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emporal dependence of the enhancement ofaterial removal in femtosecond–nanosecond

ual-pulse laser-induced breakdown spectroscopy

on Scaffidi, William Pearman, J. Chance Carter,ill W. Colston, Jr., and S. Michael Angel

Despite the large neutral atomic and ionic emission enhancements that have been noted in collinear andorthogonal dual-pulse laser-induced breakdown spectroscopy, the source or sources of these significantsignal and signal-to-noise ratio improvements have yet to be explained. In the research reported herein,the combination of a femtosecond preablative air spark and a nanosecond ablative pulse yields eightfoldand tenfold material removal improvement for brass and aluminum, respectively, but neutral atomicemission is enhanced by only a factor of 3–4. Additionally, temporal correlation between enhancementof material removal and of atomic emission is quite poor, suggesting that the atomic-emission enhance-ments noted in the femtosecond–nanosecond pulse configuration result in large part from some sourceother than simple improvement in material removal. © 2004 Optical Society of America

OCIS codes: 140.3440, 300.2140, 300.6210.

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. Introduction

aser-induced breakdown spectroscopy �LIBS�, intro-uced by Brech and Cross in 1962,1 is an analyticalechnique in which one or more high-power, short-ulse lasers are used to generate a high-temperature,harge-neutral laser-induced plasma �LIP� from a con-uctive or a nonconductive solid, liquid, or gas.2–5

ased on the wavelengths and intensities of neutraltomic and ionic emission in that ablative plasma, theomposition of the ablated sample can be determinedor a wide range of industrially6–10 and environmen-ally important elements.11–15 Unfortunately, LIBSas yet to fulfill its potential. Limits of detectionhen a single nanosecond ablative pulse is used �here-

J. Scaffidi and S. M. Angel �[email protected]� are withhe Department of Chemistry and Biochemistry, University ofouth Carolina, Columbia, South Carolina 29208. W. Pearman isith the Department of Chemistry, U.S. Military Academy, Westoint, New York 10996. When this research was performed, J. C.arter and B. W. Colston, Jr., were with the Medical Technologyrogram, Lawrence Livermore National Laboratory, Livermore,alifornia 94550. They are now with the M Division�Forensiccience Center and the Biodefense Knowledge Center, respec-ively, also at the Lawrence Livermore National Laboratory.

Received 20 February 2004; revised manuscript received 6 Sep-ember 2004; accepted 21 September 2004.

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492 APPLIED OPTICS � Vol. 43, No. 35 � 10 December 2004

fter referred to as ns SP LIBS� are generally limitedo the low parts per million or the high parts per billionor many elements, relative standard deviations arereater than those seen in other elemental analysisechniques �atomic absorption or inductively coupledlasma mass spectroscopy, for example�, and matrixffects can be quite significant.2–5 At the same time,owever, the technique’s great potential continues toraw researchers’ attention: As a purely optical tech-ique that theoretically is able to analyze virtually anyolid, liquid, or gaseous sample in environments asiverse as the deep sea or the Martian surface,16 LIBSas the potential to become a means of rapid, remote,irtually nondestructive on-site, on-line, and in situultielemental microanalysis.17–21 As a result, thereave been many attempts to improve the technique’snalytical utility by ultrashort picosecond23,24 andemtosecond25,27 �fs� pulses and by manipulating sam-le temperature,28 atmospheric pressure,29–31 andtmospheric29–31 composition.Given the practical difficulty of controlling these fac-

ors and of using ultrashort pulses in on-site and initu applications, however, research has turned towardnding other means of improving LIBS’s limits of de-ection. Examinations of the advantages of firingultiple nanosecond �ns� pulses within several micro-

econds of one another have been promising,32–41 forxample, showing 30-fold neutral atomic emission en-
ancement for silicon and as much as 100-fold ionic

