A novel double-pulse laser plasma spectroscopic technique for H analysis in metal samples utilizing transversely excited atmospheric-pressure CO2 laser-induced metastable He atoms

The analysis of hydrogen in a metal sample (zircaloy-4), which is usually difficult to perform using conventional laser-induced breakdown spectroscopy (LIBS) techniques, has been achieved using a double-pulse technique under He gas at atmospheric pressure. In this technique, a transversely excited atmospheric-pressure (TEA) CO₂ laser (1.5 J, 200 ns) was focused onto the metal surface to induce a strong He gas plasma whilst simultaneously focussing a Nd-doped yttrium aluminum garnet (Nd:YAG) laser (120 mJ, 8 ns), synchronized with the TEA CO2 laser, onto the metal to ablate atoms into the resulting He gas plasma. The emission spectrum obtained shows a narrow H linewidth with low background intensity and long lifetime emission, thereby indicating that excitation takes place via metastable He atoms. The H emission from H₂O can be suppressed by a careful pretreatment involving heating the sample in a vacuum chamber.     

Hydrogen analysis in metal samples by selective detection method utilizing TEA CO2 laser-induced He gas plasma

When a Transversely Excited Atmospheric (TEA) CO₂ laser (energy of 1.5 J, pulse duration of 200 ns) was focused on a metal sample surface containing hydrogen (H) in He gas at 1 atm, a strong helium gas plasma was produced and only H atoms came out of the sample. The H atoms then moved into the helium gas plasma to be excited through meta-stable helium atoms. Using this technique, an excellent linear calibration curve with zero intercept was made using zircalloy-2 samples containing H (100–600 ppm), where the compensation method was made using an emission intensity of O I 777.1 nm in order to subtract the H emission intensity coming from unwanted H₂O. It should be emphasized that this technique has a possibility to realize highly sensitive analysis of H with a detection limit of less than 1 ppm because of its selective detection.     

Production of laser-heated plasma in high- pressure Ar gas and emission characteristics of vacuum ultraviolet radiation from Ar2 excimers

A new pump scheme for the realization of a practical Ar₂ excimer laser operating at 126 nm has been proposed and investigated experimentally. In this scheme, pre-ionized high-pressure Ar gas was excited by an intense transversely-excited atmospheric (TEA) CO₂ laser irradiation. A 100-mm-long line plasma was successfully produced at an argon gas pressure of 2 MPa. The vacuum ultraviolet emission at 126 nm from the Ar₂ excimers was observed and its emission characteristics were investigated under various experimental conditions. Received: 10 December 2001 / Published online: 14 March 2002     

Review of Laser-Induced Plasma,Its Mechanism,and Application to Quantitative Analysis of Hydrogen and Deuterium

Abstract

A comprehensive review of important progress achieved over the last 30 years regarding knowledge of laser-induced plasmas generated by CO₂ and Nd:YAG lasers in a variety of ambient gases is presented in this article, as well as research results on the extension of laser-induced breakdown spectroscopy (LIBS) for quantitative analysis of light elements, especially hydrogen and deuterium. First, the formation of shock wave–induced expanding secondary plasma in low-pressure ambient gases is discussed along with the dynamic characteristics of the secondary plasma expansion process. The unique advantages of low-pressure gas plasma are explained in relation to the successful detection of the sharp H and D emission lines. The experimental results using helium ambient gas are presented with emphasis on the role of He gas plasma in introducing an additional delayed excitation mechanism involving the helium metastable excited state, which resulted in the complete resolution of H and D emission lines, separated by only 0.18 nm. The development of a laser precleaning treatment and special double-pulse techniques further produced a linear calibration line with zero intercept applicable to quantitative H and D analyses of zircaloy sample, with either low- or high-pressure ambient He gas. More recent use of a transversely excited atmospheric (TEA) CO₂ laser in place of an Nd:YAG laser has demonstrated the much desired larger excited helium plasma and thereby resulted in significant emission enhancement and improved detection sensitivity.     

On the repetitive operation of a self-switched transversely excited atmosphere CO<Subscript>2</Subscript> laser

The repetition rate capability of self-switched transversely excited atmosphere (TEA) CO₂ laser was studied for different gas flow configurations. For an optimized gas flow configuration, repetitive operation was achieved at a much smaller gas replenishment factor between two successive pulses when compared with repetitive systems energized by conventional pulsers.     

