Time-resolved measurement of copper emission spectrum excited by a low-pressure argon laser-induced plasma.

A laser-induced plasma generated with a pulsed Nd:YAG laser under evacuated conditions has complicated structures both temporally and spatially. The time-resolved spectra of copper in three different wavelength regions were observed in detail for elucidating the excitation mechanisms of many atomic/ionic copper emission lines. The emission intensities of copper emission lines, measured in a time-resolved mode, were strongly dependent on the kind of copper lines: ionic or atomic lines, and their excitation energies. Generally, copper ionic lines were rapidly decayed and dominantly emitted from the initial breakdown zone, because the copper ions requiring larger excitation energies were produced mainly in the hot breakdown zone. On the other hand, the atomic lines were emitted during prolonged periods, implying that they could also be excited in the expanded plasma zone. The excitation phenomena occurring in the laser-induced plasma could be better understood by analyzing the time-resolved copper spectra.     

Emission characteristics of nickel ionic lines excited by reduced-pressure laser-induced plasmas using argon, krypton, nitrogen, and air as the plasma gas

The emission characteristics of nickel ionic lines in low-pressure laser-induced plasmas are investigated when argon, krypton, nitrogen, or air gas was employed as the plasma gas. The spectrum patterns and the relative intensities of the ionic lines are measured with and without a blind cylinder surrounding the sample surface to separate the detected emission area into two portions roughly: an initial breakdown zone and an expansion zone of the plasma. Their emission intensities are strongly dependent on both the kind and the pressure of the plasma gas. Different major ionic lines are observed in the argon and the krypton plasmas: for example, the Ni II 230.010-nm line (8.25 eV) for argon and the Ni II 231.604-nm line (6.39 eV) for krypton. The excitation mechanism of these ionic lines is considered to be a resonance charge-transfer collision with argon or krypton ion due to good energy matching to the corresponding energy levels of nickel ion. These ionic lines measured with the blind cylinder at reduced pressures of around 1300 Pa give the largest signal-to-background ratios; therefore, the analytical application under such optimum plasma conditions is recommended.     

Measurement of excited state level populations for atomic and ionic iron in the inductively coupled plasma

Excited state level populations have been measured for FeI and FeII species in the ICP discharge. The measurement was accomplished by using a 4096 pixel linear photodiode array spectrometer to detect emission intensities from 22 atomic and 26 ionic lines. Relative level populations were generated from these measurements and their variation with respect to rf input power and spatial position investigated. The results show that excited state populations for atom levels are non-linear with respect to excitation energy, whereas a linear relationship is found for ion levels. These results are discussed in terms of a p-LTE model.     

Characterization of the ion beam inside the skimmer cone of an inductively coupled plasma mass spectrometer by laser excited atomic and ionic fluorescence

Laser-induced atomic and ionic fluorescence have been used to characterize the material extracted from an inductively coupled plasma through a differentially pumped interface of the type used in inductively coupled plasma mass spectrometry. Measurements were made in the 8 mm downstream from the tip of the skimmer cone. Reference measurements were also made outside the interface at the tip of the sampling cone. Sc and Ba ions and Pb atoms were used as test analytes. Sc⁺ densities drop more rapidly than either Ba⁺ or Pb densities. The transmission efficiencies of both Sc⁺ and Ba⁺ are suppressed by the addition of either Mg or Pb to the analyte solution. The effects of the two matrix additions are approximately the same. Based on the magnitude of the fluorescence signal for Ba⁺, a lower limit of 0.3% for the transmission of Ba⁺ from the plasma into the second vacuum stage has been calculated.     

Production of vibrationally excited hydrogen molecules by atom recombination on Cu and W materials

We have measured vibrational population of H(2) and D(2) molecules produced by atom (H or D) recombination on tungsten and copper material. The vibrational spectroscopy, based on the properties of dissociative electron attachment to hydrogen molecule, was used. The vibrationally excited molecules were produced by atom recombination in a cell where the studied sample is exposed to hydrogen atoms, from hot tungsten filament. Vibrational populations were obtained for the studied materials, which can be well described by the Boltzmann distribution, with specific vibrational temperatures for each material. The experimentally obtained vibrational populations for copper approximately agree with the theoretical predictions, whereas the experimentally obtained vibrational temperature for tungsten is higher and thus showing a considerable overpopulation of highly excited vibrational states than predicted. We propose that the origin of this higher excitation is related to the existence of high hydrogen surface coverage on tungsten, where hydrogen is occupying binding sites with different desorption energies. In order to obtain an insight into the recombination mechanism with more than one binding site per unit cell, a Monte Carlo simulation was performed, where it was assumed that the main production of molecules proceeds through the hot-atom recombination with an adsorbed atom. The results show that the recombination proceeds mainly through the weak binding sites, once they are occupied.     

Contribution of the charge-transfer process to the excitation mechanisms in inductively coupled plasma atomic emission spectroscopy

Among the various possible processes in an argon ICP, the charge-transfer process from argon ions has been studied by following the behavior of ionic lines with a total energy near the ionization energy of argon. This behavior has been studied along the central channel of the plasma. Changes of line-intensity behavior above and below 16 eV cannot be explained by temperature variations. Addition of foreign gases (nitrogen and hydrogen) can modify the charge-transfer process.     