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mission enhancement for aluminum when paired nsulses are used in a collinear configuration35 and asuch as 33-fold neutral atomic emission and signal-

o-noise ratio enhancement for copper when two nsulses are used in an orthogonal configuration.37 Un-ortunately, there is as yet no single explanation thatan entirely account for both the intensity and theongevity of the emission enhancements that are seenhen multiple ns pulses are used in either collinear orrthogonal dual-pulse LIBS �hereafter referred to ass–ns DP LIBS�. Theories that rely solely on ener-etic coupling between the first and second pulses ig-ore the large neutral atomic emission enhancementseen in the orthogonal ns–ns case at long interpulseelay,22,37–39 and explanations that rely entirely on theotential for reduction in atmospheric pressure andample heating effects due to the first LIP do not ap-ear to explain fully the 100-fold ionic emission en-ancements seen at short interpulse delay in theollinear configuration.35 In all probability, all threef these �and, potentially, other� factors combine toield the large neutral atomic and ionic emission en-ancements reported in ns–ns DP LIBS.Following recent research showing approximately

hreefold neutral atomic emission enhancement forluminum in bulk aluminum and for copper in brasshen a fs air spark and a ns ablative pulse are used

n fs–ns orthogonal preablation spark dual-pulseIBS40,41 �hereafter referred to as fs–ns DP LIBS�,nd, given the greatly increased crater volumes seenn the orthogonal ns–ns configuration,38,39 we havexamined the significance of improved sample re-oval in generation of the neutral atomic emission

nhancements seen in previous research with fs–nsP LIBS, with the hope of at least partial applicabil-

ty in understanding analytical improvements in col-inear and orthogonal ns–ns DP LIBS. Large per-hot depth and mass removal improvements arenteresting in their own right, showing a strong de-endence on the delay between the fs preablative airpark and the ns ablative pulse �referred to as inter-ulse delay or td�. Additionally, whereas mass re-oval calculations for the holes produced in this

esearch indicate that as much as ten times morenalyte is removed by every ns ablative pulse in fs–nsP LIBS than in ns SP LIBS, the poor correlationetween material removal enhancement and atomicmission enhancement suggests that although im-rovement in mass removal may play a role in theeutral atomic emission enhancements seen in therthogonal fs–ns pulse configuration, it is likely nothe primary source of emission enhancement in fs–nsP LIBS.

. Experiment

. Equipment

igure 1 shows the hardware setup for this study. A0-mJ, 100-fs, 800-nm pulse from a Ti:sapphire laserdescribed in full detail elsewhere42� operating at 5z was focused with a 100-mm focal-length fused

ilica lens to generate a short-lived LIP above the w

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urface of a solid aluminum or brass sample, with aitrogen and oxygen emissive lifetime of approxi-ately 6 �s. Air spark formation was followed–200 �s later �noted hereafter as an interpulse de-

ay of 0 to �200 �s, following the convention estab-ished in previous research by use of the orthogonalual-pulse configuration39� by ablation of the solidample with a 150-mJ, 5-ns, 1064-nm pulse from ad:YAG laser �Surelite III, Continuum�, also focusedith a 100-mm focal-length fused silica lens. Theptimal ablative focus and interspark crossing wereetermined by maximizing the signal-to-noise ratio ofackground-corrected neutral atomic emission at 396m for aluminum and at 521 nm for copper in brass

n both ns SP LIBS and fs–ns DP LIBS,40,41 resultingn a pulse configuration in which the fs air spark wasocused 0.6 mm above the sample surface and the nsblative pulse was focused to a 150-�m spot on theample surface, crossing the fs air spark approxi-ately halfway along its length.Atomic emission from the dual-pulse plasma was

ollected coaxially with the ns ablative pulse �Fig.�. Following collimation with the 100-mm focal-ength fused silica lens used to focus the ablativeulse and transmission through a 1064-nm dichroicirror, plasma emission was focused onto a 2-mm-

iameter light guide �N.A., 0.51; Edmund Scientific�y a pair of 100-mm focal-length fused silica lensesnd spectrally resolved with a 0.25-m spectrograph250IS�RF, Chromex� with a 1200-groove�mm grat-ng blazed at 500 nm. Background-corrected emis-ion intensity was then measured as a function of

ig. 1. Experimental setup: A fs pulse from laser 1 forms areablative air spark above and parallel to the surface of a solidample several microseconds before ablation with a ns pulse fromaser 2. Neutral atomic and ionic emission from the resultantblative plasma is collected along the axis of the ablative pulse,ocused onto a light guide or an optical fiber, spectrally resolvedith a spectrometer �Spec�, and quantified with an intensifiedated CCD �ICCD� and a computer.

avelength with an intensified gated CCD � I-MAX

0 December 2004 � Vol. 43, No. 35 � APPLIED OPTICS 6493

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024E 18�G�II P43, Princeton Instruments� withts gate width set to 200 ns and its gate delay set toegin acquisition 5 �s after ablation. This gateelay was not only found to yield the optimal signal-o-noise ratio in the current study but was also aelay at which background-corrected atomic emis-ion intensity enhancements were comparable withignal-to-noise ratio enhancements. All spectralntensity measurements are the result of five rep-icate measurements of ten shots each.

