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AIAA-2003-3752

Correlation Between Electron Density and LIBS Signal of Magnesium in water using a Meinhard Nebulizer

Akshaya Kumar, Fang-Yu Yueh, and Jagdish P. SinghDiagnostic, Instrumentation and Analysis Laboratory

Mississippi State UniversityStarkville, MS 39759 (USA)

Abstract

Double pulse laser induced breakdown spectroscopy of magnesium in water using Meinhard Nebulizer in mist mode as well as thin jet mode have been studied. Electron density of the laser produced plasma has been monitored using Stark broadening of the hydrogen alpha line. Effect of the time delay between the two laser pulses on LIBS signal as well as on electron density has been elucidated.

Introduction

Understanding the plasma dynamics is becoming more and more important because of its large number of technological applications in different area of science and technology. A number of research fields that include the plasma physics are material science, fluid dynamics, Chemical physics, biology, space propulsions, and Cosmo physics1-5. Basic goal in different areas of research is to discover the different plasma parameters and control it to harness for the useful purposes.

Now-a-days laser produced plasma of solid and liquid material is of utmost interest especially in the field of laser diagnostics, laser fusion, thin film growth and chemical analysis. Laser Induced

Breakdown spectroscopy (LIBS) is an analytical technique in which laser light is used to create the plasma and the emission of the plasma is used to recognize the composition of the material.1,2 The understanding of the basic plasma dynamics in laser produced plasma can help to optimize LIBS signal. LIBS is a well-recognized tool to diagnose atomic composition of solid, liquid and gas samples in their own state. This technique allows real-time, on-line, on-site measurement without any sample preparation. LIBS of liquids have more problem than of solids because of higher breakdown threshold and splashing of liquid and turbulences causing loss of laser energy. Different technique such as LIBS on liquid jet both thick and thin mode6, application of magnetic field around the jet7 , use of two sequential laser pulses, have been adopted to improve the technique for liquid application.Using double pulse technique a significant enhancement8 in the LIBS signal (approximately 7 times) and consequently increase in sensitivity (i.e. decrease in LOD) of trace metals in water have been observed.

The present work deals with the Nd: YAG laser ablation of a liquid jet target of the Mg solution using 532 nm radiations. Two laser pulses with few microsecond inter-pulse delay have been used to produce and re-excite the plasma. The correlation

34th AIAA Plasmadynamics and Lasers Conference23-26 June 2003, Orlando, Florida

AIAA 2003-3752

Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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Fig. 1

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between the electron density in the laser produced plasma at various time delay between two lasers and the corresponding LIBS signal was studied. The electron density is determined by using the Stark broadening of Hα line9. In the study we used a Meinhard Nebulizer to introduce the liquid sample to the laser plasma. Meinhard Nebulizer is originally developed for the ICP applications which can work either in very fine thin jet mode or mist mode depending on the requirement.

Experimental

The experimental setup to record the plasma emission of Mg in 2% acidic solution is shown in Figure-1.

Figure 1. Experimental setup to record the LIBS spectrum of liquid solution.

In brief, a Q-switched, frequency doubled Nd: YAG laser that gives 532 nm radiation of 8 ns duration was used to ablate the laser jet. To enhance the LIBS signal, another Q-switched, frequency double Nd:YAG laser is used to re-ablate and excite the laser produced plasma. The emission from the plasma was focused using a 20-cm focusing lens on the liquid jet/mist stream of a Nebulizer (Meinhard TR-30-C6) with nozzle diameter of about 0.3 mm. Meinhard nebulizer is originally made for ICP measurements and it creates a mist and optimized to work in condition of 30 psi gas pressure with argon gas flow rate of about 1000 ml/min and the liquid flow rate of 5.7 ml/min. But, in the case of LIBS we observed that the liquid sample solution flow rate of 3.5 ml/min and gas (argon) flow rate of 300 ml/min gave best signal.10 Also, we have used the same Nebulizer to produce very fine and stable liquid jet at the liquid flow rate of 17.5 ml/min. For circulating the solution we have used the closed loop system. Emission is collected in the backward direction by coupling UV grade quartz lenses of focal length 50 cm and 10 cm lenses on the optical fiber bundle. Fiber bundle is a collection of 80 single fibers of 0.1 mm core diameter. The other end of the fiber in connected to a remote spectrograph equipped with a 2400- l/mm diffraction grating. The emission signal was recorded with an Intensified CCD camera, connected to the exit end of the spectrograph. The signal is finally processed and stored in a computer. Hundred shots of spectra are accumulated for one spectrum. Thirty such spectra were recorded under the same condition for good statistical average.

