Fast Imaging of Laser-induced Plasma Emission from a ZnO Target

The spatio-temporal evolution of plasma plume laser ablation zinc oxide target was investigated by ICCD camera fast imaging. The plasma was created by a KrF excimer laser of 248 nm wavelength and 25 ns pulse. The laser fluence was set at 2 J/cm². This study was performed under vacuum and oxygen atmosphere at a pressure range of 10^− 6 to 10 mbar.Free expansion, splitting and stopping of the plume were observed at different pressures and time delays following the laser pulse. Moreover, depending on the gas pressures, the photography shows some turbulence for given time delays in the front edge of the plasma while at 5 and 10 mbar the whole plasma edge is perturbed. Rayleigh–Taylor instability is proposed as an explanation to this observed effect. A time integrated emission spectroscopy diagnostic has been also used to identify plasma species. A plasma emission spectrum shows the presence of Zn⁺, Zn and O emission lines both in vacuum and in O₂ atmosphere. As the distance from the target surface increases the Zn⁺ emission line disappears.     

Comparison in the analytical performance between krypton and argon glow discharge plasmas as the excitation source for atomic emission spectrometry

The emission characteristics of ionic lines of nickel, cobalt, and vanadium were investigated when argon or krypton was employed as the plasma gas in glow discharge optical emission spectrometry. A dc Grimm-style lamp was employed as the excitation source. Detection limits of the ionic lines in each iron-matrix alloy sample were compared between the krypton and the argon plasmas. Particular intense ionic lines were observed in the emission spectra as a function of the discharge gas (krypton or argon), such as the Co II 258.033 nm for krypton and the Co II 231.707 nm for argon. The explanation for this is that collisions with the plasma gases dominantly populate particular excited levels of cobalt ion, which can receive the internal energy from each gas ion selectively, for example, the 3d⁷4p ³G₅ (6.0201 eV) for krypton and the 3d⁷4p ³G₄ (8.0779 eV) for argon. In the determination of nickel as well as cobalt in iron-matrix samples, more sensitive ionic lines could be found in the krypton plasma rather than the argon plasma. Detection limits in the krypton plasma were 0.0039 mass% Ni for the Ni II 230.299-nm line and 0.002 mass% Co for the Co II 258.033-nm line. However, in the determination of vanadium, the argon plasma had better analytical performance, giving a detection limit of 0.0023 mass% V for the V II 309.310-nm line.     

Atomemissionsspektroskopie mit laseranregung an graphit unter erhöhtem Druck—I. Plasmaeigenschaften

The paper describes the vaporization of graphite, the plasma form, the plasma properties and the interaction of laser radiation with the plasma for when the beam of a free running laser is focused on graphite in air at atmospheric pressure and in He, O₂, or Ar at pressures in a range from (1–40) × 10⁵ Pa.The power densities of the focused laser radiation (focal distances of condenser lens 150 to 50 mm) reach 10⁷$\text{W}\text{cm}{}_2$. The extent of vaporization depends on the pulse energy (under 40 × 10⁵ Pa not unambiguously, however) and the ambient pressure.The plasma is observed with normal, framing (to 10⁷ frames per second), and streak photography (to 25 $\text{mm}\text{μs}$). The plasma form depends, among other things, on the ambient gas, the power density of the laser pulse on the sample, and the ambient pressure. Increasing the pressure in the range from 25 × 10⁵ to 40 × 10⁵ Pa modifies the plasma form from a “plume” to a “fungus” with toroidal head. The average velocity of the plasma particles in air is about 3 × 10⁵$\text{cm}\text{s}$, which is 10 times higher than the velocity of the luminous plasma front. Under increased pressure a strong interaction between the laser radiation and the plasma takes place resulting in absorption by inverse bremsstrahlung (>50% in 40 × 10⁵ Pa He). In this way the luminosity of the plasma is enhanced by a factor of 100 (delay to the start of the laser pulse about 50 μs). This absorption of the laser beam by inverse bremsstrahlung becomes the dominating mechanism of excitation of the plasma and causes a strong temperature gradient from the point of absorption to the fringe.     