. Procedures

hin metal sheets �0.20-mm-thick commercial-gradeluminum and 0.40-mm commercial-grade brass,oth of indeterminate composition� were attached toissimilar mounting blocks �aluminum to a brasslock and vice versa� with double-sided tape �Fig.�a��, and the number of shots required for sampleenetration �defined as the point at which neutraltomic emission associated with the brass or alumi-um mounting block first became visible in the LIBSlasma� was monitored as interpulse delay was var-
ed from 0 to �200 �s. Scanning electron micro- t

raphs �SEMs� were subsequently taken of both theront �ablation side� and the back �nonablation side�urfaces of the thin metal samples �Fig. 3� to permits to determine hole radii for the five holes sequen-ially drilled at each interpulse delay examined inhis study. Using volumes found from these radiind the number of shots for sample penetration �Fig.�, we calculated per-shot mass removal rates as aunction of td �Fig. 5�. Finally, we compared theemporal dependence of the enhancement of neutraltomic emission and material removal �Fig. 6� to ex-mine the potential role of increased analyte massntroduction into the ablative plasma as a source ofhe approximately threefold neutral atomic emissionnhancements previously observed in the orthogonals–ns dual-pulse configuration.

. Hole Volume and Per-Shot Mass Removalalculations

ecause of the conical crater profiles described in theingle-pulse and dual-pulse LIBS literature, andiven evidence suggesting a similar hole profile in theurrent study, we chose to calculate hole volumesased on a conical model. SEMs of both the top and

ig. 2. Hole profiling and volume calculation: �a� Thin metalamples �S� were anchored to dissimilar mounting blocks �B� withouble-sided tape. Crater volume and per-shot mass removalere calculated as described in the text, by use of the flat-bottomed

ig. 3. Hole size dependence on interpulse delay: SEMs of theront �ablation� and back �nonablation� sides of the holes drilled inhin aluminum �A, B� and brass �C, D� sheets illustrate the strongependence of hole radius on interpulse delay. Although little, ifny, change in hole size is visible at td � 0, both the front �A, C� andhe back �B, D� hole radii show rapid increases up to td � �10 �s.he hole diameter appears to decrease slowly after that point, andignificant substantial back-side scarring of the aluminum sample

he bottom surfaces of the thin metal samples al-

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owed us to measure the laser-drilled hole radii �Fig.�b�, rtop and rbottom�. We then calculated hole vol-mes based on these hole radii and the sample thick-ess T �0.20 mm for aluminum and 0.40 mm forrass�, given that

tan � � T��rtop � rbottom�, (1)

here T is the sample thickness and � is the angleetween the hole wall and the top ablative surface ofhe sample, as shown in Fig. 2�c�. By similarity,

ig. 4. Sample penetration versus interpulse delay: A generallyimilar trend is seen for fs–ns DP LIBS of both aluminum �a� andrass �b�. Introduction of the fs air spark before the ns ablativeulse shows little change at an interpulse delay of 0, followed by arecipitous drop at short interpulse delay. After a minimum of 12hots at td � �2.5 �s for aluminum and 170 shots at td � �5 �sor brass near the time the fs plasma ceases neutral atomic emis-ion �6 �s after fs plasma formation�, the number of shots requiredor sample penetration begins a slow return toward ns single-pulseevels. Error bars represent 2 standard deviations, n � 5, andhe number of single nanosecond pulses required for sample pen-tration is shown to the right of td � 0 for visual comparison withhe dual-pulse data.