Hydrogen and Deuterium Analysis Using Laser‐Induced Plasma Spectroscopy

Abstract

Hydrogen emission in laser plasma has been studied by focusing a TEA CO₂ laser and Nd‐YAG lasers on various types of samples, such as glass, quartz, and zircaloy pipes doped with hydrogen. It was found that H_α emission with a narrow spectral width occurs with high efficiency when the laser plasma is produced in low‐pressure host gas. In contrast, the conventional well‐known laser‐induced breakdown spectroscopy (LIBS), which operates at atmospheric air pressure, cannot be applied for the analysis of hydrogen as impurity. The specific characteristic of hydrogen emission in low‐pressure plasma is interpreted on the basis of our shock wave model, taking account of the fact that the hydrogen mass is extremely light compared to that of the host target. Another experimental study on gas analysis was conducted using an Nd‐YAG laser and helium host gas at atmospheric pressure on a sample of mixed water (H₂O) and heavy water (D₂O) in vapor form. It was shown that completely resolved hydrogen (H_α) and deuterium (D_α) emission lines that are separated by only 0.179 nm could be obtained at a properly delayed detection time when the charged particles responsible for the strong Stark broadening effect in the plasma have mostly disappeared. It is argued that a helium metastable excited state plays the important role in the hydrogen excitation process.     

Numerical solution of Boltzmann tranport equation for TEA CO2 laser having nitrogen-lean gas mixtures to predict laser characteristics and gas lifetime

Selective laser isotope separation by TEA CO₂ laser often needs short tail-free pulses. Using laser mixtures having very little nitrogen almost tail free laser pulses can be generated. The laser pulse characteristics and its gas lifetime is an important issue for long-term laser operation. Boltzmann transport equation is therefore solved numerically for TEA CO₂ laser gas mixtures having very little nitrogen to predict electron energy distribution function (EEDF). The distribution function is used to calculate various excitation and dissociation rate of CO₂ to predict laser pulse characteristics and laser gas lifetime, respectively.Laser rate equations have been solved with the calculated excitation rates for numerically evaluated discharge current and voltage profiles to calculate laser pulse shape. The calculated laser pulse shape and duration are in good agreement with the measured laser characteristics. The gas lifetime is estimated by integrating the equation governing the dissociation of CO₂. An experimental study of gas lifetime was carried out using quadrapole mass analyzer for such mixtures to estimate the O₂ being produced due to dissociation of CO₂ in the pulse discharge. The theoretically calculated O₂ concentration in the laser gas mixture matches with experimentally observed value. In the present TEA CO₂ laser system, for stable discharge the O₂ concentration should be below 0.2%.     

Theoretical study of hybrid TEA–CO2 lasers

Temporal and spatial analysis of dynamics of hybrid transversely excited atmospheric pressure (TEA)–CO₂ lasers is studied using two different models with four and eight energy levels. These models are used for simulation of the laser and computing the output energy. Effects of several parameters such as input energies and gas mixture concentrations (especially presence of CO molecules) are also studied.     

Short delay time UV preionized TEA CO2 laser operating with binary CO2/H2 and CO2/He gas mixtures

In order to obtain short tail-free output laser pulses from a TEA CO₂ laser, parametric study of the laser operation with CO₂/H₂ and CO₂/He binary gas mixtures containing high CO₂ concentrations was carried out. A small scale UV preionized short delay time TEA CO₂ laser was employed. In terms of the maximum extractable output pulse energy and power, the more conventional CO₂/He gas mixture was found to be inferior in comparison with the CO₂/H₂ mixture proposed here.     

CO2-laser-induced multiphoton absorption of CF2Cl2

Energy absorption by CF₂Cl₂ from an intense TEA CO₂ laser pulse is measured as a function of the pressure of CF₂Cl₂ and the pressure of Ar bath gas for different laser energy fluences. The fraction of the molecules excited by the laser field and the average energy of the molecular ensemble are determined by a simple experimental method.     