Comparison Between Two Sample Emulsification Procedures for the Determination of Mo, Cr, V and Ti in Diesel and Fuel Oil by ICP OES Along with Factorial Design

Inductively coupled plasma optical emission spectrometry (ICP OES) was used to determine Mo, Cr, V and Ti, in diesel and in used fuel oil. Samples were introduced into the ICP as emulsions to reduce interferences and allow the use of inorganic standards for quantification. A comparative study between one Triton X-100 emulsion and one detergentless emulsion was made. A 2³ factorial design was applied to elucidate and establish the relationship between three experimental variables: presence of HNO₃, amount of diesel fuel oil (between 5 and 25%) and the presence or O₂ into the Ar plasma gas flow rate. Results indicated that best performance were achieved using 10% sample (w/w) together with concentrated HNO₃ (0.5 mL) and using O₂ as auxiliary gas (0.047 L min⁻¹). The use of O₂ minimized both carbon deposits at the injector tip and plasma background. The addition of HNO₃ resulted in good correlation between inorganic standards used for calibration, and metallo-organic standards used for sample enrichment. Analyte enriched diesel and SRM 1634b were analyzed using the optimized conditions. Recoveries from 90.1 to 106.5% were achieved, with better results for detergent emulsions which enabled limits of detection at the ng g⁻¹ range for Mo, Cr, V and Ti and at smaller background.     

Study of nitrogen isotope-selective two-stage IR + UV dissociation of ammonia molecules

The one-photon IR excitation and subsequent UV dissociation of ammonia molecules selective with respect to nitrogen isotopes were studied. The selectivity of vibrational excitation is achieved by tuning CO₂ laser radiation to resonance with ¹⁴NH₃ or ¹⁵NH₃ molecules. The dependences of the yield of dissociation for each isotopic component and the selectivity on the buffer gas (N₂, O₂, Ar) pressure, the partial pressure of ammonia, and the time of delay between IR and UV laser pulses were established. At low pressures (67–270 Pa) of the isotopic mixture with a ¹⁵N concentration of 4.8%, the dissociation selectivity for ¹⁵N was 17. The mechanisms responsible for the selectivity of IR + UV-initiated dissociation are discussed. The phenomenological model has been developed that takes into consideration the effect of the interisotopic V-V exchange and V-T relaxation on the formation of the yield and selectivity of the two-stage IR+UV dissociation of ammonia.     

Line broadening mechanisms in the low pressure laser-induced plasma

The spectral profiles of Ca and Rb lines have been studied in a laser induced plasma as a function of pressure (1–10 torr) and delay time with respect to the plasma initiation (1–10 μs). Measurements were made in a plasma induced by the 1064-nm output of a Nd:YAG laser on a calcium carbonate matrix, doped with Rb. Spectral profiles were measured in absorption using a narrow-band cw Ti:Sapphire laser. It was shown that in the case of a trace element (Rb in a CaCO₃ matrix), the broadening mechanism was Doppler-dominant, whereas for a major matrix component (Ca), resonance broadening was the main contributor to the line shape. The plasma was shown to be non-equilibrium provided by the difference between the kinetic (3000 K) and the excitation (8000 K) temperatures. The electron number density at delay times of 5–10 μs and pressures of 1–10 torr was estimated not to exceed 10¹⁵ cm⁻³. The number densities of Ca atoms in the ground and the excited (23 652 cm⁻¹) states were evaluated by measuring line width and peak absorption at 732.6 nm. They were found to be in the range of (1.5–2.2)×10¹⁷ cm⁻³ for the ground state and (1.5–33)×10¹¹ cm⁻³ for the excited state.     

Time-resolved optical emission spectroscopic measurements of He plasma induced by a high-power CO2 pulsed laser

Laser-induced breakdown spectroscopy of helium plasma, initially at room temperature and pressures ranging from 12 to 101 kPa was investigated using a transverse excitation atmospheric CO₂ pulsed laser (λ = 9.621 and 10.591 μm, a full width at half maximum of 64 ns, and an intensity from 1.5 to 5.36 GW cm⁻²). The helium breakdown spectrum is mainly due to electronic relaxation of excited He, He⁺ and H. Plasma characteristics were examined in detail on the emission lines of He and He⁺ by the time-integrated and time-resolved optical emission spectroscopy technique. Optical breakdown threshold intensities, ionization degree and plasma temperatures were obtained. An auxiliary metal mesh target was used to analyze the temporal evolution of the species in the plasma. The results show a faster decay of the continuum emission and He⁺ species than in the case of neutral He atoms. The velocity and kinetic energy distributions for He and He⁺ species were obtained from time-of-flight measurements. Electron density in the laser-induced plasma was estimated from the analysis of spectral data at various times from the laser pulse incidence. Temporal evolution of electron density has been used for the estimation of the three-body electron-ion recombination rate constant.     