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here h is the height of a theoretical cone with basengle � and base radius rtop. The volume �Vcone� ofhat theoretical cone, then, is

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nd the volume of the hole is the volume of the the-retical cone minus the volume of a cone with basengle � and base radius rbottom:

Vcrater � Vcone � ��3�rbottom2�h � T�

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e calculated the total mass removal by multiplyinghe hole volume by the sample’s density �2.75 g�cm3

or aluminum, 8.47 g�cm3 for brass� and found per-hot mass removal by dividing that total mass by the

ig. 5. Per-shot mass removal versus interpulse delay: As in thease of the number of shots required for sample penetration �Fig.�, aluminum �a� and brass �b� show a generally similar trend inerms of dependence of mass removal on interpulse delay. Fol-owing a slight decrease in per-shot mass removal at td � 0, bothluminum and brass demonstrate approximately an eightfold im-rovement in mass removal �with maxima at td � �2.5 �s and td �5 �s, respectively� before a slow fall toward ns single-pulse mass

emoval rates. Error bars represent 2 standard deviations, n �, and the number of single nanosecond pulses required for sampleenetration is shown to the right of td � 0 for visual comparisonith the dual-pulse data.

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he presumption that ablation rates were reasonablyonstant regardless of hole depth owing to the thin-ess of the metal samples used in this study.

. Results and Discussion

s discussed above, although use of multiple ns laserulses in both collinear and orthogonal pulse config-rations results in quite pronounced analytical im-rovements in LIBS, the cause or causes of the largeeutral atomic35,37 and ionic35 emission enhance-ents have yet to be fully explained. Previous stud-

es with fs–ns DP LIBS showed as much as threefoldeutral atomic emission enhancement for copper inulk copper in the fs–ns DP LIBS configuration, andore recent, more thorough research41 has shown

omparable neutral atomic emission enhancementor both aluminum in bulk aluminum and copper inrass at interpulse delays as large as �140 �s, sug-esting that one or more of the sources of atomicmission enhancement thought to be present at longnterpulse delay in the ns–ns DP configuration maylso be present in fs–ns DP LIBS. As a result, andiven the large ablation improvements described

ig. 6. Dependence of neutral atomic emission enhancement onnterpulse delay for aluminum and brass: Neutral atomic emis-ion at 396 nm for aluminum in bulk aluminum �a� and at 521 nmor copper in brass �b� are both significantly enhanced by introduc-ion of a fs preablation air spark in fs–ns DP LIBS, but the tem-oral dependence of those enhancements differs greatly as aunction of interpulse delay.

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nhancement in the ns–ns case,37–39 an examinationf their relevance in the generation of fs–ns DP LIBStomic emission enhancements has potential appli-ability to explaining the cause or causes of atomicmission enhancement in ns–ns DP LIBS.

. Temporal Dependence of Hole Diameter

EMs were taken of the ablation holes produced inhis study as part of the volume calculation procedureescribed above, and representative images arehown in Fig. 3. Front and back images of alumi-um and brass at an interpulse delay of 0 �s showole profiles similar to those seen in ns SP LIBS. Asarly as an interpulse delay of �0.5 �s, however,ncreased hole diameters become visible in both frontnd back micrographs for aluminum and brass, indi-ating the beginnings of increased hole volume in thes–ns pulse configuration.

Hole diameters for both materials continue theirrowth until a plateau is reached around an inter-ulse delay of �10 �s, after which hole diametersuch larger than those generated in the ns single-

ulse configuration can be seen until an interpulseelay of at least �100 �s. Hole profiles at td � �200s �not shown� essentially match those seen in ns SPIBS, with the exception of localized back-side scar-ing of the 0.20-mm-thick aluminum sample �similaro that seen in Fig. 3�b� at interpulse delays of �50nd �100 �s�. The cause of this scarring has yet toe determined, but it is suspected that sample heat-ng by the fs air spark and the ns ablative LIP maylay a role.

