Time-Resolved Emission Spectroscopic Study of Laser-Induced Steel Plasmas

Plasma Science and Technology, Vol.15, No.6, Jun. 2013 Time-Resolved Emission Spectroscopic Study of Laser-Induced Steel Plasmas M. L. SHAH, A. K. PULHANI, B. M. SURI, G. P. GUPTA Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai 400 085, India Abstract Laser-induced steel plasma is generated by focusing a Q-switched Nd:YAG visible laser (532 nm wavelength) with an irradiance of 1 10 9 W/cm 2 on a steel sample in air at atmospheric pressure. An Echelle spectrograph coupled with a gateable intensified charge-coupled detector is used to record the plasma emissions. Using time-resolved spectroscopic measurements of the plasma emissions, the temperature and electron number density of the steel plasma are determined for many times of the detector delay. The validity of the assumption by the spectroscopic methods that the laser-induced plasma (LIP) is optically thin and is also in local thermodynamic equilibrium (LTE) has been evaluated for many delay times. From the temporal evolution of the intensity ratio of two Fe I lines and matching it with its theoretical value, the delay times where the plasma is optically thin and is also in LTE are found to be 800 ns, 900 ns and 1000 ns. Keywords: laser-induced plasma, steel, emission spectroscopy, plasma temperature and electron number density PACS: 52.25.Os, 52.38.Mf, 52.50.Jm, 52.70.Kz DOI: 10.1088/1009-0630/15/6/11 1 Introduction Laser-induced plasma (LIP) is generated when a high-power pulsed laser is focussed on a solid, liquid or gaseous sample. The generated LIP on a sample in air at atmospheric pressure from a wide variety of samples with a laser at irradiances near the plasma ignition threshold is a weakly-ionized, low-temperature and high-density transient plasma. It is extensively studied in relation to chemical analysis of samples using laser-induced breakdown spectroscopy (LIBS) [1 3]. The LIBS is a well-known analytical technique owing to its ability to yield time- and space-resolved analytical results from a variety of samples with no or little sample preparation. The emission spectrum from LIP, consisting of atomic and ionic lines, forms the basis of chemical analysis of the sample using the LIBS technique. The characteristics of LIP depend on laser irradiance, wavelength, pulse duration, sample material, ambient conditions, space and time. Two LIP parameters, the plasma temperature T and the electron number density n e widely characterize the plasma and their knowledge is essential for understanding and exploitation of LIP as a valuable and versatile spectroscopic source. These parameters also characterize whether the plasma is in local thermodynamic equilibrium (LTE) and is optically thin, necessary for chemical analysis of samples using LIBS. The most widely used spectroscopic method for the determination of T of an optically thin and LTE plasma is the Boltzmann plot method [4] which employs the ratio of integrated line intensities for two or more atomic lines. The plasma electron density n e is commonly measured using the Stark broadening of spectral lines or the Saha-Boltzmann equation if the value of T is known [4]. The Saha-Boltzmann equation is also used for the determination of T, provided that the value of n e is known [5]. Owing to the economic and technical importance of steel in a wide range of industrial applications, home appliances and medical technology, steel manufacturers are bound to provide customers with the chemical composition of steel. Because of the growing demand for real-time, in-situ analytical results in the steel industry, LIBS is receiving increasing attention of several research groups for compositional analysis of steel [6 12]. DAVIES et al. [6] have carried out quantitative analysis of steel samples in air at atmospheric pressure using remote LIBS with a Q-switched Nd:YAG laser (1064 nm wavelength) at an irradiance of about 10 9 Wcm 2, without characterizing LIP. AGUILERA et al. [7] have measured line intensities of neutral atom and ion emission from LIPs produced with a Q-switched Nd:YAG laser (1064 nm wavelength) at irradiances in the range of 7.1 10 9 7.1 10 11 W/cm 2 when laser beam was focussed on steel samples in air at atmospheric pressure and in the range of 3 10 8 3 10 9 W/cm 2 when laser beam was defocussed on the samples with the lens focal plane behind the samples. They have not characterized LIP, either. PALANCO and LASERNA [8] have developed an instrument for quantitative analysis

