Time-Resolved Emission Spectroscopic Study of Laser-Induced Steel Plasmas

Laser-induced steel plasma is generated by focusing a Q-switched Nd:YAG visible laser(532 nm wavelength) with an irradiance of 1 x 109 W/cm2 on a steel sample in air at atmospheric pressure.An Echelle spectrograph coupled with a gateable intensified charge-coupled detector is used to record the plasma emissions.Using time-resolved spectroscopic measurements of the plasma emissions,the temperature and electron number density of the steel plasma are determined for many times of the detector delay.The validity of the assumption by the spectroscopic methods that the laser-induced plasma(LIP) is optically thin and is also in local thermodynamic equilibrium(LTE) has been evaluated for many delay times.From the temporal evolution of the intensity ratio of two Fe I lines and matching it with its theoretical value,the delay times where the plasma is optically thin and is also in LTE are found to be 800 ns,900 ns and 1000 ns.     

Spectral Efficiency of Extreme Ultraviolet Emission from CO2Laser-Produced Tin Plasma Using a Grazing Incidence Flat-Field Spectrograph

A grazing incidence flat-field spectrograph using a concave grating was constructed to measure extreme ultraviolet (EUV) emission from a CO 2 laser-produced tin plasma throughout the wavelength region of 5 nm to 20 nm for lithography. Spectral efficiency of the EUV emission around 13.5 nm from plate, cavity, and thin foil tin targets was studied. By translating the focusing lens along the laser axis, the dependence of EUV spectra on the amount of defocus was investigated. The results showed that the spectral efficiency was higher for the cavity target in comparison to the plate or foil target, while it decreased with an increase in the defocus distance. The source’s spectra and the EUV emission intensity normalized to the incident pulse energy at 45 from the target normal were characterized for the in-band (2% of bandwidth) region as a function of laser energy spanning from 46 mJ to 600 mJ for the pure tin plate target. The energy normalized EUV emission was found to increase with the increasing incident pulse energy. It reached the optimum value for the laser energy of around 343 mJ, after which it dropped rapidly.     

The Diagnosis of Plasma Parameters in Surface Alloying Technique by Optical Emission Spectrometry

Electron density （Ne） in a glow discharge plasma for the surface alloying technique is diagnosed by optical emission spectrometry （OES）. With CH4 as the feeding gas, Ne is obtained by comparing the Hβ spectrum according to the Stark broadening effect. It is noticed that Ne varies with the working pressures （30 Pa to 70 Pa） and cathode voltages （500 V to 1000 V）, respectively. Due to an abnormal glow discharge, Ne is between 1. 71 × 10^15 /cm^3 to 6.64 × 10^15 /cm^3 and increases rapidly with working gas pressures and cathode voltages. The results show that OES is a useful method to measure the plasma parameters in a surface alloying glow discharge plasma.     

Optical Emission Spectroscopic Studies of ICP Ar Plasma

The ion line of 434.8 nm and atom line of 419.8 nm of Ar plasma produced by an inductively coupled plasma （ICP） were measured by optical emission spectroscopy and the influences from the working gas pressure, radio-frequency （RF） power and different positions in the discharge chamber on the line intensities were investigated in this study. It was found that the intensity of Ar atom line increased firstly and then saturated with the increase of the pressure. The line intensity of Ar^＋, on the other hand, reached a maximum value and then decreased along with the pressure. The intensity of the line in an RF discharge also demonstrated a jumping mode and a hysteresis phenomenon with the RF power. When the RF power increased to 400 W, the discharge jumped from the E-mode to the H-mode where the line intensity of Ar atom demonstrated a sudden increase, while the intensity of Ar^＋ ion only changed slightly. If the RF power decreased from a high value, e.g., 1000 W, the discharge would jump from the H-mode back to the E-mode at a power of 300 W. At this time the intensities of Ar and Ar^＋ lines would also decrease sharply. It was also noticed in this paper that the intensity of the ion line depended on the detective location in the chamber, namely at the bottom of the chamber the line was more intense than that in the middle of the chamber, but less intense than at the top, which is considered to be related to the capacitance coupling ability of the ICP plasma in different discharge areas.     

