Spectroscopic Studies of the Laser Produced Lead Plasma

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Plasma Science and Technology, Vol.13, No.2, Apr. 2011

Spectroscopic Studies of the Laser Produced Lead Plasma

M. HANIF1,2, M. SALIK1, M. A. BAIG1

1Atomic & Molecular Physics Laboratory, Department of Physics, Quaid-I-Azam University,Islamabad 45320, Pakistan

2MCS, National University of Sciences & Technology, Rawalpindi 46000, Pakistan

Abstract Spectroscopic emission of lead plasma, generated by the fundamental (1064 nm) and

second harmonics (532 nm) of a Q-switched pulsed Nd: YAG laser, is studied. The spectral lines of

neutral atoms and singly ionized lead ions were shown predominantly. The profiles of neutral lead

lines observed were used to extract the excitation temperature using Boltzmann plots, whereas

electron number density was determined from the profile of Stark broadened line. The variations

of excitation temperature and electron number density as a function of laser energy were studied.

Keywords: lead, laser ablation, optical emission spectroscopy, plasma temperature andelectron density

PACS: 52.25.Jm, 52.25.Kn, 52.25.Os, 52.38.Mf, 52.50.Jm

1 Introduction

Laser induced breakdown spectroscopy (LIBS) isa technique, which was first reported in early 1960sand then reviewed by several researchers. This sensi-tive technique is based on an optical detection for cer-tain atomic and molecular species by monitoring theiremission signals from the laser induced plasma. It pro-vides a useful tool to determine the chemical compo-sition of a wide range of materials including metals,liquids, aerosols, plastics, minerals and biological tis-sues, etc [1∼3]. It is a simple technique as comparedwith many other types of elemental analysis because ofits straightforward experimental set-up. In this tech-nique a pulsed laser is required to generate micro-plasma on the target surface and elemental analysis isaccomplished by a study of the emission from plasmaplume. The characteristics of laser induced plasma de-pend upon several parameters characterizing the fea-tures of the target, properties of the ambient medium,laser wavelength and pulse duration etc. The first di-rect spectral analysis made by LIBS can be attributedto RUNG et al [4,5]. In 1980s, this technique was in-creasingly applied as an analytical tool in research [6].

Lead is an important element and has its unique-ness due to its chemical properties. It has been ex-tensively studied using LIBS technique. The tran-sition probabilities and Stark broadening parametersof neutral and singly ionized lead lines in emissionby means of a gas-driven shock tube were measuredby MILLER et al [7]. The transition probabilities forseveral lines of ionized lead ions from the emissionlines intensities using a hollow-cathode discharge andlaser induced plasma were reported by ALONSO-MEDINA [8,9]. Time-resolved measurements and in-vestigation of spatial and temporal dependence oflead emission using LIBS were reported by CASTLE

et al [10], whereas PbIII lines were reported by COLONet al [11]. Transition probabilities for a number of PbIlines and the PbII line at 220.35 nm were reported byMEDINA et al [12,13]. LIBS-based double pulsed spec-troscopic studies of lead were reported recently [14,15].

In the present work the studies of the laser-inducedlead plasma were extended by using the fundamen-tal (1064 nm) and second harmonics (532 nm) of anNd:YAG laser. The excitation temperature and elec-tron number density are determined at different posi-tions along the direction of propagation for differentlaser energies.

2 Experimental details

The experimental system is the same as describedin our previous work [16∼21]. Briefly a Q-switchedNd:YAG (Quantel Brilliant) pulsed laser, with a pulseduration of 5 ns and a repetition rate of 10 Hz, which iscapable of delivering 400 mJ at 1064 nm, and 200 mJat 532 nm, was used. The energy of laser pulse wasvaried by the flash lamp Q-switch delay through thelaser controller, and measured by a Joule meter (Nova-Quantel 01507). The laser beam was focused on thetarget using convex lens with a focal length of 20 cm.The sample was mounted on a three-dimensional sam-ple stage, which was rotated to avoid the non-uniformpitting of the target. The distance between the focus-ing lens and the sample was kept shorter than the focallength of the lens, which prevented any breakdown ofthe ambient air in front of the target. The spot’s diam-eter ω2 can be calculated by

ω2 ≈ 2.44fλ/ω1[3],

where ω1 is the diameter of the incoming laser beambefore focusing, 0.5 cm in our case, from which one