Cooperative photoproduction of Xe2+Cl− in liquid Cl2/Xe solutions: Stimulated emission and gain measurements

Cooperative photogeneration of Xe₂⁺Cl⁻ can be accomplished by near UV laser excitation of Cl₂/Xe solutions with a high quantum efficiency. Gain measurements on the Xe₂⁺Cl⁻ (4²Γ) transition are reported. The liquid phase rare gas halides should be regarded as a family of ideal “dye” lasers.     

Resonantly laser induced plasmas in gases: The role of energy pooling and exothermic collisions in plasma breakdown and heating

The present work is a systematic experimental study of the plasma formation in cesium vapor induced by a continuous laser tuned to the resonance transition 6S_1/2–6P_3/2. Taking into account the measured absolute population densities of Cs ground and excited state atoms as well as the electron densities derived from Stark broadening of the Cs lines, complete local thermodynamic equilibrium in the laser-produced plasma was found for laser power densities ≈ 10 Wcm^− 2 at cesium ground state number densities of about 10¹⁷ cm^− 3. Direct conversion of the excitation energy or parts of the excitation energy in exothermic collisions of laser-excited atoms is concluded to be the major process for atomic vapor heating and subsequent formation of LTE plasmas.     

Characterization of laser induced breakdown plasmas used for measurements of arsenic,antimony and selenium hydrides

Studies have been performed to characterize laser induced breakdown spectroscopy (LIBS) plasmas formed in Ar/H₂ gas mixtures that are used for hydride generation (HG) LIBS measurements of arsenic (As), antimony (Sb) and selenium (Se) hydrides. The plasma electron density and plasma excitation temperature have been determined through hydrogen, argon and arsenic emission measurements. The electron density ranges from 4.5 × 10¹⁷ to 8.3 × 10¹⁵ cm^?3 over time delays of 0.2 to 15 μs. The plasma temperatures range from 8800 to 7700 K for Ar and from 8800 to 6500 K for As in the HG LIBS plasmas. Evaluation of the plasma properties leads to the conclusion that partial local thermodynamic equilibrium conditions are present in the HG LIBS plasmas. Comparison measurements in LIBS plasmas formed in Ar gas only indicate that the temperatures are similar in both plasmas. However it is also observed that the electron density is higher in the Ar only plasmas and that the emission intensities of Ar are higher and decay more slowly in the Ar only plasmas. These differences are attributed to the presence of H₂ which has a higher thermal conductivity and provides additional dissociation, excitation and ionization processes in the HG LIBS plasma environment. Based on the observed results, it is anticipated that changes to the HG conditions that change the amount of H₂ in the plasma will have a significant effect on analyte emission in the HG LIBS plasmas that is independent of changes in the HG efficiency. The HG LIBS plasmas have been evaluated for measurements of elements hydrides using a constant set of HG LIBS plasma conditions. Linear responses are observed and limits of detection of 0.7, 0.2 and 0.6 mg/L are reported for As, Sb and Se, respectively.     

Optical emission spectroscopy and modeling of plasma produced by laser ablation of titanium oxides

In the present study, the time evolution of electron number density, of electron, atom and ion temperatures, of plasma produced by KrF excimer laser ablation of titanium dioxide and monoxide targets, are investigated by temporally and spatially resolved optical emission spectroscopy over a wide range of laser fluence from 1.7 to 6 J cm⁻², oxygen pressures of 10⁻²–10⁻¹ torr and in a vacuum. A state-to-state collisional radiative model is proposed for the first time to interpret the experimental results at a distance of 0.6 mm from the target surface, in vacuum and for a time delay from 100 to 300 ns from the beginning of the laser pulse. In particular, we concentrate our attention on problems concerning the existence of the local thermodynamic conditions in the laser-induced plasma and deviation from them, as observed in our experiment. The numerical model proposed for calculating the electron number density and the population densities of atoms and ions in excited states give good quantitative agreement with the experimental results of the optical emission spectroscopy measurements.     

