Microwave Discharge Flow Research Articles

Single-photon ionization of SiC in the gas phase: experimental and ab initio characterization of SiC.

We report the first experimental observation of single-photon ionization transitions of the SiC radical between 8.0 and 11.0 eV performed on the DESIRS beamline at the SOLEIL synchrotron facility. The SiC radical, very difficult to synthesize in the gas phase, was produced through chemical reactions between CHx (x = 0-3) and SiHy (y = 0-3) in a continuous microwave discharge flow tube, the CHx and SiHy species being formed by successive hydrogen-atom abstractions induced by fluorine atoms on methane and silane, respectively. Mass-selected ion yield and photoelectron spectra were recorded as a function of photon energy using a double imaging photoelectron/photoion coincidence spectrometer. The photoelectron spectrum enables the first direct experimental determinations of the X+ 4Σ- ← X 3Π and 1+ 2Π ← X 3Π adiabatic ionization energies of SiC (8.978(10) eV and 10.216(24) eV, respectively). Calculated spectra based on Franck-Condon factors are compared with the experimental spectra. These spectra were obtained by solving the rovibrational Hamiltonian, using the potential energy curves calculated at the multireference single and double configuration interaction level with Davidson correction (MRCI + Q) and the aug-cc-pV5Z basis set. MRCI + Q calculations including the core and core-valence electron correlation were performed using the aug-cc-pCV6Z basis set to predict the spectroscopic properties of the six lowest electronic states of SiC+. Complete basis set extrapolations and relativistic energy corrections were also included in the determination of the energy differences characterizing the photoionization process. Using our experimental and theoretical results, we derived semi-experimental values for the five lowest ionization energies of SiC.

Comparative Study of Influence of Experimental Configuration on Densities of Active Species in the Early Afterglows of N2/(0–2.5%)H2 HF Flowing Plasmas

Afterglows of mixed gas of N2 and H2(0–2.5%) flowing microwave discharges in a 5 mm diameter tube connected to a 5 L reactor via a tube of 1.8 cm diameter and 50 cm long, have been studied using optical emission spectroscopy. The obtained results at the entrance of the afterglow tube of 1.8 cm diameter: Short time afterglow (SA), (10–3 s) and inside the 5 L reactor: Long time afterglow (LA), (10–2 s) were then compared. It was found that, in N2 at 2 Torr, 0.5 slpm, the active specie density ratios had a constant value of 10–2 for N/N2, but decreased respectively from 10–3 to 10–4 for N2 (X,v > 13)/N2 and from 10–6 to 10–8 for N+2 /N2. By directly connecting the discharge tube inside the 5 L reactor, the density increases by 10 for N2 (X,v > 13) and by 102 for N2+ by changing the afterglow from LA(10−2 s) to a SA(10–3 s). Moreover and by adding 1% of H2 to N2, the N/N2 and H/H2 ratios had constant values of 1% and 0.2% respectively. The SA(10–3 s) appeared to be more efficient for surface treatments than the LA (10–2 s).

Threshold photoelectron spectroscopy of the HO2 radical.

We report a synchrotron radiation vacuum ultraviolet photoionization study of the hydroperoxyl radical (HO2), a key reaction intermediate in combustion and atmospheric chemistry as well as astrochemistry, using double imaging photoelectron photoion coincidence spectroscopy. The HO2 radical is formed in a microwave discharge flow tube reactor through a set of reactions initiated by F atoms in a CH4/O2/He gas mixture. The high-resolution threshold photoelectron spectrum of HO2 in the 11 eV-12 eV energy range is acquired without interferences from other species and assigned with the aid of theoretically calculated adiabatic ionization energies (AIEs) and Franck-Condon factors. The three vibrational modes of the radical cation HO2 +, the H-O stretch, the H-O-O bend, and the O-O stretch, have been identified, and their individual frequencies are measured. In addition, the AIEs of the X3A″ ground state and the a1A' first excited electronic state of HO2 + are experimentally determined at 11.359 ± 0.003 eV and 11.639 ± 0.005 eV, respectively, in agreement with high-level theoretically computed results. Furthermore, the former AIE value provides validation of thermochemical networks used to extract the enthalpy of formation of the HO2 radical.