Characteristics of C_2 Radical in CHF_3 and CF_4 Electron Cyclotron Resonance Plasmas Studied by Optical Emission Spectroscopy

The characteristics of CHF3 and CF4 electron cyclotron resonance (ECR) plasma have been studied by optical emission spectroscopy (OES) and Langmuir probe. It is found that C2 radical is one of main compositions in both of the two plasmas. We investigated the relative concentration of C2 radical as a function of F (H) radical and ion density. The formation mechanism of C2 radical is analyzed.     

Observation of the Emission Spectra of an Atmospheric Pressure Radio-frequency Plasma Jet

An atmospheric pressure plasma jet （APPJ） using radio-frequency （13.56 MHz） power has been developed to produce homogeneous glow discharge at low temperature. With optical emission spectroscopy, we observed the excited species （atomic helium, atomic oxygen and metastable oxygen） generated in this APPJ and their dependence on gas composition ratio and RF power. O and O2（b1∑g^＋） are found in the effluent outside the jet by measuring the emission spectra of effluent perpendicular to the jet. An interesting phenomenon is found that there is an abnormal increase of O emission intensity （777.4 nm） between 10 mm and 40 mm away from the nozzle. This observation result is very helpful in practical operation.     

Trace-Rare-Gas Optical Emission Spectroscopy of Nitrogen Plasma Generated at a Frequency of 13.56 MHz

Optical emission spectroscopic measurement of trace rare gas is carried out to determine the density of nitrogen (N) atom, in a nitrogen plasma, as a function of filling pressure and RF power applied. 2% of argon, used as an actinometer, is mixed with nitrogen. In order to normalize the changes in the excitation cross section and electron energy distribution function at different operational conditions, the Ar-I emission line at 419.83 nm is used, which is of nearly the same excitation efficiency coefficients as that of the nitrogen emission line at 493.51 nm. It is observed that the emission intensity of the selected argon and atomic nitrogen lines increases with both pressure and RF power, as does the nitrogen atomic density.     

Determination of Plasma Parameters in a Dual-Frequency Capacitively Coupled CF4 Plasma Using Optical Emission Spectroscopy

Optical emission spectroscopy measurements of dual-frequency capacitively coupled CF4 plasmas were carried out.The gas temperature(Tg) was acquired by fitting the optical emission spectra of a CF B X system in 201~206 nm.The atomic fluorine concentration and the electron temperature(Te) were obtained by trace rare gas optical emission spectroscopy and a modified Boltzmann plot technique,respectively.It was found that the gas temperature was about 620±30 K at 50 mTorr and the atomic fluorine concentration increased while the electron temperature decreased with increasing gas pressure and power of high frequency(60 MHz).With increasing low frequency(2 MHz) power,the electron temperature also increased,but the atomic fluorine concentration was insensitive to this change.The generation and disappearance mechanisms of F atoms are discussed.     

Plasma Uniformity in a Dual Frequency Capacitively Coupled Plasma Reactor Measured by Optical Emission Spectroscopy

Local measurement of plasma radial uniformity was performed in a dual frequency capacitively coupled argon plasma (DF-CCP) reactor using an optical probe. The optical probe collects the light emission from a small separate volume in plasma, thus enabling to diagnose the plasma uniformity for different experimental parameters. Both the gas pressure and the low-frequency (LF) power have apparent effects on the radial uniformity of argon plasma. With the increase in either pressure or LF power, the emission profiles changed from a bell-shaped to a double-peak distribution. The influence of a fused-silica ring around the electrodes on the plasma uniformity was also studied using the optical probe. Possible reasons that result in nonuniform plasmas in our experiments are discussed.     

Measurement of electron density and electron temperature of a cascaded arc plasma using laser Thomson scattering compared to an optical emission spectroscopic approach

As advanced linear plasma sources, cascaded arc plasma devices have been used to generate steady plasma with high electron density, high particle flux and low electron temperature. To measure electron density and electron temperature of the plasma device accurately, a laser Thomson scattering(LTS) system, which is generally recognized as the most precise plasma diagnostic method, has been established in our lab in Dalian University of Technology. The electron density has been measured successfully in the region of 4.5?×?10(19)m~(-3) to7.1?×?10~(20)m~(-3) and electron temperature in the region of 0.18 eV to 0.58 eV. For comparison,an optical emission spectroscopy(OES) system was established as well. The results showed that the electron excitation temperature(configuration temperature) measured by OES is significantly higher than the electron temperature(kinetic electron temperature) measured by LTS by up to 40% in the given discharge conditions. The results indicate that the cascaded arc plasma is recombining plasma and it is not in local thermodynamic equilibrium(LTE). This leads to significant error using OES when characterizing the electron temperature in a non-LTE plasma.     