Laser isotope separation and the multiphoton dissociation of formic acid using a pulsed HF laser

The focussed beam from a single line [P₂ (5)] of a pulsed HF laser has been used to stimulate the decomposition of formic acid. The yield (Y is the number of product molecules per pulse / formic acid pressure) of the non-condensable (77 K) products, hydrogen and CO, has been studied as a function of laser radiant energy (from 25-115 mJ) and pressure (from 0.4-2.7 kPa). The intensity dependence of Y suggests that each dissociating formic acid requires the equivalent of at least 6 HF P₂(5) photons (260 kJ/mole). For pressures above about 0.6 kPa, Y_H2 = (?0.6 ± 1.7) × 10¹² + (2.4 ± 1.0) × 10¹² P and Y_CO = (?0.5 ± 6.1) × 10¹³ × (8.7 ± 3.7) × 10¹³ P. The linear dependerrce of yields indicates that a collisionally assisted decomposition process is important at these pressures. The efficiency of the conversion of photon energy to reaction products at a pressure of 2.7 kPa is ? 7% for CO and ? 0.2% for hydrogen. Selective excitation of HCOOH in equimolar mixtures of HCOOH/HCOOD, at a total pressure of 0.6 kPa, has provided a physically separated product, hydrogen gas, which is isotopically enriched in H versus D 25 fold as compared to the formic acid mixture. The degree of enrichment decreases as the total pressure of the mixture is increased. A possible mechanism accounting for isotope enrichment and the collisionally assisted dissociation is outlined.     

Chlorine isotope enrichment in CO2 TEA laser photolysis of CF2Cl2

A number of lines from a CO₂ TEA laser were used to photolyze CF₂Cl₂. Enrichment of the ³⁵Cl and ³⁷Cl isotopes in the molecular chlorine formed during the photolysis was observed using a mass spectrometer. Maximum enrichment was about 1.8. The dependence of enrichment on wavelength, reactant concentration, inert gas pressure, and the presence of SiF₄ is reported. Of particular interest is the persistence of significant enrichment at pressures up to several hundred torr (≈ 10⁵ Pa). Aside from the practical significance of this enrichment at high pressures, it suggests that there are important contributions from isotopically specific interactions after the laser pulse.     

Sub-picosecond laser ionization of free C60 and C70 at 248 nm

Delayed ionization is found to be absent for sub-picosecond laser excitation of free C₆₀ and C₇₀ at 248 nm. The autocorrelation trace obtained for C ₆₀ ⁺ in a laser time-of-flight (TOF) mass spectrometer using two time-delayed and collinear 248 nm ultrashort laser pulses has a width of 1.1 ps (715 fs for sech² pulses), in agreement with the laser pulse duration measurement in NO gas. Both above observations can be explained by direct ionization of C₆₀ via coherent two-photon absorption by the high intensity sub-picosecond 248 nm laser excitation avoiding the channel leading to delayed ionization.     

Spectroscopic and analytical characteristics of an inductively coupled argon plasma combined with hydride generation with or without simultaneous introduction of the sample aerosol for optical emission spectrometry

A radially viewed inductively coupled argon plasma was used for optical emission spectrometry of volatile species formed by reaction with NaBH₄ (hydride generation). The volatile hydrides were either introduced into the plasma alone or at the same time as a sample aerosol generated by pneumatic nebulization with a commercially available Concomitant Metals Analyzer. The effects of the forward power, the presence of pre-reducing agents [(NH₂)₂SC, KI, KBr and hot HCl], the occurrence of easily ionized elements (Ca, K, Mg and Na) in the analyte solutions on the excitation temperature (as measured via Ar atomic lines) and the electron number density were investigated for both of the sample introduction modes applied. The detection limits and the signal-to-background intensity ratios for As, Bi, Sb, Se and Sn lines were also evalutated and were observed to deteriorate with increasing power. When simultaneous hydride generation and pneumatic nebulization was employed under optimized experimental conditions, detection limits of 3.5, 2.9, 4.3, 1.5 and 2.1 μg L⁻¹ for As, Bi, Sb, Se and Sn, respectively, were obtained, and the intensities of the analytical lines for elements that do not form volatile hydrides were found to be 40% (Cd), 30% (Ni), 20% (Co, Cr, Fe, Mn and Zn) and 10% (Cu, Mg, V) greater than those obtained when only pneumatic nebulization was used.     

