An active spectroscopical study on the plasma parameters of an ICP

Various techniques to measure the electron density and temperature of a 100 MHz inductively coupled plasma (ICP) are compared with each other. Apart from passive spectroscopy of measuring the shape of (e.g. H(beta)) and the area under emission lines we explored the field of active spectroscopy. Non-intrusive and specific are the methods of diode laser absorption (DLA) of an argon 4s-4p transition and Thomson scattering (TS). Intrusive and global is the power interruption (PI) technique: the response of line (argon and analytes) and continuum emission is followed during PI. Finally, a combination of two active techniques, namely TS during PI, is studied. By using the different techniques on the same plasma condition (frequency, power and flows) this intercomparison will reveal the validity region of the various techniques. In this way a strong basis will be created for understanding plasma phenomena.     

An active spectroscopical study on the plasma parameters of an ICP

Various techniques to measure the electron density and temperature of a 100 MHz inductively coupled plasma (ICP) are compared with each other. Apart from passive spectroscopy of measuring the shape of (e.g. H) and the area under emission lines we explored the field of active spectroscopy. Non-intrusive and specific are the methods of diode laser absorption (DLA) of an argon 4s-4p transition and Thomson scattering (TS). Intrusive and global is the power interruption (PI) technique: the response of line (argon and analytes) and continuum emission is followed during PI. Finally, a combination of two active techniques, namely TS during PI, is studied. By using the different techniques on the same plasma condition (frequency, power and flows) this intercomparison will reveal the validity region of the various techniques. In this way a strong basis will be created for understanding plasma phenomena.     

微波等离子体炬光源基本特性的研究

用计算机化断层扫描成象技术研究了微波等离子体炬(MPT)放电的发射轮廓,证明该光源具有良好的对称性,并有一个有利于样品引入的中央通道,最佳分析区在炬管上方5～10 mm,用激光Thomson散射和Rayleigh散射技术测定了ArMPT和HeMPT放电的电子温度、电子密度和气体温度,证明MPT放电的电子温度很高而气体温度较低,是一种非热光源。其中的高能电子处于过布居状态,具有很高的激发能力。     

Electron production and loss processes in a spectrochemical inductively coupled argon plasma

The behavior of inductively coupled plasmas for spectroscopic purposes has been studied extensively in the past. However, many questions about production and loss of electrons, which have a major effect on this behavior, are unanswered. Power interruption is a powerful diagnostic method to study such processes. This paper presents time resolved Thomson scattering measurements of the electron density n_e and temperature T_e in an inductively coupled argon plasma during and after power interruption. In the center of the plasma the measured temporal development of n_e and T_e can be attributed to ambipolar diffusion, three-particle recombination and ionization. However, at the edge of the plasma an additional electron loss process must be involved. In addition, the high electron temperature during power interruption indicates the presence of an electron heating mechanism. The energy gain by recombination processes is shown to be insufficient to explain this electron heating. These discrepancies may be explained by the formation and destruction of molecular argon ions, which can be present in significant quantities.     

The relation between internal and external parameters of a spectrochemical inductively coupled plasma

The excitation kinetics in a spectrochemical plasma are governed by the electron density n_e, electron temperature T_e, and heavy particle (gas) temperature T_h. Therefore, knowledge of these ‘internal’ plasma parameters is important for an understanding of the relation between the sample concentration in the plasma and light emission. Because of the small size of the plasma, the internal plasma parameters are related rather directly to the ‘external’ operational parameters of the plasma, such as the plasma dimensions, power density, and pressure. This relation is established by the various particle and energy balances, and can be used to estimate the internal plasma parameters and predict trends for a change in the operational parameters. In the present work, this approach was applied to spectrochemical inductively coupled plasmas under various gas-flow, gas-composition, and plasma-power conditions, and validated by Thomson scattering experiments. The measured values and trends of the internal plasma parameters are in close agreement with those expected on the basis of the operational parameters of the plasma.     