. Temporal Dependence of Sample Penetration

ntroduction of a fs preablative air spark results in aapid reduction in the number of shots required forenetrating the thin metal samples used in this re-earch, especially at short interpulse delay. For alu-inum �Fig. 4�a��, for example, the average number

f pulses required for penetrating a 0.20-mm thickheet decreases from 80 shots for ns SP LIBS to 12hots for fs–ns DP LIBS at an interpulse delay of2.5 �s. The number of shots for penetration hov-

rs about that minimum for approximately 12.5 �sefore starting a slow climb toward ns single-pulseevels. A slightly slower rate of return toward the nsingle-pulse level is seen after a brief plateau at annterpulse delay of �40 �s, reaching the 80-pulse nshreshold at an estimated interpulse delay of approx-mately �140 �s, then exceeding it at least until annterpulse delay of �200 �s.

The behavior of 0.40 mm thick brass in fs–ns DPIBS �Fig. 4�b�� is generally similar to that seen for.20-mm aluminum �Fig. 4�b��. An initial rapid de-rease to 120 shots at an interpulse delay of �5 �srom the 400 shots needed in the ns single-pulse cases followed by a slow climb to a plateau of 170 shots atn interpulse delay of approximately �40 �s, andhis plateau is followed by an even slower climb to-ard the ns single-pulse levels. Unlike 0.20-mm-

hick aluminum, however, 0.40-mm brass does not
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200 �s and, instead, shows a 30% reduction in theumber of shots required for sample penetration athat td. This difference may be the result of differ-nt sample thicknesses, different sample composi-ions, or simply due to the large error bars forluminum in Fig. 4�a�.

. Temporal Dependence of Mass Removal

fter an initial reduction in per-shot mass removalor fs–ns DP LIBS of aluminum at td � 0 �s �calcu-ated relative to the rate of material removal seen ins SP LIBS optimized for maximal signal-to-noiseatio for background-corrected emission at 396 nm�resumed to be a result of plasma shielding by the fsir spark at short interpulse delay, a rapid improve-ent is seen until a maximum of 1.15 �g�shot at an

nterpulse delay of �2.5 �s �Fig. 5�a��. This sharpncrease in material removal is followed by a lessapid decrease toward ns single-pulse levels until alateau of 0.45 �g�shot at an interpulse delay of �40s. After this brief interruption, mass removal con-

inues to fall at a further reduced rate, reaching thes single-pulse level of 0.11 �g�shot at an interpulseelay somewhere from �100 to �200 �s.The trend for mass removal in fs–ns DP LIBS of

.40-mm-thick brass �Fig. 5�b�� generally resembleshat seen for aluminum removal at short interpulseelay. A rapid improvement in removal rate �calcu-ated with respect to the per-shot material removalate for ns SP LIBS optimized for maximal signal-to-oise ratio for background-corrected atomic emissiont 521 nm� is seen after the initial reduction at td ��s, reaching a maximum of 0.62 �g�shot at an

nterpulse delay of �5 �s. This maximum is fol-owed by a reduction in the removal rate until annterpulse delay of approximately �40 �s, at which arief plateau of approximately 0.27 �g�shot is seen.ass removal continues its decay after this plateau,

ut, unlike 0.20-mm-thick aluminum, 0.40-mm brasstill demonstrates slightly enhanced mass removal0.090 �g�shot for fs–ns DP LIBS versus 0.075 �g�hot for ns SP LIBS� at an interpulse delay of �200s.

. Correlation of Mass Removal and Neutral Atomicmission Enhancements

igures 5 and 6 show the dependence of mass re-oval enhancement �Figs. 5�a� and 5�b�� and neutral

tomic emission enhancement �Figs. 6�a� and 6�b�� onnterpulse delay for td from 0 to �200 �s. Both alu-

inum �Fig. 5�a�� and brass �Fig. 5�b�� show approx-mately eightfold material removal enhancement athort interpulse delay, with maxima of 7.4X at td �2.5 �s for aluminum and of 8.5X at td � �5 �s for

rass. These large increases in mass removal areccompanied by significant improvements in neutraltomic emission, with 3.5X enhancement in alumi-um emission at 396 nm at interpulse delays as longs td � �55 �s �Fig. 6�a�� and approximately 2.5Xopper emission enhancement at 521 nm at td � �3.5s �Fig. 6�b��. Aside from the difference in magni-

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ancements are compared, their temporalissimilarity is quite apparent. Whereas enhance-ent of mass removal shows a rapid decline for both

luminum �Fig. 5�a�� and brass �Fig. 5�b�� at inter-ulse delays greater than td � �10 �s, large neutraltomic emission enhancements persist until inter-ulse delays larger than �100 �s and display a farlower fall toward ns single-pulse levels than is seenor enhancement of material removal. The potentialignificance of these temporal dependencies are dis-ussed below.