Character of the electronic ground state and of charge-transfer excited states in ionic solids: An ab initio cluster model approach

The electronic structure of the ground electronic state and of some special charge-transfer excited states in ionic solids is examined from the ab initio cluster model approach. Different ab initio wave functions, including a frozen orbital approach, the Hartree–Fock self-consistent field, and multireference configuration interaction wave functions, are considered and analyzed using different theoretical techniques. We explicitly consider some alkaline–earth oxides such as CaO, a more difficult case such as A1₂O₃, a transition-metal oxide such as NiO, and a system with a more complicated structure such as KNiF₃. Analysis of ab initio wave functions in terms of valence bond components shows that all these compounds are largely ionic, thus supporting the simple picture arising from the ionic model. However, the nature of the excited states is more complex. Alkaline–earth oxides lowest excited states are essentially described as charge-transfer excitations dominated by a single resonant valence bond structure and the calculated energy difference is comparable to the experimental optical gap. In the case of A1₂O₃, the electronic spectra presents excitonic features and the local charge-transfer excitation excited states provide a reasonable representation of these phenomena. Finally, several different valence bond structures are present in the lowest electronic states of KNiF₃. © 1994 John Wiley & Sons, Inc.     

Rotational effect in the ionization of a highly excited atom by collision with a polar molecule

If the collisional ionization is chiefly due to energy transfer from the polar-molecule rotation to an electron in a high Rydberg state of the atom, then theory predicts that the cross section averaged over a thermal distribution of rotational states should show step-like structure as a function of the energy of the Rydberg state. This structure has been experimentally detected, and it can be considered as direct evidence of the rotational effect in the collisional ionization.     

Laser excited atomic fluorescence for studies of enhancement effects in the direct current plasma

This paper reports a preliminary use of laser excited atomic fluorescence spectrometry (LEAFS) to study analyte population enhancement caused by easily ionized elements (EIEs) in the direct current plasma (DCP). Spatial atom density profiles in the DCP were obtained using resonance fluorescence at the calcium atom line at 422.7 nm, with and without the addition of an EIE. Variations in atom density caused by an EIE were found to be far too small to account for the marked enhancements of atomic emission signals which are caused by EIEs.Direct line fluorescence of the barium ion, excited at 614 nm and detected at 455 nm, was used to probe the effect of an EIE on excited state populations. Measurements in the analytical region of the plasma close to the core revealed that enhancements of fluorescence signals at low laser powers disappeared at laser powers which were sufficient to saturate the atomic transitions. While this result does not clarify any of the mechanisms of excitation in the DCP, it does lend support to two of the fundamental postulates of a recent model of the spectrochemical excitation processes in the DCP. These are first, that the analytical region of the DCP is not in local thermodynamic equilibrium (LTE) and second, that EIE enhancement proceeds by modulating the rates of power distribution among various plasma zones.In the outer zone of the analytical region of the DCP, depressive interferences occurred. These did not disappear upon saturation which indicates that they were not rate effects but effects that resulted from atom density changes.     

Nature and location of excited charge-transfer states in the porphine-magnesium porphine dimer

The ground and excited states of a cofacial porphine-magnesium porphine dimer with a ring separation of 5.35 Å are investigated by ab initio configuration interaction calculations, using a floating gaussian basis. A pair of charge-transfer states are found ≈23000 cm^?1 above the ground state, but are lowered by ≈7400 cm^?1 upon coordination of the Mg atom with chloride ion.     

Double resonance laser-induced fluorescence of Cadmium and Zinc in the inductively coupled plasma and electrothermal atomizer

Double resonance laser-induced fluorescence (LIF) spectrometry studies of Zn and Cd have been performed in the inductively coupled plasma (ICP) and electrothermal atomizer (ETA). Stepwise excitation of Zn is accomplished at 213.856 nm and 636.235 nm with fluorescence emissions observed near 334.5 nm. Excitation of Cd is accomplished at 228.802 nm and 643.847 nm with fluorescence emissions observed near 361.1 nm and 347.6 nm. Calibration studies demonstrate that the double resonance LIF approaches provide high sensitivity and linearity over several orders of magnitude in both atomizers. Limits of detection for Zn and Cd in the ICP are 1.7 ng/ml and 5 ng/ml, respectively. Excellent sensitivity is observed in the ETA resulting in limits of detection of 70 pg/ml (700 fg absolute mass) and 4 pg/ml (40 fg absolute mass) for Zn and Cd, respectively. The Zn content of a bovine serum standard reference material (NIST SRM #1598) has been determined by ETA-LIF and found to be 940±60 ng/g, which is in very good agreement with the certified value of 890±60 ng/g. Low-level ETA-LIF measurements of Zn in these studies are strongly limited by the high background level observed for this element.     