Plasma Science and Technology, Vol.13, No.2, Apr. 2011

can calculate the area/size of the spot on the sur-face of the target. f is the focal length of the lens,20 cm in our case, and λ is the wave length of thelaser used. In our case fundamental (1064 nm) and sec-ond harmonics (532 nm) of a Nd: YAG laser were used.Therefore, the area of the spot for the first harmonic(1064 nm) and second one (532 nm) are 8.4×10−5 cm2

and 2.12×10−5 cm2, respectively. The spectra were ob-tained by an average of data from ten individual shotswith identical experimental conditions. The radiationemitted by the plasma were collected by a fiber op-tics (high-OH, with a core diameter of 600 µm) havinga collimating lens (with a field of view of 0o to 45o)placed at a right angle to the direction of the laserbeam. The optical fiber was connected with a LIBS-2000 detection system (Ocean Optics Inc.) to measurethe plasma emission. The emitted signal was correctedby subtracting the dark signal of the detector throughthe LIBS software. The LIBS-2000 detection system isequipped with five spectrometers having a slit width of5 µm for each, covering the range between 200 nm to720 nm. Each spectrometer was equipped with 2048 el-ement linear charged coupled device (CCD) array andwith an optical resolution of about 0.05 nm. In orderto record the emission spectrum, the LIBS-2000 detec-tion system was synchronized with the Q-switch of theNd:YAG laser. The flash lamp out of the Nd: YAG lasertriggered the detection system through a four-channeldigital delay/Pulse generator (SRS DG 535), and theLIBS-2000 detection system triggered the Q-switch ofthe Nd: YAG laser.

3 Results and discussion

3.1 Optical emission spectra

Plasma generated by a high intensity laser pulse ex-pands normally against the target surface due to shockwaves [22]. In the present work, a lead plasma is gener-ated using two modes of a Q-switched Nd:YAG laser,namely fundamental and second harmonic modes at1064 nm and 532 nm, respectively. In the first set of ex-periments, the fundamental (1064 nm)-mode laser witha pulse energy of 400 mJ and a pulse duration of 5 nswas focused on the lead target placed in air. The emis-sion spectrum of the plasma generated at the surfaceof the lead is recorded at different distances along thedirection of propagation of the plasma plume.

The ground state configuration of lead is 6p2 whichyields 3P0,1,2, 1D2 and 1S0 levels. The first excited state6p7s consists of four levels 3P0,1,2 and 1P1 whereas tenlevels are built on the 6p6d configuration. The emis-sion spectrum of laser-generated lead plasma mainlycontains transitions between the 6p7s and 6p6d config-urations based levels to the levels built on the 6p2 con-figuration. It is observed that the lines connected withthe 6p2 1D2 and 1S0 levels are very sharp. The observedsingly ionized lead lines originate from the transitionsof 5f 2F5/2,7/2 →6d2D3/2,5/2, 7d2D3/2,5/2 →7p2P1/2,3/2

and 7p2P1/2,3/2 →7s2S1/2, which are relatively broaderthan the neutral lead lines. Fig. 1 shows the emissionspectrum of lead plasma covering the spectral regionof 250 nm to 290 nm. The spectrum was generated byusing a laser of 1064 nm having an energy of 120 mJand the detector was placed at 1 mm from the targetsurface. Most of the lines in this region belong to transi-tions in neutral lead. The strongest lines are observedat 261.43 nm and 280.20 nm, identified as transitionsof 6p6d3D2 →6p23P1 and 6p6d3P3 →6p23P2, respec-tively. The other strong lines are at 266.32 nm and283.31 nm corresponding to the transitions of 6p7s3P2

→6p23P2 and 6p7s3P1 →6p23P0, respectively. All theobserved lines in the investigated spectral region alongwith their assignments are listed in Table 1, based onthe data listed in the tables from National Institute ofStandards (NIST) formerly known as National Bureauof Standards (NBS) [23,24].

Fig.1 Emission spectrum of neutral lead plasma gener-

ated by the 1064 nm laser covering the region of 255 nm to

290 nm

In Fig. 2, the emission spectrum covering the regionof 350 nm to 450 nm is shown. The spectrum was gen-erated by using an 1064 nm-laser with an energy of120 mJ and the detector was placed at 1 mm from thetarget surface. The dominant lines belong to neutrallead atoms, in addition to a couple of lines attachedto singly ionized lead ions. The strongest line of PbIis identified as 6p7s3P1→6p23P2 at 405.78 nm. Thesingly ionized lines at 424.68 nm and 438.85 nm areidentified as 5f2F7/2→6d2D5/2 and 5f2F5/2→6d2D3/2,respectively. In Fig. 3, the spectrum covering the regionof 500 nm to 670 nm is reproduced. The spectrum wasgenerated by using an 1064 nm-laser with an energy of120 mJ and the detector was placed at 1 mm from thetarget surface. In this region, most of the lines belongto singly ionized lead ions, in addition to a couple ofvery sharp PbI lines at 500.5 nm and 520.14 nm whichare identified as the transitions of 6p7s1P1→6p21S0 and6p8s3P1→6p21S0, respectively. Interestingly, the well-known sodium D lines at 588.99 nm and 589.59 nm arealso observed in the spectrum that supplements the cal-ibration of our spectrometers. The strongest PbII linein this region is at 560.80 nm which is identified as the