Surface modification of the Cr-based coatings by the pulsed TEA CO2 laser

The interaction of a transversely excited atmospheric (TEA) CO₂ laser with chromium oxynitride (CrON) coating deposited on a AISI 304 steel substrate was considered. The results have shown that CrON was surface-modified by the laser beam of 45 J/cm² energy density. The energy absorbed from the TEA CO₂ laser beam was partially converted into thermal energy, which has generated a series of effects such as melting, vaporization of the molten material, and shock waves in the vapor and in the solid. Morphological manifestations on the CrON coating surface can be summarized as follows: non-uniform features with ablation and appearance of crater-like form (central zone of interaction); appearance of three damaged areas and presence of hydrodynamic effects with resolidified droplets (periphery zone of interaction). In case of applied energy density the interaction of laser radiation with CrON has been always followed by plasma creation in front of the coating. PACS 79.20.Ds; 61.80.Ba     

Arc-discharge monitoring system of a high-power repetitively-pulsed TEA CO2 laser

High-power repetitively-pulsed TEA CO₂ lasers are excited by a glow discharge, and it turns out to be the arc discharge under some conditions. The arc-discharge is a disadvantageous condition and must be avoided. According to the Faraday electromagnetism induction principle, the arc-discharge monitoring system with a magnetic-field probe is designed for high-power repetitively-pulsed TEA CO₂ lasers. The magnetic-field variation induced by the discharge current can be tested, and the discharge state can be distinguished according to the output induction voltage. Experimental results show that the magnetic-field induction voltages generated by a glow discharge and an arc discharge are very different ones. The maximum induction voltage of the glow discharge is 2.0?V, while the minimum induction voltage of the arc discharge is 2.5?C4?V. Three alarm levels are set by measuring the arc-discharge intensities. At the first level, automatic filling?Cexhausting equipment starts to refresh the gas media, at the second level, the laser repetition rate is reduced, and at the third level the laser operation stops immediately. As a result, the working reliability of a high-power repetitively-pulsed TEA CO₂ laser system can be improved significantly by using the arc-discharge monitoring system.     

Surface modifications of TiN coating by pulsed TEA CO2 and XeCl lasers

Interactions of a transversely excited atmospheric (TEA) CO₂ laser and an excimer XeCl laser, pulse durations ∼2 μs (initial spike FWHM ∼100 ns) and ∼20 ns (FWHM), respectively, with polycrystalline titanium nitride (TiN) coating deposited on high quality steel AISI 316, were studied. Titanium nitride was surface modified by the laser beams, with an energy density of 20.0 J/cm² (TEA CO₂ laser) and 2.4 J/cm² (XeCl laser), respectively. The energy absorbed from the CO₂ laser beam is partially converted to thermal energy, which generates a series of effects such as melting, vaporization of the molten material, shock waves, etc. The energy from the excimer XeCl laser primarily leads to fast and intense target evaporation. The calculated maximum temperatures on the target surface were 3770 and 6300 K for the TEA CO₂ and XeCl lasers, respectively. It is assumed that the TEA CO₂ laser affects the target deeper, for a longer time than the XeCl laser. The effects of the XeCl laser are confined to a localized area, near target surface, within a short time period.Morphological modifications of the titanium nitride surface can be summarized as follows: (i) both lasers produced ablation of the TiN coating in the central zone of the irradiated area and creation of grainy structure with near homogeneous distribution; (ii) a hydrodynamic feature, like resolidified droplets of the material, appeared in the surrounding peripheral zone; (iii) the process of irradiation, in both cases, was accompanied by appearance of plasma in front of the target.Target color modifications upon laser irradiation indicate possible chemical changes, possibly oxidation.     

Surface modification of the titanium implant using TEA CO2 laser pulses in controllable gas atmospheres - Comparative study

Interaction of a TEA CO₂ laser, operating at 10.6 μm wavelength and pulse duration of 100 ns (FWHM), with a titanium implant in various gas atmospheres was studied. The Ti implant surface modification was typically studied at the moderate laser beam energy density/fluence of 28 J/cm² in the surrounding of air, N₂, O₂ or He. The energy absorbed from the TEA CO₂ laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following titanium implant surface changes and phenomena were observed, depending on the gas used: (i) creation of cone-like surface structures in the atmospheres of air, N₂ and O₂, and dominant micro-holes/pores in He ambient; (ii) hydrodynamic features, most prominent in air; (iii) formation of titanium nitride and titanium oxide layers, and (iv) occurrence of plasma in front of the implant. It can be concluded from this study that the reported laser fluence and gas ambiences can effectively be applied for enhancing the titanium implant roughness and creation of titanium oxides and nitrides on the strictly localized surface area. The appearance of plasma in front of the implants indicates relatively high temperatures created above the surface. This offers a sterilizing effect, facilitating contaminant-free conditions.     