Non-thermal features of atmospheric-pressure argon and helium microwave-induced plasmas observed by laser-light Thomson scattering and Rayleigh scattering

Laser-light Thomson scattering and Rayleigh scattering have been measured from a microwave-induced plasma sustained at atmospheric pressure, using both argon and helium as a support gas. The measurements were performed at several spatial positions in each plasma, and at forward microwave power levels of 350 W for argon, and at 350 W and 100 W for helium. It was found from these measurements that both argon and helium plasmas deviate substantially from local thermodynamic equilibrium (LTE), Measured electron temperatures range from 13 000–21 500 K, whereas gas temperatures are generally lower by a factor of 2 to 10, depending on the support gas and the spatial position in the discharge. At the same forward microwave power, the electron temperature of the helium plasma is about 3500–7000 K higher than that of the argon plasma. Yet, the argon plasma has a higher electron number density than the helium plasma. Electron number densities in both argon and helium plasmas are roughly two to three orders of magnitude lower than what LTE would predict, based on the measured electron temperatures and the Saha Equation. Even more interestingly, signals in the far-wing portion of the Thomson-scattering spectrum were found to be significantly higher than are predicted by a fitted Maxwellian curve, indicating that there exists an over-population of high-energy electrons. It is concluded that, compared to the inductively coupled plasma, the microwave-induced plasma is highly non-thermal and remains in an ionizing mode in the analytical zone.     

Temperature and density dependence of the light and heavy water ultraviolet absorption edge

Characteristics of the ultraviolet absorption band edge of high-pressure light and heavy water are reported over the temperature range of 25-400 degrees C, extending into the supercritical regime. A gradual redshift in the absorption band edge of approximately 0.6 eV is observed with increasing temperature. This shift cannot be explained by vibrational hot band growth or changes in the degree of Rayleigh scattering with increasing temperature, and is ascribed to a shift of the electronic transition energy. The density dependence for the absorption edge in 400 degrees C supercritical water was also examined, and showed a surprising approximately 0.1 eV blueshift over a factor of 3.5 decrease in density. This shift may be due to a narrowing of the absorption spectrum with decreasing density. It is proposed that the previously reported "red tail" of the water absorption extending into the near ultraviolet and visible could be attributed to preresonant Rayleigh scattering, and that the true onset of liquid water absorption is approximately 5.8 eV at 25 degrees C.     

Study of early laser-induced plasma dynamics: Transient electron density gradients via Thomson scattering and Stark Broadening,and the implications on laser-induced breakdown spectroscopy measurements

To further develop laser-induced breakdown spectroscopy (LIBS) as an analytical technique, it is necessary to better understand the fundamental processes and mechanisms taking place during the plasma evolution. This paper addresses the very early plasma dynamics (first 100 ns) using direct plasma imaging, light scattering, and transmission measurements from a synchronized 532-nm probe laser pulse. During the first 50 ns following breakdown, significant Thomson scattering was observed while the probe laser interacted with the laser-induced plasma. The Thomson scattering was observed to peak 15–25 ns following plasma initiation and then decay rapidly, thereby revealing the highly transient nature of the free electron density and plasma equilibrium immediately following breakdown. Such an intense free electron density gradient is suggestive of a non-equilibrium, free electron wave generated by the initial breakdown and growth processes. Additional probe beam transmission measurements and electron density measurements via Stark broadening of the 500.1-nm nitrogen ion line corroborate the Thomson scattering observations. In concert, the data support the finding of a highly transient plasma that deviates from local thermodynamic equilibrium (LTE) conditions during the first tens of nanoseconds of plasma lifetime. The implications of this early plasma transient behavior are discussed in the context of plasma–analyte interactions and the role on LIBS measurements.     

Effect of an inductively coupled plasma mass spectrometry sampler interface on electron temperature,electron number density,gas-kinetic temperature and analyte emission intensity upstream in the plasma

Thomson scattering, Rayleigh scattering and line-of-sight emission intensities of Ca ion and Sr ion from the inductively coupled plasma were measured in the presence and in the absence of an inductively coupled plasma mass spectrometry sampler interface. When present, the sampler interface was located 13 mm above the load coil (ALC); optical measurements were made 6, 7 and 8 mm ALC. The experimental results suggest that both the electron temperature (T_e) and gas-kinetic temperature (T_g) dropped in the presence of the sampler interface, with the change in T_g seemingly greater than that in T_e, suggesting a faster cooling process for the heavy particles. In contrast, electron number density (n_e) seemed to be generally increased in the outer regions of the discharge but went down in the central channel, a reflection that n_e is possibly dominated by ambipolar diffusion which becomes less efficient as T_e drops. Assuming these results, the plasma decays more gradually ALC and deviates from local thermodynamic equilibrium even more significantly in the presence of the sampler interface. Analyte line emission intensity was either depressed or enhanced in the presence of the interface, depending on the element being observed and the operating conditions. In addition, the change in emission intensity caused by the sampler interface became much more dramatic when a matrix element, such as Li or Zn, was introduced.     