M. L. SHAH et al.: Time-Resolved Emission Spectroscopic Study of Laser-Induced Steel Plasmas of steel samples using LIBS and evaluated capability for a fast quality assessment in steel factories. They have used a Q-switched Nd:YAG laser (1064 nm wavelength) at an irradiance of 5.7 10 9 W/cm 2 on steel samples without mentioning the value of the plasma temperature. BASSIOTIS et al. [9] have carried out LIBS analysis on steel samples in air at atmospheric pressure. They have performed measurements with a Q-switched Nd:YAG laser (1064 nm wavelength) and optimized experimental parameters to improve the linearity of the calibration curves for LIBS analysis. They have reported the values of T equal to 6500 K for a 4 µs delay time and 6000 K for a 7 µs delay time of the signal detector without mentioning the laser irradiance used in their LIBS experiments. ISMAIL et al. [10] have performed a detailed study of the plasma parameters T and n e in LIBS experiments on aluminium and steel alloys using a Q-switched Nd:YAG laser (1064 nm wavelength). Although they have not mentioned the pulse irradiance on the samples, they have determined T and n e at a delay time of 1.5 µs in the two alloys. The values of T obtained from the Boltzmann plot method and n e obtained from the Stark broadening method are 11000 K and 9.05 10 16 cm 3 respectively in their steel alloy. LOPEZ-MORENO et al. [11] have studied the quantification of elemental compositions of steel samples using LIBS with a higher-power microchip Nd:YAG laser (1064 nm wavelength) for enhancing the applicability of the LIBS technique to process monitoring in the steel-making industry. They have considered non-gated operation of the signal detector. The laser irradiance used in their experiments is estimated to be 10 11 W/cm 2. The value of T obtained from the Boltzmann plot method is reported to be 11000 K at this laser irradiance. VRENEGOR et al. [12] have analyzed high-alloy steel samples using LIBS, investigating matrix effects in LIPs generated with a Q-switched Nd:YAG laser (1064 nm wavelength) irradiating the steel samples. Although they have not mentioned the value of laser irradiance employed, they have determined the values of T from the Boltzmann plot method and n e from the Stark broadening method at different delay times. Typically, the values of T equal to 10300 K and n e = 5.5 10 16 cm 3 at a delay of 6 µs are reported in their LIP studies of steel samples. Thus, a Q-switched Nd:YAG infrared laser (1064 nm wavelength) is used in LIBS experiments on steel samples placed in air at atmospheric pressure. Use of an infrared laser on samples at atmospheric air pressure generates plasmas with a high electron density, leading to problems in reproducibility of the plasma emission characteristics [13]. For producing LIPs with improved emission characteristics, alternate approaches such as the use of argon atmosphere [13], ultraviolet laser [14] and multiple-pulse laser [15] have been proposed. In the present paper, we have extended the studies of laser-induced steel plasma by employing a Q-switched Nd:YAG visible laser (532 nm wavelength) focussed on a steel sample in air at atmospheric pressure. Timeresolved spectroscopic measurements of atom and ion emissions of the LIP have been carried out with an intensified charge-coupled device (ICCD) camera coupled with an Echelle grating spectrometer. All the emission measurements are space-integrated along the line of sight. The plasma temperature T using the Boltzmann plot method and the electron number density ne using the Saha-Boltzmann equation method have been determined at different delay times of the ICCD relative to the onset of the laser pulse. The validity of the assumption by the above-mentioned diagnostic methods that the LIP is optically thin and is in LTE has been evaluated for many delay times. 