. Implications

iven the difference in sample thickness �0.20 mmor aluminum and 0.40 mm for brass� and the well-ocumented dependence of per-shot mass removal onrater depth in both ns and fs single-pulse LIBS,43

etailed conclusions would be premature despite theow aspect ratio of the holes in this study �2 for brass,

for aluminum�. Some general conclusions, how-ver, can reasonably be reached. First, based on theumber of shots required for sample penetration, cra-er diameters, and per-shot mass removal rates, in-erpulse delay clearly plays an important role inaterial removal in fs–ns DP LIBS, as it does ins–ns DP LIBS.39 Second, despite differences inaterial thickness, a generally similar mass removal

nhancement profile is seen for both aluminum andrass at short interpulse delays �td from 0 to �40 �s�,uggesting that generally similar causes of materialemoval enhancement may exist for the two materi-ls at those interpulse delays. The substantial de-iations seen after �40 �s cannot yet be explained,ut the temporal dependence shown in Figs. 5 and 6ndicates that although increased material removal

ay play some role in the atomic emission enhance-ents seen in fs–ns DP LIBS at short interpulse

elay, atomic emission enhancements at longerelays—at which fairly large neutral atomic emissionnhancements remain visible for both copper in brassnd aluminum in bulk aluminum while mass re-oval has fallen to ns SP LIBS levels—cannot be

ttributed to increased analyte concentration in thes–ns DP LIBS LIP. As a result, we must concludehat some other factor or factors cause the neutraltomic emission enhancements seen at those longnterpulse delays.

This third conclusion raises some concern as to whyIP emission in the fs–ns dual-pulse configuration isot directly proportional to the mass removed by thes ablative pulse—the apparent implication �espe-ially given the lack of obvious plasma temperatureffects or of self-reversal associated with self-bsorption�41 is that the fraction of analyte mass thatesults in neutral atomic emission at the interpulseelay and yields optimal mass removal in the fs–nsonfiguration is only one fourth of the mass fractionesulting in neutral atomic emission in ns SP LIBS.his observation further suggests that although ma-erial removal is greatly improved by use of the fs airpark, much of that material is either not atomized or
ot excited by the ns ablative pulse—almost as though

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uch of the enhanced mass removal may be in someonemitting form rather than in that of atoms and

ons. Thermal imaging of the sample surface, laser-nduced fluorescence imaging, and scattering studiesf the region above the solid sample should permiturther examination of this possibility. Additionally,se of multiple samples of similar thinness will permitore-direct correlation of per-shot mass removal with

roperties such as heat capacity, thermal conductivity,hermal diffusivity, and sample reflectivity while itliminates concerns regarding the use of samples ofifferent thicknesses.

. Conclusions

irst, as described above, per-shot depth and massemoval show both large enhancement at the optimalnterpulse delay and a strong dependence on td foroth aluminum and brass in the fs–ns dual-pulseonfiguration. Second, the close similarity of massemoval profiles at short delay suggests similarthough currently undetermined� sources of material-emoval enhancement at interpulse delays between 0nd �40 �s. Lack of close correlation between ma-erial removal enhancement and atomic emission en-ancement at longer td values suggests that theominant cause or causes of these two phenomenaay not be directly related at td greater than �40 �s.astly, the nonlinear correlation between mass re-oval and atomic emission enhancement suggests

hat a significant fraction of the mass removed in thes–ns dual-pulse configuration may be ejected in someonemitting form, such as fragments and droplets,ather than as atoms and ions.

We acknowledge support of this research underrant CHE-0316069 by the National Science Foun-ation. The authors are grateful to Dwight Price,laus Widmann, and John Boyett of the Ultrashortulse Laser Facility, Lawrence Livermore Nationalaboratory, for providing laser facilities and techni-al assistance. Additionally, we thank Noel Moorend the Continuum Laser Corporation for continuingssistance and technical support.

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temporal dependence of the enhancement of material removal in femtosecond-nanosecond dual-pulse...

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