Thomson scattering from analytical plasmas

Electrons play an essential role in energy transfer and many other processes within analytical plasmas, so a complete characterization of any plasma requires spatially and temporally resolved data for the electron number density and temperature, as well as the electron energy distribution function. Thomson scattering provides a reliable non-intrusive means of determining inherently radially resolved absolute values for these fundamental parameters, without the assumptions about cylindrical symmetry or thermodynamic conditions in the plasma that are required by many other techniques. However, because of the relatively complex and expensive instrumentation required to perform successful Thomson scattering measurements, this method has not been widely applied in the field of analytical chemistry. The goal of this review was to clarify the history and theory of Thomson scattering, discuss the instrumental details that must be considered in a Thomson scattering experiment, and summarize the results of studies in which the technique has been applied to the characterization of analytical plasmas, in order to demonstrate the power of Thomson scattering as a tool for plasma characterization.     

Fundamental comparison between non-equilibrium aspects of ICP and MIP discharges

A characterization theory is constructed to relate external control parameters to averaged values of internal plasma properties. It is based on a study on the effect of transport fluxes on elementary balances. This theory is used to compare the inductively coupled plasma (ICP) with the TIA (`torche a injection axiale', a microwave plasma) and to predict properties of He plasmas. The theory is validated by means of laser experiments on the ICP and TIA. Thomson (TS), Rayleigh (RyS) and Raman (RnS) scattering are used to get insight into spatially resolved plasma parameters. TS gives the electron number density and temperature, whereas the combination of RyS and RnS provides the densities of (entrained) air molecules and argon atoms (main gas). In particular, the influence of steep gradients and the air entrainment turn out to be of crucial importance for those situations in which the characterization theory does not properly predict the values of plasma parameters.     

Determination of Indole Alkaloids by High-Performance Liquid Chromatography with Resonance Rayleigh Scattering Detection

A novel resonance Rayleigh scattering (RRS) detection approach combined with high–performance liquid chromatography (HPLC) was developed for the determination of reserpine, deserpidine, and ajmalicine. The resonance Rayleigh scattering signal increased when the analytes were bound to diiodofluorescein in Britton–Robinson buffer (pH 4.2). Separation was performed using a C18 column with a mobile phase consisting of acetonitrile–5 millimolar ammonium acetate buffer at pH 4.2 (44:56, volume by volume). Resonance Rayleigh scattering was measured at excitation and emission wavelengths of 320 nanometers. The experimental parameters affecting the separation and the scattering intensity were carefully optimized. Possible mechanisms for the resonance Rayleigh scattering enhancement of the indole alkaloid–diiodofluorescein system were explored by scanning electron microscopy, nuclear magnetic resonance, and ultraviolet-visible absorption spectroscopy. The method was employed for the determination of the analytes in human urine and pharmaceutical tablets.     

Comparison of electrothermal atomization diode laser Zeeman- and wavelength-modulated atomic absorption and coherent forward scattering spectrometry

Atomic absorption and coherent forward scattering spectrometry by using a near-infrared diode laser with and without Zeeman and wavelength modulation were carried out with graphite furnace electrothermal atomization. Analytical curves and limits of detection were compared. The magnetic field was modulated with 50 Hz, and the wavelength of the diode laser with 10 kHz. Coherent forward scattering was measured with crossed and slightly uncrossed polarizers. The results show that the detection limits of atomic absorption spectrometry are roughly the same as those of coherent forward scattering spectrometry with crossed polarizers. According to the theory with bright flicker noise limited laser sources the detection limits and linear ranges obtained with coherent forward scattering spectrometry with slightly uncrossed polarizers are significantly better than those obtained with crossed polarizers and with atomic absorption spectrometry. This is due to the fact that employing approaches of polarization spectroscopy reduce laser intensity fluctuations to their signal carried fractions.     