2 Experimental details Fig. 1 shows the schematic diagram of the experimental set-up for LIBS studies. A Q-switched Nd:YAG (Quantel TG980) visible laser (532 nm wavelength) with a pulse duration of 7 ns and a repetition rate of 20 Hz was focussed at right angles to the surface of a steel sample, which was placed in air at atmospheric pressure on a X-Y-Z translation stage rotated manually to avoid pitting of the sample. The certified elemental composition of the steel is: Fe (72.4%), Cr (18.3%), Ni (8.03%), Mn (0.82%) and Si (0.33%). The laser beam with 70 mj pulse energy was focussed by a bi-convex lens of 10 cm focal length to produce a focal spot diameter of about 1 mm. This yields the incident laser irradiance equal to 1 10 9 W/cm 2, sufficient for inducing the plasma on the steel surface but insufficient for air breakdown in front of the sample. This laser irradiance, which is the same as used in Ref. [6], is above the plasma ignition threshold since the line emission from plasma formed during laser ablation of steel in air has been measured for laser irradiances 3 10 8 W/cm 2 in Ref. [7]. An Echelle grating spectrometer coupled with a gateable ICCD camera (Andor Mechelle ME5000- DH734-18U-03PS150) was used to record the emission spectrum. The spectrometer covers a wide wavelength range of 200 975 nm in a single spectrum with a good wavelength resolution (0.05 nm). A Hg-Ar lamp, which provides sharp lines from 200 1000 nm, was used for wavelength calibration of this system. Intensity calibration of the Echelle spectrometer-iccd system was done using a NIST certified Deuterium-Quartz- Tungsten-Halogen lamp (Ocean Optics, USA). The plasma radiation, space-integrated along the line of sight, was collected and imaged on to the spectrometer slit using an optical fibre-based collection system. This emission collection assembly (ECA) was positioned at a distance of about 20 cm from the plasma, making an angle of 45 to the laser beam. The spectrally dispersed light from the spectrograph was collected by a thermoelectrically cooled ICCD camera which is sensitive in the whole UV-VIS-NIR region, converting the spectral signal into digital signal. The detector was gated in synchronization with the laser pulse 547

Plasma Science and Technology, Vol.15, No.6, Jun. 2013 to record time-resolved emission spectrum. The detector integration time was limited to 1000 ns to avoid saturation of the detector whereas four delay times of 800 ns, 900 ns, 1000 ns and 1200 ns were chosen for recording the plasma emission spectra at different delays, avoiding the high intensity continuum emission at the early times of the plasma formation. the following equation [3,4,17] : ln ( Iλ ki ) 1 = g k A ki k B T E k + ln ( hcln), (1) 4πP where I is the integrated intensity of a spectral line occurring between an upper energy level k and a lower energy level i, A ki is the transition probability, λ ki is the transition wavelength, E k and g k are the energy and degeneracy of the upper energy level k respectively, n is the total number density, h is the Planck constant, c is the speed of light, L is the characteristic length of the plasma, k B is the Boltzmann constant and P is the partition function. The plot of the left-hand side of Eq. (1) versus E k for several transitions is called the Boltzmann plot which yields a straight line with a slope of 1/k B T. Thus, the value of T can be determined from the slope of the Boltzmann plot. BS Beam splitter, DO Digital oscilloscope, ECA Emission collection assembly, FPD Fast photo diode, M Mirror Fig.1 Schematic diagram of the experimental set-up for LIBS studies 3 Results and discussion 3.1 Emission spectra In order to obtain enough reproducibility of the emission spectra from the LIP and compensate for any expected sample inhomogeneity, emission from 