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Investigation of Influence of Hydrostatic Pressure on Double-Pulse Laser-Induced Breakdown

Spectroscopy for Detection of Cu and Zn in Submerged Solids

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2013 Appl. Phys. Express 6 042403

(http://iopscience.iop.org/1882-0786/6/4/042403)

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Investigation of Influence of Hydrostatic Pressure on Double-Pulse Laser-Induced Breakdown

Spectroscopy for Detection of Cu and Zn in Submerged Solids

Tomoko Takahashi�, Blair Thornton, and Tamaki Ura

The University of Tokyo, Meguro, Tokyo 153-8505, Japan

E-mail: takahas@iis.u-tokyo.ac.jp

Received February 14, 2013; accepted March 22, 2013; published online April 11, 2013

The effects of pressure on double-pulse laser-induced breakdown spectroscopy (LIBS) for analysis of the composition of solids submerged in

water have been investigated. It has been found that while an increase in water pressure results in an overall reduction in plasma temperature and

increased broadness in the observed spectra, analytically useful spectra can be observed up to 5MPa (50 atm). The results suggest that double-

pulse laser-induced breakdown spectroscopy may be suitable for in situ measurement of the chemical composition of solids submerged in lakes,

rivers, and shallow seas. # 2013 The Japan Society of Applied Physics

The technique of laser-induced breakdown spectros-copy (LIBS) is suitable for in situ elemental analysisbecause it does not require any sample preparation

and the results are obtained in real time. LIBS is a form ofatomic emission spectroscopy that analyzes light emittedfrom atoms and ions of ablated material in a plume createdby focusing a high-power pulse laser on a sample. Since theatoms and ions of the laser-ablated material emit specificwavelengths of light, elemental analysis of solids immersedin a transparent liquid, such as water, should be fundamen-tally possible. The authors previously demonstrated thatwell-resolved emission spectra from solids submerged inwater can be observed after a low-energy single pulse of�10 ns duration at pressures up to 30MPa (300 atm).1) It isoften reported that improved spectral features can beobserved by using a double-pulse laser excitation techniqueunderwater at atmospheric pressure because the first laserpulse can produce a cavitation bubble in the liquid allowingthe plume generated by the second pulse to spread insidethe bubble.2) Since a plume with low optical density canbe obtained in this way, well-resolved spectra can beobserved.3) It has been reported, however, that the double-pulse method is not effective at pressures greater than 10 and14.6MPa for liquids and immersed solids, respectively.4,5)

Experiments performed by the authors also found noobservable signal enhancement using a double pulse atpressures over 10MPa. A possible mechanism for this is thatthe optical densities of the plumes generated by the secondpulse increase at high pressure since the cavitation bubblegenerated by the first laser pulse becomes smaller as thehydrostatic pressure increases.

In this study, the effectiveness of double-pulse LIBS foranalysis of the composition of solids submerged in water athigh pressure is discussed by evaluating the relation betweenthe spectra and the density of material ablated from thetarget in the plume (hereafter, density). The results arediscussed in comparison with those that are obtained usinga single pulse.

The experimental setup for imaging and spectroscopicmeasurements at high pressure is shown in Fig. 1. The laserpulses are generated using two 1064 nm Nd:YAG Q-switched lasers, each delivering a pulse of 8 ns durationvia the same 1000 �m fused-silica fiber. The lasers arefocused using a pressure-resistant fused silica objective lenswith a focal length of 7mm in water. The solid targets areplaced in a sample holder that is attached to the objective

lens so that the distance to the target remains fixed regardlessof changes in hydrostatic pressure. The system is put intoa water-filled chamber that can be pressurized using apump. Shadowgraph imaging of the plumes was performedusing an intensified charge-coupled device (ICCD; PrincetonInstruments PiMAX 1024i) together with a long-rangeobjective (Union Optics DZ4-T ZC15). Measurements ofthe amount of material ablated from the target after laserirradiation were made using a 3D laser scanning microscope(Keyence color VK-9700 series). Spectroscopy was per-formed by observing the light emitted from the plumethrough the same fiber used for laser delivery to observethe plumes normal to the surface of the target in order tomaximize the visible cross section. The spectrograph has afocal length of 150mm with a light throughput of f =4 (ActonResearch Spectra Pro 2150). A 1200 groove/mm grating anda 50 �m wide entrance slit are used, and the spectra arerecorded using an ICCD camera as a detector.