Optical detection of laser-induced ionization in the inductively coupled plasma for the study of ion-electron recombination and ionization equilibrium

The recombination kinetics between the strontium ions and the electrons have been studied in an inductively coupled plasma. Two excimer lasers have been used to pump two dye lasers, which were made spatially and temporally coincident in the plasma region investigated. With the first laser system the strontium atoms were photoionized and the second laser was then used to probe the ions formed by measuring the resulting ionic fluorescence. Since the second laser was delayed by external triggering with respect to the ionizing laser, the temporal fate of the ions could be continuously monitored. The recombination time constant was found to be 15.5 μs, indicating the absence of fast ion chemistry (in the tens of ns range) which was observed in an air-acetylene flame. Moreover, it was found that: (i) the addition of an easily ionizable element (K, Li) increased the rate of recombination; and (ii) the recombination time constant was directly proportional to the ion/atom ratio of strontium. It was concluded that a change in the electron energy distribution is more relevant to the rate of recombination than changes in the absolute number density of electrons.     

Photophoresis of small-size particles in a gas excited by resonance radiation

The motion of small aerosol particles in uniform gas excited by a resonance radiation is caused by the specific mechanism. In the case of a small deviation of the laser radiation frequency from that of the electronic or the vibration-rotation transition, due to the Doppler effect, only the molecules whose velocity projection on the radiation propagation direction lies within a certain velocity range can be excited. For this reason, the relationship between the excited and the nonexcited molecules in the aforesaid velocity range differs from the corresponding relationship in the velocity range, which is symmetrical with regard to the center of the Maxwell distribution. A resultant force, which is applied by the gas to aerosol particles, arises due to the different scatterings of the excited and nonexcited molecules by the surface of aerosol particles.     

Simulation of atomic and ionic absorption and emission spectra for thermal plasma diagnostics: application to a volatilisation study in a plasma jet

Simulation of atomic emission and absorption spectra has been carried out in order to improve and to check the validity of measurement techniques and data processing for plasma diagnostic. Doppler, Van der Waals, Stark and instrumental broadenings and shifts have been taken into account. The effect of self-absorption has also been considered. The input parameters are pressure, temperature, electron density, and emitting species number density. In such calculations data needed for the determination of collisional and Stark broadenings are not always available. An empirical method of data evaluation is described and discussed. The limits and the precision of this modelling are discussed with experimental examples. This simulation method has been applied to the diagnostic of volatilisation phenomena in thermal plasma spraying processes. It has been used to assist in the choice of diagnostic lines, instrumental parameters and data processing parameters. It has also been used to evaluate the accuracy of the temperature and density distributions calculated from spectroscopic measurements.     

Detection of boron nitride radicals by emission spectroscopy in a laser-induced plasma

Several vibrational bands of boron nitride radicals have been observed in a plasma produced by pulsed-laser ablation of a boron nitride target in low-pressure nitrogen or argon atmospheres. Using time- and space-resolved emission spectroscopic measurements with a high dynamic range, the most abundant isotopic species B¹¹N have been detected. The emission bands in the spectral range from 340 to 380 nm belong to the Δυ =−1, 0, +1 sequences of the triplet system (transition A³Π–X³Π). For positive identification, the molecular emission bands have been compared with synthetic spectra obtained by computer simulations. Furthermore, B¹⁰N emission bands have been reproduced by computer simulation using molecular constants which have been deduced from the B¹¹N constants. Nevertheless, the presence of the lower abundant isotopic radical B¹⁰N was not proved due the noise level which masked the low emission intensity of the B¹⁰N band heads.     

An operational pH in aqueous dimethylsulfoxide based upon the acidity dependence of the rate of a simple ionic recombination reaction in the lowest excited singlet state

7-Hydroxy-1-naphthalenesulfonate (2-naphthol-8-sulfonate) monoanion demonstrates proton transfer in the lowest excited singlet state in DMSO-water mixtures as well as in pure water. The dissociation reaction of the directly excited monoanion is strongly solvent-dependent, and independent of solution acidity. The reprotonation of the conjugate base, however, depends predominantly on the acidity of the solution and only on the continuum properties of the solvent. The separability of the dissociation and reprotonation reactions, using steady-state methodology, allows the reprotonation to be treated independent of the dissociation. The linear relationship between the ratio of the relative fluorescence efficiencies of acid and conjugate base, and the hydrogen ion concentration is obtained only if proper Br?nsted activity factors are included in the relationship. These factors can be calculated from classical electrostatics and are the fourth powers of the activity coefficients necessary to convert the formal hydrogen ion concentration to hydrogen ion activity. Using this approach, pH was calculated from hydrogen ion concentration in DMSO-water solutions, containing a mole fraction of DMSO up to about 0.4.     

Emission efficiency for particulate forms of iron and aluminum in rain-water measured by inductively-coupled plasma atomic emission spectrometry

The bulk emission efficiency for particulate forms of iron and aluminum in rain-water was evaluated. The efficiencies for iron and aluminum were >80% for particles 0.4–1 μm in diameter and about 17% for particles >8 μm; relative standard deviations were <20%. A procedure is described by which the concentrations of particulate forms of iron and aluminum in rain-water are determined. Fluctuation of particles in rain-water during a rainfall event can be monitored in detail.