120 laser pulses was accumulated by addition of the corresponding ICCD readings, with background subtraction, to record one emission spectrum at a given delay time. Four such emission spectra were recorded at a given delay time. In all, we have collected the emission spectra at four delay times (800 ns, 900 ns, 1000 ns and 1200 ns). Fig. 2 shows the typical emission spectra of laser-induced steel plasma at a delay time of 1000 ns. Since Fe is a major element with a content of 72.4%, its rich atomic and ionic transitions make the spectrum much crowded, overlapped with lines of the other elements and making the spectral analysis much tedious. As mentioned in Fig. 2, seven Fe I and three Fe II emission lines, which are well resolved, non-resonant and have minimal spectral interference with other spectral lines in a complex steel spectrum, are chosen for the plasma characterization in the present work. These lines along with their spectroscopic parameters, taken from the NIST atomic database [16], are shown in Table 1. 3.2 Plasma temperature The Boltzmann plot used to determine the plasma temperature T is applicable to LTE and optically thin plasmas and is obtained from the graphical spectral analysis of many lines of a species simultaneously using Fig.2 LIBS spectra of a steel sample at a delay time of 1000 ns, showing (a) Fe I lines and (b) Fe II lines used for the plasma characterization We have generated the Boltzmann plots at four delay times using seven Fe I lines mentioned in Table 1 for determining the values of T at different delays. A typical Boltzmann plot at a delay of 1000 ns is shown in Fig. 3, where the data were fitted with the leastsquare approximation. The slope of the plot gives T = 0.875 ev (10150 K). The values of T determined from the plots at four delay times are presented in Table 2. As evident from the table, the temperature increases initially, reaching a maximum (10150 K) at 1000 ns delay and decreases thereafter. 548

M. L. SHAH et al.: Time-Resolved Emission Spectroscopic Study of Laser-Induced Steel Plasmas Table 1. Wavelength, lower and upper energy levels, upper level degeneracy, transition probability for the Fe I and Fe II emission lines Atom/ion Wavelength Upper level Lower level Upper level Transition (nm) energy (ev) energy (ev) degeneracy probability (s 1 ) Fe I 346.59 3.686 0.110 3 1.19 10 7 Fe I 355.49 6.319 2.832 13 1.40 10 8 Fe I 361.88 4.415 0.990 7 7.22 10 7 Fe I 396.93 4.608 1.485 7 2.26 10 7 Fe I 426.05 5.308 2.399 11 3.99 10 7 Fe I 432.58 4.473 1.608 7 5.16 10 7 Fe I 492.05 5.352 2.832 9 3.58 10 7 Fe II 233.28 5.361 0.048 6 1.31 10 8 Fe II 233.80 5.408 0.107 4 1.13 10 8 Fe II 241.33 5.257 0.121 4 1.02 10 8 expressed as [4,17,18] Fig.3 Boltzmann plot made by using seven Fe I transitions, considering the intensities at a delay time of 1000 ns. The continuous line represents the results of a linear best fit. The slope gives the temperature equal to 0.875 ev 3.3 Electron number density Among several diagnostic methods for measuring the electron number density n e, plasma spectroscopy based on either Stark broadening of spectral lines or Saha-Boltzmann equation is considered as a simple method [4]. Instead of using the Stark line broadening method which is usually considered [10], we have used the Saha-Boltzmann equation method, as considered in Refs. [17,18], for determining n e. Although both the methods are similar in accuracy, the Stark line broadening method involves the electron impact parameter along with the spectral line profile whereas the Saha- Boltzmann equation method involves only the spectral line intensities. Using the spectral line intensities of a species in two consecutive charge states Z and Z + 1 of a particular element, the electron number density of the LIP is determined from the Saha-Boltzmann equation n e = I Z I Z+1 6.04 10 21 (T ) 3 2 exp[( E k,z+1 +E k,z χ Z )/k B T ] cm 3, (2) I Zλ ki,z g k,z A ki,z where IZ = and χ Z is the ionization energy of the species in the ionization stage Z. Here, the subscript Z is used to denote the ionization