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Table 1. Spectroscopic parameters of the PbI lines

Wavelength λ Transitions Statistical weight Transition probability Upper level energy(nm) gk gi Aki(s

−1) Ek (cm−1)

217 6p6d3D1 → 6P23P0 3 1 1.84×108 46068.44224.68 6p7d3D2 → 6P23P1 5 3 4.49 × 106 53211.32239.37 6p7d3F3 → 6P23P2 7 5 6.47 × 107 52412.33240.19 6p7s1P1 → 6P23P1 3 3 2.80 × 107 49439.62241.17 6p7d3F2 → 6P23P2 5 5 1.86 × 107 52101.66244.62 6p8s3P1 → 6P23P1 3 3 2.45 ×107 48686.93247.64 6p7d3D1 → 6P23P1 3 5 3.78 ×107 48188.63257.73 6p8s3P1 → 6P23P1 3 3 6.68 ×107 49439.62261.43 6p6d3D2 → 6P23P1 5 3 1.9 ×108 46060.84266.32 6p7s3P2 → 6P23P2 5 5 7.1 ×107 48188.63280.20 6p6d3F3 → 6P23P2 7 5 1.6 ×108 46328.67283.31 6p7s3P1 → 6P23P0 3 1 5.8 ×107 35287.224287.33 6p6d3F2 → 6P23P2 5 5 3.7 ×107 45443.17322.05 6p7d3D1 → 6P21D2 3 5 1.66 ×107 52499.619357.27 6p7s1P1 → 6P21D2 3 5 9.9 ×107 49439.62363.95 6p7s3P1 → 6P31P1 3 3 3.4 ×107 35287.227367.15 6p8s3P1 → 6P31D2 3 5 4.4 ×107 48686.93368.35 6p7s3P0 → 6P31P1 1 0 1.5 ×108 34959.91373.99 6p7s3P2 → 6P21D2 5 5 7.30 ×107 48188.63401.97 6p6d3F3 → 6P23D2 7 5 3.5 ×106 46328.67405.78 6p7s3P1 → 6P21D2 3 5 8.9 ×107 35287.22416.80 6p6d3F2 → 6P21D2 5 5 1.2 ×106 45443.17500.54 6p7s1P1 → 6P21S0 3 1 2.7 ×107 49439.93520.14 6p8s1P1 → 6P21S0 3 1 1.9 ×107 48686.93

transition of 7p2P3/2 →7s2S1/2 and its other fine struc-ture component of 7p2P1/2→7s2S1/2 is observed at666.09 nm. The PbII line observed at 537.18 nm isidentified as 5f2F7/2→6s6p24P5/2. The other PbII linesdue to 7d2D3/2 →7p2P1/2 and 7d2D5/2 →7p2P3/2 wereobserved at 504.2 nm and 554.4 nm, respectively. ThePb II lines originating from the transitions of 7s2S1/2

→6p2P3/2 and 8s2S1/2 →7p2P1/2 were observed at220.4 nm and 678.8 nm, respectively. It is noticed thatthe lines of neutral lead atoms are strong and sharp,whereas those of the singly ionized lead ions are broad.

Fig.2 Emission spectrum of predominantly neutral and

ionized lead plasma generated by the 1064 nm laser cov-

ering the region of 350 nm to 450 nm

It is interesting to compare the widths of the linesoriginating from a common upper level 6p7s3P1 butdecay to the fine structure components of 6p23P0,1,2.These lines were observed at 283.31 nm, 363.95 nm and

405.78 nm, respectively. In accordance with the orbital-spin (LS) coupling selection rules, their relative inten-sities should be in accord with the ratio of their statis-tical weights i.e., 1:3:5. Indeed, their intensities followthis trend. This intensity behavior indicates that theplasma can be considered in the state of local thermo-dynamic equilibrium (LTE). The width of these linesat 283.31 nm was determined as 0.19 nm, which be-longs to the transition of 6p7s3P1→6p23P0, the lowestfine structure component. The width of other line at363.95 nm (6p7s3P1→6p23P1) is extracted as 0.25 nmand that of the line at 405.78 nm (6p7s3P1→6p23P2)as 0.33 nm. Their widths increase as the J-value of thelower state increases.