High H-atom Density in R(He,Ar)/x%(N2–5%H2) Early Afterglows

Absolute densities of N atoms and N2(A) molecules and estimated densities of H atoms and N2(X,v > 13) and NH molecules are determined in R(He,Ar)/x%(N2–5%H2) flowing microwave discharges by a band intensity ratio method after calibration of the N atom density by NO titration. By varying the plasma parameters: pressure of 8 and 4 Torr, flow rates of 0.5, 1.0 and 2.0 slpm, HF power up to 150 W, a high N + N recombination fraction (> 80%) has been observed with He dilution in the early afterglow (t = 5 × 10–3 s). A maximum value of the H-atom density has been found in the He/2%(N2–5%H2) gas mixture at 8 Torr, 0.5 slpm and 150 W, with a [H]/[N] ratio of 0.3. With Ar dilution, it is found that the Ar/5%(N2–5%H2) gas mixture at 4 Torr, 0.5 slpm, 100 W brought a high H2 dissociation (80%), with a [H]/[N] ratio of about 0.6. From the variation of the H-atom density along the quartz tube, the wall destruction probability of the H-atoms was calculated to be $${\gamma }_{H}^{He}$$ = $${\gamma }_{H}^{Ar}$$ = (1–4) × 10–3 in helium and argon mixtures. Such results are of interest for the enhancement of surface nitriding by the combined effects of N and H atoms inclusion inside a TiO2 surface.

Vacuum ultraviolet photodynamics of the methyl peroxy radical studied by double imaging photoelectron photoion coincidences.

The vacuum ultraviolet photoionization of the methyl peroxy radical, CH3O2, and unimolecular dissociation of internal energy selected CH3O2 + cations were investigated in the 9.7-12.0 eV energy range by synchrotron-based double imaging photoelectron photoion coincidence. A microwave discharge flow tube was employed to produce CH3O2 via the reaction of methyl radicals (CH3) with oxygen gas. After identifying and separating the different sources of CH3 + from photoionization of CH3 or dissociative photoionization of CH3O2, the high resolution slow photoelectron spectrum (SPES) of CH3O2 was obtained, exhibiting two broad bands superimposed with a complex vibrational structure. The first band of the SPES is attributed to the X3A″ and a1A' overlapped electronic states of CH3O2 + and the second is assigned to the b1A' electronic state with the help of theoretical calculations. The adiabatic ionization energy of CH3O2 is derived as 10.215 ± 0.015 eV, in good agreement with high-accuracy theoretical data from the literature. The vertical ionization energy of the b1A' electronic state is measured to be 11.5 eV and this state fully dissociates into CH3 + and O2 fragments. The 0 K adiabatic appearance energy (AE0K) of the CH3 + fragment ion is determined to be 11.15 ± 0.02 eV.

Structural analysis of hydrogenated amorphous carbon nitride films formed from the decomposition of CH3CN in the microwave discharge flow of Ar

Hydrogenated amorphous carbon nitride (a-CNx:H) films were formed from the decomposition of CH3CN in the microwave discharge flow of Ar. During the film formation, the radio-frequency (RF) bias voltages (−VRF) were applied to the substrate stage in the range of 0 V ≤ −VRF ≤ 100 V. The change of the chemical bonding states upon the application of −VRF was investigated based on the X-ray photoelectron spectroscopy (XPS), FTIR, and Raman scattering spectroscopy. The [N]/([N]+[C]) ratios of films were in the range of 0.14–0.26 with the negative dependence on −VRF, which is the result of the chemical sputtering by the Ar+ bombardment induced by −VRF. From the deconvolution analyzes of the XPS and Raman spectra, the development of the two-dimensional carbon network and of the one--dimensional −C=N− network is suggested upon the application of −VRF.

Active species densities in R/x%N2 and R/x%(N2-5%H2) (R = Ar or He) microwave early afterglows

Afterglows of R/x%N2 and R/x%(N2-5%H2) (R = Ar or He) flowing microwave discharges are characterized by optical emission spectroscopy. Absolute densities of N-atoms, N2(A) and N2(X,v>13) metastable molecules and N2+ ions and evaluated densities of NH and H are determined after calibration of the N-atom density by NO titration. New results on NH radical and H-atom relative densities are obtained by considering that the excitation of the NH(A) radiative state in the afterglow is produced by N2(X,v>13) + NH collisions. The interest of these results concerns the enhancement of surface nitriding by combined effects of N and H atoms inclusion in afterglow conditions.