Since most of the energy of the secondary pulse isabsorbed within the volume of ejecta produced by theprimary pulse in the double-pulse LIBS method,6) thedensity of the plume observed during double-pulse LIBSmeasurements is assumed to be nearly equal to the density ofthe ablated material in the cavitation bubble just before thesecondary pulse is irradiated. The following experimentswere carried out: shadowgraph imaging to measure thevolume of the cavitation bubbles, measurement of materialablated from the target using a three-dimensional (3D) laserscanning microscope, and spectroscopy. By comparing thedensity of the cavity calculated from the results of the first

Fig. 1. Experimental setup.

Applied Physics Express 6 (2013) 042403

042403-1 # 2013 The Japan Society of Applied Physics

http://dx.doi.org/10.7567/APEX.6.042403

two experiments with the spectra obtained, the effects ofthe density are analyzed as a factor influencing the qualityof the spectra.

Shadowgraph imaging was performed to determine thevolumes of the cavities generated after irradiation of a singlepulse for pulse energies of 5, 15, 25, and 35mJ, respectively,at pressures of 0.1, 2, and 5MPa using the method describedby the authors in Ref. 2. The mass of the material ablatedwas determined for each energy and pressure by scanningthe volume of craters formed in the target after focusing onehundred pulses onto the same spot under each condition. Themass of the material ablated by a double pulse was alsomeasured to confirm that the energy from the second pulse isprimarily absorbed by the material ablated by the first pulse.From the results, it was found that, for the pulse energiesat which experiments were performed, the second pulsedoes not increase the total amount of ablated material, inaccordance with the results of Forsman et al.6)

To discuss the influence of density on the quality ofthe spectra observed after the second pulse, we calculatedthe densities at the moment the second pulse is fired bydividing the amount of material ablated by the volume of thecavitation bubble. Figure 2 shows density as a function oftime after irradiation of a single pulse. It can be seen that thedifference of densities between each pressure is negligibleat 2 �s, shortly after pulse irradiation. However, beyond thistime, the volumes of cavities reduce with increased hydro-static pressure.7) Since the effect of external pressures on themass ablated is negligible,8) the density of the material in thecavity increases with increased pressure.

Spectroscopic measurements were performed for thedifferent pulse energies where the time differences betweenirradiation of the first and second pulses were set based onthe plume densities determined from the previous experi-ments. A time difference of 2 �s was chosen to study plumesof high density. The time difference when the density is itsminimum under each energy and pressure condition waschosen for low density plumes, and the half of this timedifference was chosen for comparison. The hydrostatic pres-sure was varied between 0.1, 2, and 5MPa. The energy ofthe first laser pulse, E1 (mJ), and that of the second, E2 (mJ),were set as follows: ðE1; E2Þ = ð15; 25Þ, ð25; 25Þ, andð35; 25Þ, and the average of ten measurements under each

condition was used. In each case, the delays were optimizedto achieve the largest signal to noise ratio. The gate widthwas set at 1000 ns in all cases, and the energy of the secondpulse was set to 25mJ. Figure 3 shows the spectra observedfrom plumes generated on a metallic plate immersed inwater. Figures 3(a) to 3(c) are the spectra obtained by doublelaser pulse at pressures of 0.1, 2, and 5MPa, respectively,and Fig. 3(d) is a superimposed plot for the spectra observedafter a single laser pulse at pressures of 0.1, 2, and 5MPa. Inthe experiments using a single pulse, the delays of 600 nsand the gate width of 1000 ns were used. The pulse energyfor the single-pulse measurements was set to 5mJ sincewell-resolved spectra cannot be seen if the energy of thesingle pulse is more than 10mJ. The lines seen in thespectra correspond to Zn I at 481 nm 3S1 ! 3P2, Cu I at511 nm 2P3=2 ! 2D5=2, 515 nm

2D3=2 ! 2P1=2, and 522 nm2D5=2 ! 2P3=2. Concerning the spectra observed for doublelaser pulses, the peaks are generally broader at high pressurethan those at atmospheric pressure. However, well-resolvedlines can be seen at all pressures tested, and the signals aremore intense than those measured using a single pulse upto a pressure of 5MPa, which is related to the differenceof densities between each pressure. Giacomo et al.5) andLawrence-Snyder et al.4) reported that when solids andliquids are pressurized over 14.6 and 10MPa, respectively,the spectra deteriorated with no observable benefits of usinga double pulse. Meanwhile, for measurements using a singlepulse, the influence of the pressure on the peak signals is notseen as previously reported, since the effects of pressure onthe size of the cavities and their densities are negligibleduring the early stages of ablation during which measure-ments are made after a single pulse.1,8) Nevertherless, thepeaks have lower intensities than for the double pulse at allconditions in this study. The temperature of the plumesduring spectroscopic measurements can be calculated usingthe two-line method with the Cu I peaks at 522 and 511 nm,

0 50 100 150 200 250 300

10−4

10−3

10−2

10−1

time, μs

dens

ity, g

/cm

3

0 5 10 15

10−4

10−3

10−2

10−1

0.1 MPa2 MPa5 MPa

Fig. 2. Density of the plume as a function of time after the pulse is fired,

E1 ¼ 15mJ.