On the electron temperatures and densities in plasmas produced by the “torche à injection axiale”

The electron temperature and the electron density of plasmas created by the “Torche à Injection Axiale” (TIA) are determined using Thomson scattering. In the plasma with helium as the main gas, temperatures of around 25 000 K and densities of between 0.64 and 5.1 × 10²⁰m⁻³ are found. In an argon plasma the electron temperature is lower and the electron density is higher: 17 000 K and around 10²¹ m⁻³ respectively. From these results it can be established that the ionisation rates of both plasmas are much larger than the recombination rates, which means that the plasmas are far from Saha equilibrium. However, deviations from a Maxwell electron energy distribution function, as reported for the “Microwave Plasma Torch” (MPT), are not found in the TIA. The excitation and ionisation power of the TIA appears to be stronger than that of the MPT.     

Rayleigh scattering study and particle density determination of a high refractive index TiO2 nanohybrid polymer

We have experimentally carried out a Rayleigh scattering study of a high refractive index TiO(2) nanohybrid polymer. By employing the Rayleigh scattering technique with at least three different wavelengths, we can obtain the Rayleigh ratio of the TiO(2) nanohybrid polymer at each utilized wavelength. These measured Rayleigh ratios are then used to estimate the size of the nanoparticle and determine the number of nanoparticles per unit volume or particle density. Furthermore, this technique can be used to evaluate the dominant size of nanoparticles in the nanohybrid polymer mainly contributed to Rayleigh scattering.     

Diode laser emission linewidth determination: application to H2O line profile studies in the 5 and 1.4 microm regions

In order to study absorption line profiles using the stabilized diode laser spectrometer of Laboratoire de Physique Moléculaire et Applications (LPMA), a reliable determination of the emission line shape of different diodes laser is needed. In the near infrared region (1.39 and 1.66 microm) we used Distributed Feed Back diode lasers which operate around room temperature and in the middle infrared (5 and 8 microm) we used lead salt diode lasers cooled in a helium closed cycle cryostat or in a liquid nitrogen dewar. Some results obtained in H2O line profile studies in the 1.39 and 5 microm regions are presented as examples demonstrating how absorption line profile measurements can lead to erroneous values of the spectroscopic parameters when the contribution of the diode laser emission line width is neglected.     

LD激发的ZnWO4:Er3+晶体上转换发光

采用Czochralski法生长出ZnWO₄:Er³⁺(Er=0.02mol)单晶,测量了吸收光谱和激发光谱。在波长966nm功率500mW的激光二极管(LD)激发下观察到上转换发光。强度最大的发射峰位于547和558nm,发射光谱分析表明,上转换激发过程与双光子步进吸收有关。     

Fundamental studies on a planar-cathode direct current glow discharge. Part I: characterization via laser scattering techniques

A laser-scattering-based instrument was used to study an argon d.c. planar-diode glow discharge. The gas-kinetic temperature (T_g) was determined via Rayleigh scattering and the electron number density (n_e), electron temperature (T_e), and shape of the electron energy-distribution function were determined by Thomson scattering. Axial profiles of these parameters were obtained as the discharge current, voltage, and pressure were varied. Trends in the profiles of T_g and in the other parameters show the interdependence of these plasma species and properties. The results will be compared with current theoretical computer models in order to improve our understanding of the fundamental processes in glow discharges sustained under conditions appropriate for spectrochemical analysis.     

Real-time monitoring of Yb vapor density using an extended cavity violet diode laser

Based on laser absorption spectroscopy (LAS), we developed a vapor density monitor for controlling the vaporization rate of Yb using a tunable diode laser. The laser source consisted of an extended cavity violet diode laser which has an emission wavelength of 398.8 nm coincident with the Yb absorption transition line, 6s(2) 1S(0)-6s6p 1P(1). The light emitted from the diode laser was transmitted across an atomic vapor column generated by heating the Yb metal, while the laser frequency was scanned across the atomic transition line. By comparing the amount of incident light to the amount of light transmitted after the light passed through the vapor column, the vapor density was determined using the Beer's law. From the experimental results, we demonstrated that the diode-laser-based LAS operated successfully for the real-time monitoring of the Yb vapor density.     

An apparatus for measuring Thomson scattering from an inductively coupled plasma

An experimental arrangement is presented which has been proven capable of measuring Thomson scattering from an inductively coupled plasma (ICP). This system has been shown to reject stray light to a sufficient extent that useful scattering signals can be measured as near as 0.3 nm from the incident laser wavelength. In this paper, design considerations are discussed and a viable experimental system is described. Preliminary Thomson scattering results are presented.