Results

Comparison of double pulse LIBS signal in mist and thin jet mode

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It has been widely reported that two pulse LIBS experiment is beneficial than one pulse. Normally, after the creation of the plasma by one pulse, it expands and the various species in their ionized state returns back to their neutral state by the electron-ion recombination process. Normal life-time of the plasma is of the order of 20-30 s. Light emission from the plasma is sum of the two distinct processes11. First process is recombination of free electrons and the free ions that contribute to the background

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Figure 2 LIBS signal at various time delay between lasers [Mg 20ppm, 1us gate delay, 10us gate width, L1=140mJ (Big Sky), L2=160mJ (Surelite)].

emission. Second process is the emission from the atoms and ions due to the transition from one bound state to the other, contributes the line emission. After creation of the laser induced plasma both kind of emissions decay with the time but with different rate. Background emission, which is stronger in initial few microseconds, decay faster than the emissions from the atoms and ions. The bounds states emissions are used to characterize the elements present in the plasma/material. The purpose second laser pulse re-excite the plume created by first laser pulse which after a certain time delay re-ionizes the expanding plume. The delay timing between the lasers is selected in such a manner that the ionization of the

concerned species (heavy metal atoms which expand little slower than the lighter atomic components of water and surrounding gas) is more than the unwanted species. As a result we observed the enhancement in the LIBS signal of the element of interest (Mg in present Case).Double pulse LIBS spectra of 20 ppm Mg in water in jet mode and mist mode using Meinhard Nebulizer is shown in Figure-2. In Figure-2 the laser energy of two lasers are 150 mJ and 130 mJ respectively. The gate delay and gate width are 3µs and 10 µs respectively which are optimized initially. It is evident from the Figure that as we increase the time delays between the two lasers the LIBS signal increases in the case of Jet mode and decreases in the case of mist mode. It is also evident from Figure-2 that on increasing the time delays between the lasers the LIBS signal of Mg increases and acquires maximum around time delay of 3 µs and then starts decreasing. In case of mist the behavior is different than in case of jet. In mist mode the LIBS signal goes on decreasing on increasing the delay between the lasers. In fact the situation in mist is very different than in case of jet. In mist the liquid particles are coming out in the form of fine droplets immersed in the gas (Argon in present case). When the laser beam is focused on the mist, each droplets behave as a tiny lens that can focus the laser beam at random locations. The second laser, which is supposed to re-excite the plasma plume created by the first pulse may not find the right spot. Also the fluctuation in mist particle size distribution at the laser focal point may cause the additional problems.

Comparison of plasma density in double pulse LIBS in mist and thin jet mode

In Figure-3, we have compared the variation of electron density on the time delay between the lasers. To estimate the electron

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density the Hα line at 656.2 nm is recorded in same experimental conditions as it was used to record the LIBS spectra of magnesium lines. Stark broadening of Hαline was used to estimate the electron temperature9. It is clear from the Figure-3, that initially when the time delay between the two lasers is zero, both lasers behave as a single laser; the electron density in mist mode is higher than that in the jet mode. The higher electron density in the mist mode believed due to the low ionization potential of the purging gas used in the mist mode. The laser plasma was generated at argon

Figure-3. A plot of variation of plasma density by changing the time delay between the two lasers in jet mode and Mist mode of Nebulizer. [1us gate delay, 10us gate width, L1=140mJ (Big Sky), L2=160mJ (Surelite)].