Densities of Active Species in R/x%(N2–5%H2) (R = Ar or He) Microwave Flowing Afterglows

Afterglows of R/x(N2–5%H2) (R = Ar or He) flowing microwave discharges are characterized by optical emission spectroscopy. Absolute densities of N atoms and N2(A) and N2(X,v > 13) molecules and estimated densities of NH and H atoms are determined after calibration of the N atom density by NO titration. It is determined the variations of active species densities along the post-discharge tube from the early afterglow to the late afterglow region. Conditions allowing to obtain a high concentration of H-atoms in dominant N-atoms have been found in the early afterglow region of the R/x(N2–5%H2) mixture for x < 5%. From these densities, the destruction probabilities of the H-atoms on the afterglow quartz tube wall is found to be: $$\gamma_{H}^{He}$$ = (2–3) × 10−4 and $$\gamma_{H}^{Ar}$$ = (3–4) × 10−3. The interest of these results concerns the enhancement of surface nitriding by combined effects of N and H atoms inclusion in afterglow conditions.

Communication: On the first ionization threshold of the C2H radical.

The slow photoelectron spectrum of the ethynyl radical has been recorded for the first time by using the DESIRS beamline of the SOLEIL synchrotron facility. Ethynyl was generated using a microwave discharge flow tube. The observation of the X+Π3←XΣ+2 transition allowed the first direct measurement of the adiabatic ionization threshold of this radical (EI = 11.641(5) eV). The experimental results are supported by ab initio calculations. Our preliminary investigation of the cationic ground state potential energy surfaces predicts a non-negligible Renner-Teller effect which has not been discussed previously.

Synchrotron-based valence shell photoionization of CH radical.

We report the first experimental observations of X(+) (1)Σ(+)←X (2)Π and a(+) (3)Π←X (2)Π single-photon ionization transitions of the CH radical performed on the DESIRS beamline at the SOLEIL synchrotron facility. The radical was produced by successive hydrogen-atom abstractions on methane by fluorine atoms in a continuous microwave discharge flow tube. Mass-selected ion yields and photoelectron spectra were recorded as a function of photon energy using a double imaging photoelectron/photoion coincidence spectrometer. The ion yield appears to be strongly affected by vibrational and electronic autoionizations, which allow the observation of high Rydberg states of the neutral species. The photoelectron spectra enable the first direct determinations of the adiabatic ionization potential and the energy of the first triplet state of the cation with respect to its singlet ground state. This work also brings valuable information on the complex electronic structure of the CH radical and its cation and adds new observations to complement our understanding of Rydberg states and autoionization processes.

Fabrication of hydrogenated amorphous silicon carbide films by decomposition of hexamethyldisilane with microwave discharge flow of Ar

Hydrogenated amorphous silicon carbide films have been fabricated by the decomposition of hexamethyldisilane with a microwave discharge flow of Ar. Mechanically hard films were obtained by applying radio-frequency (RF) bias voltages to the substrate. The atomic compositions of the films were analyzed by a combination of Rutherford backscattering and elastic recoil detection, X-ray photoelectron spectroscopy (XPS), and glow discharge optical emission spectroscopy. The chemical structure was analyzed by carbon-K near-edge X-ray absorption fine structure spectroscopy, high-resolution XPS, and Fourier transform infrared absorption spectroscopy. The structural changes upon the application of RF bias were investigated, and the concentration of O atoms near the film surface was found to play a key role in the mechanical hardness of the present films.