Fig.3 Emission spectrum of neutral atoms and predom-

inantly ionized lead ions generated by the 1064 nm laser

covering the region of 500 nm to 670 nm

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3.2 Electron temperature andelectron number density

Electron temperature was estimated through theBoltzmann plot method under the assumption thatthe plasma is in local thermodynamic equilibrium(LTE) [25∼28] by using

ln(

Iki λki

Akigk

)= ln

(N(T )U(T )

)− Ek

kT, (1)

where, Iki is the integrated line intensity of the tran-sition taking place between an upper level (k) and alower level (i), λki is the transition wavelength, Aki isthe transition probability, gk is the statistical weight oflevel (k), N(t) is the total number density, U(T ) is thepartition function, Ek is the energy of the upper level, kis the Boltzmann constant and T is the excitation tem-perature. A plot of the expression on the left hand sideagainst the upper level energy Ek yields a straight linewith a slope of (−1/kT ). Thus, the electron tempera-ture can be determined without the knowledge of thetotal number density or the partition function. The sys-tematic errors are bound to be present, therefore, thetemperature is extracted with an uncertainty of about10%, coming mainly from the transition probabilitiesand the measurement of the integrated intensity of thespectral lines.

Four lead lines at 357.27 nm, 363.95 nm, 368.52 nmand 373.99 nm are used to determine electron tem-perature. The electron temperature calculated for the1064 nm laser is 13450 K, and that for the 532 nm laseris 13550 K. The electron temperature has also been cal-culated as a function of the distance from the targetsurface for 1064 nm as well as for 532 nm laser that isshown in Fig. 4. For the 1064 nm laser, the temper-ature varies from 13450 K to 12850 K as the distanceis varied from close to the target surface up to 2 mm,whereas it varies from 13550 K to 12950 K in the caseof 532 nm laser as the distance is varied from close tothe target surface up to 1.5 mm. The electron temper-ature was also determined by varying the laser energyfrom 85 mJ to 125 mJ for the 1064 nm laser and from35 mJ to 75 mJ for the 532 nm laser. The tempera-ture increases from 14500 K to 16550 K in the first case(1064 nm) and from 14500 K to 17250 K for the secondcase (532 nm).

The electron density was determined from the lineprofiles of the isolated lead neutral lines at 363.95 nmand 373.99 nm and the singly ionized line at 537.18 nmusing the following relation [2,3,25] by neglecting thecontribution of the ion impact broadening:

∆λ1/2 = 2ω

(Ne

1016

), (2)

where ∆λ1/2 the width of the spectral line and ω theimpact broadening factor, Ne the electron number den-sity. The value of ω corresponding to different plasmatemperatures is obtained from reference data [29]. Theline profile of the PbI line at 363.95 nm recorded from

the plasma generated by the fundamental 1064 nm laseris shown in Fig. 5(a). The laser energy was varied froma minimum up to 135 mJ. The width of the line profileincreases with the increase in laser energy.

Fig.4 Variation of the excitation temperature along the

direction of propagation of the plume using fundamental

(1064 nm) and second harmonic (532nm) of a Nd:YAG laser

Fig.5 (a) Variation in the signal intensity and width of the

PbI line at 363.95 nm using 1064 nm laser, (b) Stark broad-

ened profile of the transition of PbI 6p6d3D2 →6p23P1 at

261.43 nm using 1064 nm laser. The full line represents the

Lorentzian fit to the experimental data points. The full

width half maxima of the spectra are used to estimate the

electron density

The Stark broadened profile of PbI 6p6d 3D2→6p23P1transition at 261.43 nm recorded using the 1064 nmlaser is shown in Fig. 5(b). The full line representsthe Lorentzian fit to the experimental data points.

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The full width half maxima of the spectra are usedto estimate the electron density. The electron densityclose to the target, for the fundamental 1064 nm laserwith a pulse energy of 135 mJ and the second har-monic 532 nm laser with a pulse energy of 75 mJis 7.3×1016 cm−3 and 9.2×1016 cm−3, respectively.It decreases to 4.3×1016 cm−3 and 8.0×1016 cm−3

at distances of 1.5 mm and 1.0 mm from the targetsurface, respectively, for PbI line at 363.95 nm, asshown in Fig. 6. The electron density calculated forthe PbII line at 666.09 nm, for fundamental (1064 nm)and second harmonic (532 nm) is 1.7×1017 cm−3 and3.4×1017 cm−3 and decreases to 1.30×1017 cm−3 and1.25×1017 cm−3 at distances of 1 mm and 1.5 mm fromthe target surface, respectively. It is observed that Ne

close to the target is maximum and decreases withthe increase in the distance from the target. Both thetemperature and the electron number density reachmaximum at the position close to the target, sincethe region close to the surface absorbs laser radiationcontinuously during the laser pulse. When the plasmaexpands, it transfers its energy to the surroundings.Moreover, it is transparent to the laser pulse; there-fore, the temperature and the density decrease alongthe direction of propagation of the plume. The tem-perature and density are different for the two modesof the Nd:YAG laser, because of the difference in theenergy per photon for each mode.