atmosphere in mist mode while it is generated in water surrounded by the ambient gas is the jet mode. Also, for jet mode on increasing the time delay the electron density initially decreases then starts increasing. After the plasma generated by the first laser expanded to certain size, the second laser can effectively re-excite the plasma and increase the plasma density. Initially when the time delay between the two lasers is zero, they work as a single laser with higher laser intensity. At laser delay time equals 200 ns, LIBS signal intensity increases on the other hand the electron density goes down. The plasma density goes

down because the plasma density generated by the first laser is lower than the case when the two laser pulses were together. It seems that initially the size to plasma plume is so small and electron density is so high that second laser is not able to reach the material and create its own ablation and plasma. Rather, it re-excites the plasma created by the first laser and results an increase in the LIBS signal. With time, the size of the plasma plume is getting bigger and bigger and the area of the plume of the plasma between the influence of the second laser is becoming less and less. As a result of that the density of the plasma is reduced. Beyond a critical time (Delay=1.5 µs) the plasma created by the first laser gets diluted such that the second laser is able to penetrate the plume and it creates the plasma from the material by its own, As a result the plasma density starts increasing and it reaches to a maximum. Further, on increasing the time delay the influence of the first plasma becomes weaker only the characteristic of the second laser plasma remains. Comparing double pulse LIBS signal in jet mode and mist mode it is very obvious that jet mode is beneficial for the enhancement in the signal.

ACKNOWLEDGEMENT

This work is supported by Department of Energy contract no. DE- FC26- 98FT-40395.

References

1. Rusak, D.A., Castle, B.C., Smith, B.W., and Wineforder, J.D. 1997 “Fundamentals and Applications of Laser-Induced Breakdown Spectroscopy”, Critical Review in Analytical Chemistry, 27, 257-290.

2. Singh, J.P., Yueh, F.Y., Zhang H., 1997. “Study of Laser induced breakdown spectroscopy as a process monitor and control tool for hazardous waste remediation”, Process control and quality 10, 247-258.

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3. Cremers, D. A. and Radziemski, L. J. "Laser Plasmas for Chemical Analysis" in Laser Spectroscopy and its Application, L.J. Radziemski, R.W. Solarz, J.A. Paisner, eds. (Marcel Dekker, New York, NY, 1987), Chap. 5, p.351.

4. Yueh, F.Y., Singh, J.P., and Zhang, H., “Elemental Analysis with Laser-induced Breakdown Spectroscopy”, Encyclopedia of Analytical Chemistry, (John Wiley and Sons, 2000) p.2066-2087.

5. Margetic V., Ban T., Leis F., Niemax K., Hergenroder R. , “ Hydrodynamic expansion of a Femtosecond Laser Produced Plasma” , Spectrochimica Acta B 58 (2003)415-425.

6. Kumar A. ,Yueh F.Y., and Singh J.P., “Study of Double Pulse Laser Induced Breakdown Spectroscopy Using Different Thickness Jets”, Applied Optics (Communicated)

7. Rai V.N., Zhang H., Yueh F.Y., Singh J.P. and Kumar A.’ “Effect of steady magnetic field on laser induced breakdown spectroscopy” , Applied Optics (2003 ) Accepted Manuscript No. 18383

8. Rai V.N., Zhang H., Yueh F.Y. and Singh J.P., 2003, ”Study Of Laser Induced Breakdown Emission From Liquid Under Double Pulse Excitation”, Appl.Opt. ( in press).

9. Byi.C.A. and Scheeline, A., 1993, “Saha-Boltzman Statistics for Determination of Electron Temperature and Density in Spark discharge using an Echelle/ CCD system”, Appl. Spectrosc. 47 2022.

10. Kumar A., Yuesh F. Y., Miller T. and Singh J.P., Determination of Trace Elements in Liquid using Meinhard Nebulizer by Laser Induced Breakdown Spectroscopy, Applied Optics (Communicated)

11. Rai, V.N., Singh, J.P., Yueh, F.Y. andCook,R.L,.2001.“Dynamics, Stability and Emission of Radiation from Laser Produced Plasma Expanding Across an External Magnetic Field”. AIAA-2001- 2812.

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