Changes of chemical structure of hydrogenated amorphous silicon carbide films with the application of radio-frequency bias voltages during chemical vapor deposition

Hydrogenated amorphous silicon carbide films were fabricated using the decomposition of Si(CH3)4 with the microwave discharge flow of Ar. The changes of the chemical structure and the mechanical hardness of films were investigated upon the application of the radio-frequency bias voltages (−VRF) to the substrate. The analysis was based on a combination of Rutherford backscattering and elastic-recoil detection analysis, the X-ray photoelectron spectroscopy, the glow discharge optical emission spectroscopy, the nano-indentation measurements, and Fourier-transformed infrared spectroscopy. The fraction of the C–Si bonding state has a positive dependence on −VRF, whereas that of the C–C bonds in the sp3 hybridized state is negligible. In addition, hydrogen terminations were removed effectively under the low −VRF conditions of 0–20V. The films show the penetration of O atoms from the atmospheric origin into the bulk region. The mechanical hardness of the present films is the result of the balance among these factors.

Production of CH(A2Δ) radicals from the dissociative excitation reaction of C2H2 with microwave discharge flow of Ar

Optical emission spectra of the CH(A2Δ–X2Π) transition were observed using the dissociative excitation reaction of C2H2 with the microwave discharge flow of Ar at the pressure of Ar, PAr, in the range of 0.20–0.40 Torr. The mechanism of the production of the CH(A2Δ) state was investigated by the following two methods. The first is the comparison of the PAr dependences of the emission intensity, of the intensity of the laser-induced fluorescence spectra of the 3P1–3P0 transition of Ar, and of the density and temperature of free electrons. The second is the steady-state chemical kinetic analysis with several assumptions of the relevant reaction rate constants. It was concluded that CH(A2Δ) radicals were formed by charge transfer from Ar+ followed by ion–electron recombination with the significant contribution of the impact of free electrons. Energetic considerations revealed that the ions participating in the recombination reaction are excited vibrational levels of C2H2+.

Synchrotron-based double imaging photoelectron/photoion coincidence spectroscopy of radicals produced in a flow tube: OH and OD

We present a microwave discharge flow tube coupled with a double imaging electron/ion coincidence device and vacuum ultraviolet (VUV) synchrotron radiation. The system has been applied to the study of the photoelectron spectroscopy of the well-known radicals OH and OD. The coincidence imaging scheme provides a high selectivity and yields the spectra of the pure radicals, removing the ever-present contributions from excess reactants, background, or secondary products, and therefore obviating the need for a prior knowledge of all possible byproducts. The photoelectron spectra encompassing the X(3)Σ(-) ground state of the OH(+) and OD(+) cations have been extracted and the vibrational constants compared satisfactorily to existing literature values. Future advantages of this approach include measurement of high resolution VUV spectroscopy of radicals, their absolute photoionization cross section, and species/isomer identification in chemical reactions as a function of time.

Measurements of population densities of metastable and resonant levels of argon using laser induced fluorescence

We present a new approach to measure population densities of Ar I metastable and resonant excited states in low temperature Ar plasmas at pressures higher than 1 Torr. This approach combines the time resolved laser induced fluorescence technique with the kinetic model of Ar. The kinetic model of Ar is based on calculating the population rates of metastable and resonant levels by including contributions from the processes that affect population densities of Ar I excited states. In particular, we included collisional quenching processes between atoms in the ground state and excited states, since we are investigating plasma at higher pressures. We also determined time resolved population densities of Ar I 2 p excited states by employing optical emission spectroscopy technique. Time resolved Ar I excited state populations are presented for the case of the post-discharge of the supersonic flowing microwave discharge at pressures of 1.7 and 2.3 Torr. The experimental set-up consists of a pulsed tunable dye laser operating in the near infrared region and a cylindrical resonance cavity operating in TE111 mode at 2.45 GHz. Results show that time resolved population densities of Ar I metastable and resonant states oscillate with twice the frequency of the discharge.