stage of the species (Z = 0 for neutral atoms, Z=1 for singly ionized atoms, etc.). The lowering of the ionization energy due to interactions in the plasma is negligibly small which has been omitted in Eq. (2). It is worth pointing out that the Saha-Boltzmann equation given by Benmansour et al. [5] is somewhat different from Eq. (2). T is in unit of ev. Using the measured intensity ratio of Fe I and Fe II lines at a given delay time, the electron number density n e is determined from Eq. (2). We have considered five intensity ratios, Fe I 346.59 nm and Fe II 233.28 nm, Fe I 355.49 nm and Fe II 233.28 nm, Fe I 361.88 nm and Fe II 233.28 nm, Fe I 346.59 nm and Fe II 233.80 nm and Fe I 346.59 nm and Fe II 241.33 nm and determined five values of n e at a given delay time. The arithmetic mean of the five values of n e is taken as the average value of n e at that particular delay time. Similarly, we have determined the average values of n e at four delay times. These average values are presented as the electron density at four delay times in Table 2. As evident from the table, the electron number density follows the similar temporal evolution to that of the temperature. It increases initially, reaching a maximum (9.8 10 16 cm 3 ) at the delay time of 1000 ns and decreases thereafter. As seen in Table 2, both the plasma temperature and the electron number density are found to be maximum at 1000 ns of the delay time and decrease thereafter. When the plasma expands, it transfers its energy to the surroundings and cools down, resulting in a decrease in T. During the plasma expansion the electron-ion recombination occurs, causing the decrease in n e. The increase in T and n e initially may be ascribed to inelastic 549

Plasma Science and Technology, Vol.15, No.6, Jun. 2013 collisions occurring in high density and high temperature plasma formed in the beginning. Similar behavior of these plasma parameters was seen in laser-induced copper plasma [17]. Table 2. Plasma temperature and electron number density as a function of delay time of the detector relative to the onset of the laser pulse Delay time Plasma temperature Electron number (ns) (K) density (cm 3 ) 800 7550 0.1 10 16 900 8600 4.2 10 16 1000 10150 9.8 10 16 1200 8680 0.5 10 16 3.4 Criterion for optically thin and LTE Plasma The above-mentioned emission spectroscopic methods for characterizing the plasma require the plasma to be optically thin and in LTE, also necessary for the quantitative chemical analysis of a sample using the LIBS technique. It is thus necessary to know the time window for time-evolving LIPs where the plasma is optically thin as well as in LTE. The condition for the validity of the LTE alone is often checked with the McWhirter s [19] criterion which provides the lowest value of the electron number density necessary for the plasma to be in LTE. The LTE plasma satisfying the McWhirter s criterion may or may not be optically thin. The criterion whether the LIBS plasma is optically thin as well as in LTE is obtained from the intensity ratio of two lines of a particular element in the same ionization stage Z which is expressed as [17] I 1 = ( λ nm,z )( A ki,z )( g k,z ) ( E k,z E n,z ) exp, I 2 λ ki,z A nm,z g n,z k B T (3) where I 1 is the line intensity from the k-i transition and I 2 is that from the n-m transition. If we consider two emission lines having the same upper level or as close as possible, the temperature effect of the Boltzmann factor on the reproducibility of the line intensity ratio is minimized and at the same time the consideration of the efficiency factor of the collecting system is avoided. Neglecting the exponential factor in that condition, one can find out the theoretical value of the intensity ratio of the two lines by using the atomic parameters of the transitions. By matching this theoretical intensity ratio with the measured values at different delay times, one finds out the time window where the plasma is optically thin and in LTE. To infer this time window, we have considered two Fe I emission lines, one at 425.08 nm