Fig.6 Variation of the electron number density along the

direction of propagation of the plume using fundamental

(1064 nm) and second harmonic (532 nm) of Nd:YAG laser

Both electron temperature and electron number den-sity were also determined for different values of thelaser energy. In both modes of the Nd:YAG laser,a same trend of the temperature variation as a func-tion of the laser energy was observed. The variationof the electron number density with the laser energyalso shows a similar behaviour. For the fundamen-tal mode (1064 nm) and the laser energy in a rangeof 85 mJ to 135 mJ, the electron temperature variesfrom 14500 K to 16840 K while the electron numberdensity varies from 5.7×1016 cm−3 to 7.2×1016 cm−3.The variation of the electron temperature as a func-tion of laser energy for the 1064 nm laser is shown inFig. 7. Using the second harmonic (532 nm) and vary-ing the laser energy from 35 mJ to 75 mJ, the electron

temperature varies from 14500 to 17500 K while theelectron number density varies from 7.7×1016 cm−3 to9.3×1016 cm−3. The electron density as a function ofthe laser energy is shown in Fig. 8. As the laser energyincreases, the observed increase in Ne and Te is dueto the absorption and/or reflection of the laser pho-ton by the plasma, which depends upon the plasmafrequency. In our experiment, the corresponding fre-quencies against two modes of the Nd: YAG laser are2.8×1014 Hz and 5.6×1014 Hz, respectively, whereasthe plasma frequency is νp =8.9×103

√Ne. For an

electron number density Ne of about 1016 cm−3, thenνp =3.6 ×1012 Hz which is below the laser frequencyof 1014 Hz, indicating that the energy loss due to thereflection of the Nd: YAG laser from the plasma is in-significant.

Fig.7 Variation of the excitation temperature with the

laser energy using fundamental (1064 nm) of Nd: YAG laser

Fig.8 Variation of the electron number density with the

laser energy using second harmonic (532 nm) of Nd:YAG

laser

The use of the emission spectroscopy to measureboth temperature and electron number density requiresoptically thin spectral lines. The self absorption de-pends on the oscillator strength, level energies degen-eracy, broadening parameters as well as the plasmaparameters. The lead plasma is observed to be op-tically thin. For self absorption a strong line wouldhave a dip at the central frequency (self absorption),however, in the present work no dip at the central fre-quency of the observed emission lines was found. The

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McWhirter’s [30] criterion was used to check the condi-tion for the validity of the local thermodynamic equilib-rium (LTE). The electron number densities determinedin our experiments are well within the range in whichthe LTE conditions are valid [2]:

Ne ≥ 1.6× 1012T 1/2(∆E)3, (3)

with Ne (cm−3) the electron number density, T (K) theplasma temperature and ∆E (eV) the difference in theenergy between the upper and lower state of all theinvestigated transitions.

4 Conclusion

A Q-switched Nd:YAG laser at its fundamental(1064 nm) and second harmonic (532 nm) was used tostudy the laser produced lead plasma. The emissionspectrum of the plasma reveals transitions of neutralatoms and singly ionized lead ions. The electron tem-perature and the electron number density were been de-termined along the axial positions of the plasma plume.It is observed that both electron temperature and elec-tron density reach maximum at the position close tothe target, and decrease along the distance from thetarget. Variations of the electron temperature and theelectron number density with the laser energy indicatethat both increase with the increase in laser energy.The electron temperature calculated for the 1064 nmand the 532 nm lasers is 13450 K and 13550 K, respec-tively, which is in close agreement with that reportedby CASTLE et al. [10] and ALONSO-MEDINA [29] as14000 K and 11200 ± 250 K, respectively.

Acknowledgements

The research project was financially supported bythe Higher Education Commission (HEC) and PakistanScience Foundation (PSF-134). M. HANIF is thankfulto MCS, National University of Sciences & Technology(NUST) for encouragement in terms of provision of timeand financial support to carry out research work.

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(Manuscript received 19 August 2010)(Manuscript accepted 14 February 2011)E-mail address M. HANIF:hanif muhammad@yahoo.com,drhanif-mcs@nust.edu.pk

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