Single transverse mode operation of a pulsed volume excited atmospheric pressure CO2 laser using an unstable resonator

The operation of a volume excited TEA CO₂ laser using an unstable resonator configuration is described. Considerable improvement over conventional single mode operation is achieved. The theoretical and experimental far field patterns are compared.     

CO2 Laser-induced plasma formation on a copper surface covered by dielectric particles

The results of an experimental study of TEA CO₂ laser plasma formation in front of a copper surface covered by crystallites of sodium chloride are reported. It is found that the threshold laser-beam intensities for plasma formation depend upon the mean surface concentration of sodium chloride. The results obtained are explained within the frame of a thermal model of surface-plasma formation.     

An injection-locked single-mode tea CO2 laser for SMM laser pumping

A D₂O laser has been developed for collective Thomson scattering measurements of ion temperature in high temperature plasmas. A pulse duration and a spectral width of a high power D₂O laser has been successfully controlled for this purpose, by using a TEA CO₂ laser injection-locked by an etalon-tuned TEA CO₂ laser as a pump source.     

Sub-target effect in film analysis using TEA CO2 laser-induced plasma

A TEA CO₂ laser beam (500 mJ, 200 ns) was focused on film samples, under low pressure surrounding gas at around 1 Torr. It has been found that to generate the laser plasma for the sample of oil or powder, the sample should be attached in the form of thin film on the surface of a metal, such as copper plate, which acts as a sub-target. The plasma has favorable characteristics for spectrochemical analysis due to its low background and sharp line spectrum, and hence an optical multichannel analyzer (OMA) without gated function can be used for spectrum acquisition. Using the sub-target effect we have performed for analyses on water molecular layer and water impurities, where the water was condensed by heating process or electrolysis process on the sub-target so that impurities in the water were attached to the metal surface. It should be emphasized that in this case the sub-target itself has never been ablated and no any damaged on its surface. Another application of the sub-target effect is used for the analysis of oil contamination in soils. We have succeeded to detect clearly the emission line of C I 247.9 nm from the carbon as a major constituent of the oil. To derive the carbon emission intensity coming only from oil, compensation was made to cancel the contribution from other organic species using the emission of Ca, which inherently contains in other organic species in soil. As result, a good linear relationship between carbon emission intensity and oil concentration was obtained.     

Modeling of helicopter-borne tunable TEA CO2 DIAL system employment for detection of methane and ammonia leakages

The characteristics of a helicopter-borne lidar based on tunable TEA CO₂ laser and its third harmonic designed for remote detection of methane and ammonia leakages from pipelines are analyzed numerically. The spectral range near 3 μm was shown to be most promising for remote sensing of methane emissions. Parameters of radiation of the tunable pulse-periodic mini-TEA CO₂ laser and generators of harmonics to be utilized in the helicopter-borne differential absorption lidar are estimated. Emissions of different gas intensities are analyzed for possible detectability at a distance of up to 1 km. The use of the third harmonic of the TEA CO₂ laser allows methane emissions from a pipeline to be detected and measured with mean measurement error from 10% to 15% for methane concentrations varying from the background level to the explosion-hazardous one. The optimal pair and possibilities of the ammonia remote sensing on the base of the first harmonic of TEA CO₂ laser were determined as well.     

Rapidly tuning miniature TEA CO2 laser – rotating mirror and grating mechanism

In this research, directed toward using differential absorption lidar (DIAL) for measuring concentrations of pollutant gases, a device for rapidly tuning a transversely excited atmospheric-pressure (TEA) CO₂ laser is presented. It is shown that it is possible to utilize a rotating six-sided scanning mirror and a fixed diffraction grating to rapidly switch wavelength over randomly selected lasing transitions in the 9–11 μm region of the spectrum. The scanning mirror and an optical encoder are driven by a hysteresis synchronous motor at a speed of 1500 rpm. A surface-wire-corona preionization was utilized in a cavity. The laser system is highly automated with microprocessor-controlled laser line selection. Single-branch emission at two wavelengths with time interval ⩽10 ms has been obtained from a single cavity TEA CO₂ laser. An accurate line selection has been demonstrated in over 40 transitions at a pulse repetition frequency of up to 100 Hz. The laser energy at first-order couple output was up to 20 mJ per pulse and the pulse width is about 60 ns in an active volume of 36 cm³.