Measurement of the O2 Dissociation Fraction in RF Low Pressure O2/Ar Plasma Using Optical Emission Spectrometry

Measurement of the oxygen dissociation fraction in RF low pressure oxygen/argon plasma using optical emission spectrometry is presented. The oxygen dissociation...     

Optical Emission Spectroscopy Analysis of the Early Phase During Femtosecond Laser-Induced Air Breakdown

Single-pulse and double-pulse optical emission spectroscopy （OES） analyses were carried out in air by using ultrashort laser pulses at atmospheric pressure. The aim of this work is to use spectroscopic methods to analyze the early phase of laser-induced plasma after the femtosecond laser pulse. The temporal behavior of emission spectra of air plasma has been characterized. In comparison with the single-pulse scheme, the plasma emission obtained in the double-pulse scheme presents a more intense continuum along with several additional ionic lines. As only one line is available in the single-pulse scheme, the plasma temperature measurements were performed using only the relative line-to-continuum intensity ratio method, whereas the relative line-to-line intensity ratio method and the relative line-to-continuum intensity ratio method were used simultaneously to estimate the electron temperature in the double-pulse scheme. The results reveal that the temperature values obtained by the two methods in the double-pulse scheme agree. Moreover, this shows that the relative line-to-continuum intensity ratio method is suitable for early phase of laser-induced plasma diagnostics. The electron number density was estimated using the Stark broadening method. In the early phase of laser-induced plasma, the temporal evolution of the electron number density exhibits a power law decrease with delay time.     

Vibrational Temperature of Excited Nitrogen Molecules Detected in a 13.56 MHz Electrical Discharge by Sheath-Side Optical Emission Spectroscopy

Optical emission spectroscopy parallel to the axis of a low-pressure radio-frequency discharge in nitrogen was realized,i.e.end-on instead of customary side-on measurements.A specially designed optical feedthrough was fabricated and installed co-planar with the anode electrode,allowing non-invasive observations perpendicular to the plasma sheath.The principal excited species flowing towards the grounded electrode were identified and,by assuming Boltzmann distributions,the vibrational temperature of the molecules on the X1∑+g and C3Πu levels was estimated under various operating conditions.     

Characterization of Nitrogen Glow Discharge Plasma via Optical Emission Spectrum Simulation

Optical emission spectroscopy in nitrogen glow discharge plasma is simulated, and the collision excitations and characteristic emissions of the species （N2, N2^＋, N^＋, N） are investigated by a Monte Carlo model for nitrogen molecular gas discharge. The excitation rates of the main excited states are calculated and the corresponding relation and relative magnitude between the distribution of excitation rate of a certain excited state and the distributions of the emission rates of various lines originating from this excited level are also explored. The simulated results are compared with the experimental measurements in two typical discharge conditions. The luminescence mechanism of the line N2^＋： 391.4 nm is explained based on the microscopic plasma processes. The cathode glow in N2 discharge is found to be mainly caused by N^＋ impact excitation and the intensity of cathode glow decreases with the voltage. The corresponding relation between the emission rate or intensity of the 391.4 nm line and the production rate and the density of N2^＋ is also examined.     

Experimental Research on Plasma Induced by TEA CO2 Laser Propulsion

Results in the air-breathing propulsion experiments with a parabolic light craft and a self-made UV-preionized 100 J TEA CO2 laser device are presented. Air disturbance and the spectrum of the plasma after the interaction of pulsed laser radiation with the light craft were studied. It was found that the focal length of the parabolic light craft had a significant effect on the air-disturbance. Two shock waves were detected for the longer focal length, while only one shock wave detected for the short focal length. The spectrum of the laser-induced plasma, the distribution of the characteristic lines, and the temporal behaviors of the air plasma were studied in detail. The results showed that, the evolution of the laser-induced plasma lasted 20μs, and the plasma spectrum would reach the maximum intensity at 7μs.     