480 500 520

1000

2000

3000

4000

5000

wavelength, nm

inte

nsity

, arb

. uni

t ΔT = 180 μs

480 500 520400

600

800

1000

1200

1400

wavelength, nm

ΔT = 9 μs

480 500 520

500

1000

1500

2000

2500

ΔT = 7 μs

inte

nsity

, arb

. uni

t

wavelength, nm480 500 520

400

600

800

1000

1200

wavelength, nm

0.1 MPa

2 MPa

5 MPa

Zn

Cu

Cu

Cu

(b)(a)

(d)(c)

Fig. 3. Comparison of spectra measured for brass at (a) 0.1, (b) 2, and

(c) 5MPa, and (d) a 5mJ single pulse. The E1 (mJ) is 15mJ, E2 (mJ) is

25mJ and the time difference between two pulses, �T , is the time when the

density is its minimum.

T. Takahashi et al.Appl. Phys. Express 6 (2013) 042403

042403-2 # 2013 The Japan Society of Applied Physics

which correspond to transitions at different energy levels.It can be seen that for measurements using a double pulse atatmospheric pressure, the ratio of the intensities of the 522and 511 nm peaks is high. Since the energy levels of thetransition at 522 nm are higher than at 511 nm, this indicatesthat the temperature of the plume for the double pulse atatmospheric pressure is higher than the temperature at highpressure and when using a single pulse.

Figure 4 shows the temperature of the plume and the fullwidth at half maximum (FWHM) of the peak correspondingto Cu I at 522 nm as functions of density. It can be seen thatas the density of the cavity at the time at which the secondpulse is fired increases, the FWHM of peaks becomesbroader and the temperature of the plume is reduced. It canalso be seen that measurements made at different pressuresand energies fit the same trend on the density plot, and it

is suggested that these parameters do not directly influencethe signal quality, but only affect the signal quality throughthe resulting change in density which they cause. Therelationship between temperature of the plume and signalbroadness with density is nonlinear, and the effect ofchanges in density on the temperature of the plumes andFWHM of the peaks becomes less pronounced at higherdensities, which explains why the effects of changes inpressure from 0.1 to 2MPa are larger than that of thechanges seen between 2 and 5MPa. Furthermore, ourexperiments demonstrate that, at pressures of up to 5MPa,the double-pulse LIBS method is superior to single-pulseLIBS. The results suggest that double-pulse LIBS is asuitable technique for in situ elemental analysis up to50 atm, such as in rivers, lakes, and continental shelves.

1) B. Thornton and T. Ura: Appl. Phys. Express 4 (2011) 022702.

2) V. Lazic, F. Colao, R. Fantoni, V. Spizzichino, and S. Jovicevic:

Spectrochim. Acta, Part B 62 (2007) 30.

3) T. Sakka, H. Oguchi, S. Masai, and Y. H. Ogata: Chem. Lett. 36 (2007) 508.

4) M. Lawrence-Snyder, J. Scaffidi, S. M. Angel, A. P. M. Michel, and A. D.

Chave: Appl. Spectrosc. 61 (2007) 171.

5) A. De Giacomo, A. De Bonis, M. Dell’Aglio, O. De Pascale, R. Gaudiuso,

S. Orlando, A. Santagata, G. S. Senesi, F. Taccogna, and R. Teghil: J. Phys.

Chem. C 115 (2011) 5123.

6) A. C. Forsman, P. S. Banks, M. D. Perry, E. M. Campbell, A. L. Dodell, and

M. S. Armas: J. Appl. Phys. 98 (2005) 033302.

7) K. Sasaki, T. Nakano, W. Soliman, and N. Takada: Appl. Phys. Express 2

(2009) 046501.

8) B. Thornton, T. Takahashi, T. Ura, and T. Sakka: Appl. Phys. Express 5

(2012) 102402.

Fig. 4. Temperature and peak FWHM as functions of density.

T. Takahashi et al.Appl. Phys. Express 6 (2013) 042403

042403-3 # 2013 The Japan Society of Applied Physics