not mentioned in Table 1 and the other at 432.58 nm mentioned in Table 1. The first emission line is chosen owing to its upper energy level being the same as that of the second emission line. The transition probability of the first emission line is 1.02 10 7 s 1 and that of the second emission line is 5.16 10 7 s 1 [16]. The intensity ratio for this couple of lines, calculated from Eq. (3), is 0.2. The experimentally measured intensity ratios of this couple of lines are 0.17, 0.23, 0.25 and 0.13 at delay times of 800 ns, 900 ns, 1000 ns and 1200 ns, respectively. Thus, the LIP produced in this work is optically thin as well as in LTE at delay times of 800 ns, 900 ns and 1000 ns. 4 Conclusions A Q-switched Nd:YAG visible laser (532 nm wavelength) was used to study the laser-induced steel plasma using a laser irradiance of 1 10 9 W/cm 2 on a steel sample placed in air at atmospheric air. The timeresolved spectroscopic measurements of atom and ion emissions of the LIP have been carried out with an intensified charge-coupled device (ICCD) camera coupled with an Echelle grating spectrometer. All the emission measurements are space-integrated along the line of sight. The plasma is characterized using the spectral intensities of iron atomic and ionic lines. The plasma temperature T using the Boltzmann plot method and the electron number density n e using the Saha-Boltzmann equation method have been determined at different delay times of the ICCD relative to the onset of the laser pulse. The validity of the assumption by the above-mentioned diagnostic methods that the LIP is optically thin and is also in LTE has been evaluated at four delay times. From the temporal evolution of the intensity ratio of two Fe I lines and matching it with its theoretical value, the delay times where the plasma is optically thin and is also in LTE are found to be 800 ns, 900 ns and 1000 ns. Acknowledgments The authors are thankful to Dr. L. M. GANTAYET, Director, Beam Technology Development Group and Dr. A. K. DAS, Head, Laser and Plasma Technology Division, Bhabha Atomic Research Centre for their encouragement and support. References 1 Radziemski L J, Cremers D A. 1989, Laser-Induced Plasmas and Applications. Marcel Dekker Inc., New York 2 Miziolek A W, Pallesschi V, Schechter I. 2006, Laser- Induced Breakdown Spectroscopy. Cambridge University Press, Cambridge 3 Cremers D A, Radziemski L J. 2006, Handbook of Laser-Induced Breakdown Spectroscopy. John Wiley & Sons Ltd, West Sussex, England 4 Griem H R. 1997, Principles of Plasma Spectroscopy. Cambridge University Press, Cambridge 5 Benmansour M, Nikravech M, Morvan D, et al. 2004, J. Phys. D: Appl. Phys., 37: 2966 550

M. L. SHAH et al.: Time-Resolved Emission Spectroscopic Study of Laser-Induced Steel Plasmas 6 Davies C M, Telle H H, Montgomery D J, et al. 1995, Spectrochimica Acta Part B, 50: 1059 7 Aguilera J A, Aragon C, Penalba F. 1998, Applied Surface Sci., 127: 309 8 Palanco S, Laserna J J. 2000, J. Anal. At. Spectrom., 15: 1321 9 Bassiotis I, Diamantopoulou A, Giannoudakos A, et al. 2001, Spectrochimica Acta Part B, 56: 671 10 Ismail M A, Imam H, Elhassan A, et al. 2004, J. Anal. At. Spectrom., 19: 489 11 Lopez-Moreno C, Amponsah-Manager K, Smith B W, et al. 2005, J. Anal. At. Spectrom., 20: 552 12 Vrenegor J, Noll R, Sturm V. 2005, Spectrochimica Acta Part B, 60: 1083 13 Aragon C, Aguilera J A, Penalba F. 1999, Appl. Spectrosc., 53: 1259 14 Kagawa K, Kawai K, Tani M, et al. 1994, Appl. Spectrosc., 48: 198 15 Sattmann R, Sturm V, Noll R. 1995, J. Phys. D: Appl. Phys., 28: 2181 16 NIST Atomic Spectra Database Data, http://physics. nist.gov 17 Unnikrishnan V K, Alti K, Kartha V B, et al. 2010, Pramana - J. Phys., 74: 983 18 Gomba J M, Angelo C D, Bertuccelli D, et al. 2001, Spectrochimica Acta Part B, 56: 695 19 McWhirter R W P. 1965, Plasma Diagnostic Techniques. Edited by R. H. Huddlestone, S. L. Leonard, Academic Press, New York (Manuscript received 27 June 2011) (Manuscript accepted 13 December 2011) E-mail address of corresponding author G. P. GUPTA: gpgupta@barc.gov.in 551