Quantitative Determination of Density of Ground State Atomic Oxygen from Both TALIF and Emission Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Two experimental techniques have been used to quantify the atomic oxygen density in the case of hot air plasma generated by a microwave (MW) resonant cavity. The latter operates at a frequency of 2.45 GHz inside a cell of gas conditioning at a pressure of 600 mbar, an injected air flow of 12 L/min and an input MW power of 1 kW. The first technique is based on the standard two photon absorption laser induced fluorescence (TALIF) using xenon for calibration but applied for the first time in the present post discharge hot air plasma column having a temperature of about 4500 K near the axis of the nozzle. The second diagnostic technique is an actinometry method based on optical emission spectroscopy (OES). In this case, we compared the spectra intensities of a specific atomic oxygen line (844 nm) and the closest wavelength xenon line (823 nm). The two lines need to be collected under absolutely the same spectroscopic parameters. The xenon emission is due to the addition of a small proportion of xenon (1% Xe) of this chemically inert gas inside the air while a further small quantity of H₂ (2%) is also added in the mixture in order to collect OH(A¬X) and NH(A-X) spectra without noise. The latter molecular spectra are required to estimate gas and excitation temperatures. Optical emission spectroscopy measurements, at for instance the position z=12 mm on the axis plasma column that leads to a gas measured temperature equal to 3500 K, an excitation temperature of about 9500 K and an atomic oxygen density 2.09×1017± 0.2×10¹⁷ cm ⁻³ . This is in very good agreement with the TALIF measurement, which is equal to 2.0×10¹⁷ cm ⁻³ .     

Optical Emission Spectroscopic Measurement of Hydroxyl Radicals in Air Discharge with Atomized Water

Effects of discharge mode, voltage applied, size of the nozzle discharge electrode and flow rate of water on the generation of hydroxyl radical were investigate...     

Spectral Characterization of Laser Induced Plasma from Titanium Dioxide

Optical emission from TiO2 plasma, generated by a nanosecond laser is spectroscopically analysed. The main chemical species are identified and the spatio-temporal distribution of the plasma parameters such as electron temperature and density are characterized based on the study of spectral distribution of the line intensities and their broadening characteristics. The parameters of laser induced plasma vary quickly owing to its expansion at low background pressure and the possible deviations from local thermodynamic equilibrium conditions are tested to show its validity.     

Study of Non-Thermal DC Arc Plasma of CH_4/Ar at Atmospheric Pressure Using Optical Emission Spectroscopy and Mass Spectrometry

Non-thermal C/H/Ar plasmas are widely applied to carbonaceous material production and processing.In this work,plasma parameters and gaseous species of the atmospheric non-thermal C/H/Ar plasmas produced by an atmospheric-pressure DC arc discharge generator in CH_4/Ar were investigated.The voltage-current characteristics were measured for different CH_4/Ar ratios.Optical emission spectroscopy was employed to analyze the electron excitation temperature,gas temperature and electron density under various discharge conditions.The hydrocarbon molecules produced in the CH_4/Ar plasmas were detected with photoionization mass spectrometry.The optical spectral results demonstrated that the electron excitation temperature was 0.4-1 eV,the gas temperature was 2800-4200 K and the electron density was in the range of(5-20)×10~(15) cm~(-3).The mass spectrum indicated that a variety of unsaturated hydrocarbons(C_2H_4,C_3H_6,C_6H_6,etc.) and several highly unsaturated hydrocarbons(C_4H_2,C_5H_6,etc.) were produced in the non-thermal arc plasmas.     

A Study on Optical Emission of CF4＋CH4 Plasma and Deposition Mechanisms of a-C ： F, H Films

Fluorinated amorphous carbon (a-C : F, H) films were deposited by inductively coupled plasma using CH4 and CF4 gases. Actinometrical optical emission spectroscopy (AOES) was used to determine the relative concentrations of various radicals, CF, CF2 CH, C2, H and F, in the plasma as a function of gas flow ratio R, R=[CH4]/([CH4] [CF4]). The structural evolution of the films were characterized by Fourier transform infrared transmission (FTIR) spectroscopy. The relationship between the film deposition and the precursor radicals in the plasma was discussed. It was shown that CH radical, as well as CF, CF2, C2 radicals are of the precursors, contributing to a-C : F, H film growth.