Spectroscopic characterization of a microplasma used as ionization source for ion mobility spectrometry

We report a miniaturized excitation source for soft ionization of molecules based on a dielectric barrier discharge. An atmospheric plasma is established at the end of a 500 μm diameter capillary using He as buffer gas. The plasma jet which comes out of the capillary is dependent on the gas flow rate. The mechanism of the production of N₂⁺ outside the capillary, which is relevant for the protonation of molecules and sustains the production of primary ions, is investigated by spatially resolved spectroscopic measurements throughout the plasma. Possible application of such miniaturized plasmas is the ionization of gaseous compounds under atmospheric pressure as an alternative to traditional APCI (atmospheric pressure chemical ionization). The miniaturized plasma was applied as ionization source for ion mobility spectrometry where the common sources are radioactive, thus limiting the place of installation. First measurements of gaseous compounds with such a plasma ion mobility spectrometer with promising results showed detection limits comparable or even better than those obtained using common radioactive ionization sources.     

Experimental lifetime of the metastable 5D 3/2 state in Ba+

A discrepancy exists between theoretical and experimental lifetimes of the metastable 5D _3/2 state in Ba⁺. In order to redertermine that lifetime, we probe the population of the metastable 5D _3/2 state of a Ba⁺ ion cloud stored in a Paul ion trap in the presence of He buffer gas as the function of time delay after pulsed laser excitation of this state. The measured decay rates at different buffer gas pressures are extrapolated to zero pressure and we obtain a radiative decay time of 48.0±5.9 s. This is not in agreement with theoretical predictions of about 80 s, but reduces the discrepancy from a previously reportet experimental value of 17.5 s. If the possibility of finestructure mixing to an adjacent 5D _5/2 level by collisions is considered our value represents a lower limit of the radiative lifetime for the 5D _3/2 state.     

CO2 laser excitation of triethylsilane: Time resolved luminescence of diethylsilyl radical

The infrared multiphoton excitation of triethylsilane in the gas phase, with a pulsed CO₂ laser at high intensities (I > 700 MW/cm²), produced an intense luminescence. The spectrum and time profile of this luminescence was studied as a function of pressure, and laser frequency. The radiative lifetime of this emission was 357 ± 10 ns, and the quenching rates by Cl₂ and NO were determined from lifetime measurements. A reasonable mechanism for the interpretation of this luminescence involves the initial infrared multiphoton decomposition of triethylsilane, followed by the secondary infrared multiphoton excitation of the primary photofragment diethylsilyl radical, which subsequently undergoes relaxation to an excited electronic state. The addition of O₂ resulted in a new chemiluminescence at shorter wavelengths, which corresponds to the SiO* chromophore group. © 1994 John Wiley & Sons, Inc.     

Analytical applications of laser moding caused by intracavity absorption

A continuous CO₂ laser with a reflecting mirror can operate at several wavelengths simultaneously. If an organic vapor is introduced into a separate cavity in the laser optical path, the laser will sometimes mode rapidly causing some lasing lines to diminish to zero and others to become enhanced; this has been observed even for very low amounts (10^-5 g) of organic gases. Laser intercavity absorption spectroscopy depends on overlap of a vibrational—rotational line of a sample with a laser transition line. The absorption by the sample greatly affects the laser wavetrain at that particular wavelength and interferes with the lasing action. The technique is not based on Beer's law, and the detection limits observed are orders of magnitude better than those of conventional infrared absorption spectroscopy. Two laser systems were used and various organic gases were studied. When a totally reflecting mirror which permitted free moding was used, the detection limits found were 0.14 μg, 0.95 μg and 0.60 μg for vinyl chloride, propylene, and ethylene, respectively. When a grating was used as the rear cavity optics restricting the wavelengths of the laser lines, the detection limits were 140 μg, 94 μg, 63 μg and 0.26 μg for vinyl chloride, propylene. ethylene and ethyl chloride, respectively.