 Kinetics of vibrationally excited nitrogen
5.2.1 Role played by N2(v) in nitrogen containing plasmas
5.2.2 Electron impact excitation/de-excitation of N2(v)
5.2.3 Vibrational relaxation of N2(v) in the gas phase
5.2.4 Vibrational relaxation on surfaces: motivation of the study
5.3 Study of N2(v) relaxation using infrared titration
5.3.1 Principles of the infrared titration technique
5.3.2 Experimental procedure
5.3.3 Validity of the diagnostics
5.3.3.1 Gas temperature and its influence on the relaxation measurements
5.3.3.2 Dissociation and re-association of titrating molecules
5.3.3.3 Vibrational excitation of titrating molecules
5.4 Modeling of vibrational kinetics in N2-CO2
5.4.1 Relevant relaxation processes
5.4.2 Model description
5.4.2.1 Gas phase processes
5.4.2.2 Heterogeneous processes
5.4.2.3 Numerical solution
5.4.3 Validation of the model
5.4.4 On the possibility of experimental determination of N2 dependence on v
5.4.5 Modeling results in N2 – CO2 mixtures
5.4.6 Determination of N2 for silica. The influence of CO2 admixture
5.4.7 Conclusions on the study of the N2 – CO2 system
5.4.8 Data analysis using characteristic relaxation times
5.5 Application of the IR titration for  determination on catalytic and plasmapretreated surfaces
5.5.1 The effect of plasma pretreatment
5.5.2 Vibrational relaxation on TiO2.
5.5.3 Vibrational relaxation on other catalytic surfaces
5.5.4 Vibrational relaxation on silica surface in air plasma.
5.6 Infrared titration as a plasma diagnostics tool: determination of the vibrational temperature of N2
5.6.1 Formulation of the method and its application to the N2 – CO2 system
5.6.2 On the vibrational temperature measurements in N2 – N2O and N2 – CO
5.7 Conclusions
6. Chapter VI: General conclusions and outlook
6.1 General conclusions
6.2 Summary of the main results
6.3 Implication for modelling of surface recombination
6.4 Future work
6.5 New diagnostic techniques
7. References

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