Laser-Spectroscopic Measurements of the Free Radicals Produced from the Dissociation of C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;2&lt;/sub&gt; with the Microwave Discharge Flow of Ar

Laser-Spectroscopic Measurements of the Free Radicals Produced from the Dissociation of C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;2&lt;/sub&gt; with the Microwave Discharge Flow of Ar

Determination of N and O-Atoms, of N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;(A) and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;(X, v &amp;gt; 13) Metastable Molecules and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;&amp;lt;sup style=&amp;quot;margin-left:-6px;&amp;quot;&amp;gt;+&amp;lt;/sup&amp;gt; Ion Densities in the Afterglows of Ar-N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Microwave Discharges

Early afterglows of Ar-N2 flowing microwave discharges are characterized by optical emission spectroscopy. The N and O atoms, the N2(A) and N2(X, v > 13) metastable molecules and N2+ ion densities are determined by optical emission spectroscopy after calibration by NO titration for N and O-atoms and measurements of NO and N2 band intensities. For an Ar-xN2 gas mixture with × increasing from 2 to 100% at 4 Torr, 100 Watt and an afterglow time of 3 × 10- 3 s at the 5 liter reactor inlet, it is found densities in the ranges of (2 - 6) × 1014 cm- 3 for N-atoms, one order of magnitude lower for N2(X, v > 13) and for O-atoms (coming from air impurity), of 1010 - 1011 cm- 3 for N2(A) and of 108 - 109 cm- 3 for N2+.

Determination of N and O-atoms, of N2(A) and N2(X, v>13) metastable molecules and N+2 ion densities in the afterglows of N2-H2, Ar-N2-H2 and Ar-N2-O2 microwave discharges

Early afterglows of N2-H2, Ar-N2-H2 and Ar-N2-O2 flowing microwave discharges are characterized by optical emission spectroscopy. The N and O atoms, the N2(A) and N2(X, v>13) metastable molecules and N+2 ion densities are determined by optical emission spectroscopy after calibration by NO titration for N and O-atoms and measurements of NO and N2 band intensities. In N2-(0.05-2.5 %)H2 and Ar-(1-50%)N2-(0.05-2.5%)H2 gas mixtures, the O-atoms are coming from O2 impurities in the discharge. Concentrations of these active species are compared to the ones obtained in Ar-(5-50%)N2-(0.2-2.5%)O2 gas mixtures in which a controlled amount of O2 is added.

Determination of N and O‐atoms and N2 (A) Metastable Molecule Densities in the Afterglows of N2‐H2, Ar‐N2‐H2 and Ar‐N2‐O2 Microwave Discharges

AbstractEarly afterglows of N2‐H2, Ar‐N2‐H2 and Ar‐N2‐O2 flowing microwave discharges are characterized by optical emission spectroscopy. The N and O atoms and the N2 (A) metastable molecule densities are determined by optical emission spectroscopy after calibration by NO titration for N and O‐atoms and measurements of NO and N2 band intensities. If an uncertainty of 30% is estimated on N‐atomic density, an inaccuracy of one order of magnitude is obtained on the O and N2 (A) densities. In N2‐(0.05‐2.5%)H2 and Ar‐(1‐50%)N2‐(0.05‐2.5%) H2 gas mixtures, the O‐atoms are coming from O2 impurities in the discharge. Concentrations of N and O‐atoms and of N2 (A) densities are compared to the ones obtained in Ar‐(5‐50%)N2‐(0.2‐2.5%) O2 gas mixtures in which a controlled amount of O2 is added. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Mechanism of Production of CN(X2Σ+) Radicals from the Decomposition Reaction of CH3CN with Microwave Discharge Flow of Ar

CN(X2Σ+) radicals were produced from the dissociative excitation reaction of CH3CN with the microwave discharge flow of Ar. The mechanism of the production of these radicals was studied on the basis of a combined analysis of the laser-induced fluorescence (LIF) and electrostatic-probe measurements. The densities of CN(X2Σ+) and the metastable states of Ar, Ar(3P0,2), were determined by observing the LIF spectra of the CN(A2Πi–X2Σ+), Ar(3P1–3P2), and Ar(3P1–3P0) transitions. A chemical–kinetic analysis of the above densities together with the temperature and the density of free electrons was carried out to elucidate that the dominant mechanism of the production of CN(X2Σ+) is the energy transfer from Ar(3P0,2). The rate constant of the production of CN(X2Σ+) is determined to be (4.6 ± 0.4) × 10−17 m3 molecule−1 s−1. Based on the results of the present and our previous studies (Ito et al. in Diam Relat Mater 24:121–125, 2012) , a quantitative description was made on the incorporation of N atoms into hydrogenated amorphous carbon nitride films formed in the present reaction system.