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patial and temporal dependence of intersparknteractions in femtosecond–nanosecondual-pulse laser-induced breakdown spectroscopy

on Scaffidi, William Pearman, Marion Lawrence, J. Chance Carter, Bill W. Colston Jr.,nd S. Michael Angel

A femtosecond air spark has recently been combined with a nanosecond ablative pulse in order to mapthe spatial and temporal interactions of the two plasmas in femtosecond–nanosecond orthogonal pre-ablation spark dual-pulse laser-induced breakdown spectroscopy �LIBS�. Good spatial and temporalcorrelation was found for reduced atomic emission from atmospheric species �nitrogen and oxygen� andincreased atomic emission from ablated species �copper and aluminum� in the femtosecond–nanosecondplasma, suggesting a potential role for atmospheric pressure or nitrogen�oxygen concentration reductionfollowing air spark formation in generating atomic emission enhancements in dual-pulse LIBS. © 2004Optical Society of America

OCIS codes: 140.3440, 300.2140, 300.6210.

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. Introduction

aser-induced breakdown spectroscopy �LIBS�, firstntroduced by Brech and Cross in 1962,1 has endured

somewhat turbulent history. Early demonstra-ions of its potential for ultrafast, multielemental mi-roanalysis resulted in rapid commercialization,ollowed by rapid disillusionment as users becamencreasingly aware of the matrix effects, the highelative standard deviations, and the part-per-illion limits of detection that typify LIBS.2–4 Fol-

owing a brief lull, the development of smaller, moreeliable pulsed lasers; intensified charge-coupled de-ectors; and high-quality, inexpensive fiber optics al-owed a resurgence in LIBS research,2–4 with specialmphasis on the potential for the microanalysis ofrreplaceable works of art and archaeological arti-acts,5,6 the on-site and in situ7–10 analysis of

J. Scaffidi, M. Lawrence, and S. M. Angel �angel@mail.chem.c.edu� are with the Department of Chemistry and Biochemistry,niversity of South Carolina, Columbia, South Carolina 29208.. Pearman is with the Department of Chemistry, United States.ilitary Academy, West Point, New York 10996. J. C. Carter and. W. Colston, Jr. are with the Medical Technology Program, Law-ence Livermore National Laboratory, Livermore, California4550.Received 8 December 2003; revised manuscript received 4 June

004; accepted0003-6935�04�275243-08$15.00�0© 2004 Optical Society of America

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azardous11–14 and difficult-to-reach samples,10,15

nd high-resolution spatial mapping of analyte dis-ributions.16,17 At the same time, work by variousesearchers demonstrated the potential for signifi-ant atomic emission improvements �and, therefore,ignificantly improved-limits of detection� throughhe manipulation of sample temperature,18 atmo-pheric pressure�concentration,19–22 and atmo-pheric composition.19–22 Additionally, the use ofltrashort picosecond23,24 and femtosecond ablativeulses24–27 has shown promise for improving signal-o-noise ratios and ablative reproducibility in LIBS.nfortunately, though useful in a laboratory setting,

hese means of analyte emission enhancement arensuitable for the on-site and in situ applications forhich LIBS holds its greatest promise.On the other hand, more recent work using multi-

le collinear28,29 or orthogonal30–34 laser pulses toreatly enhance atomic emission in a laser-inducedlasma �LIP� has demonstrated great potential foroth on-site and in situ elemental analysis. In 1991,ebbing et al.30 observed two- to three-fold atomic

mission enhancements for a number of analytes inlass, steel, copper, and aluminum matrices whensing a nanosecond ablative pulse followed by a nano-econd pulse parallel to the sample surface �orthog-nal to the ablative pulse� to reheat ablated materialn a second LIP. Later, Sturm et al.28 noted substan-ial enhancement for a variety of metals and nonmet-ls in steel while using up to three collinear pulses

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ver the course of 65 �s. St-Onge et al.29 recentlyeported 30-fold atomic emission enhancements foreutral silicon emission and up to 100-fold enhance-ent for singly ionized aluminum lines in a similar

anosecond–nanosecond collinear dual-pulse config-ration. The source or sources of these collinearual-pulse enhancements, unfortunately, remain un-lear.

In a deviation from other researchers’ practice, ourroup has focused on nanosecond–nanosecond or-hogonal preablation spark dual-pulse LIBS, inhich a nanosecond pulse fired parallel to and milli-eters above the sample surface forms an air spark

rior to ablation by a second nanosecond laser pulseocused on the sample surface.31–33 Atomic emissionnhancements are observed in this pulse configura-ion �33-fold enhancement relative to nanosecondingle-pulse intensity for neutral copper emission inulk copper,31 11-fold for neutral lead emission inulk lead,31 and 11-fold, 20-fold, and 20-fold for neu-ral titanium, aluminum, and iron emission inlass,32 for example�, like the collinear configurationmployed by other researchers, although the naturef these enhancements is not completely understood.ttempts to better characterize the source or sourcesf dual-pulse atomic emission enhancements in thisulse configuration eventually led to the combinationf the orthogonal geometry and greatly dissimilaraser pulses �nanosecond and femtosecond pulses, forxample, as in current and previous34 research� foreparation of the atmospheric pressure�concentra-ion, sample heating, and plasma–plasma energeticoupling effects considered to be potential sources oftomic emission enhancement in dual-pulse LIBSnd as an attempt to improve ablative reproducibilityhrough use of a high-power, ultrafast femtosecondblative pulse.34

In the work reported here, a femtosecond preabla-ion air spark has been combined with a nanosecondblative plasma in the orthogonal pulse configura-ion. Examination of spatial and temporal correla-ions shows a very close relationship between theeduced atomic emission for atmospheric �nitrogennd oxygen� species and the enhanced atomic emis-ion for ablated �copper and aluminum� species, sug-esting either that one effect causes the other or thatoth are caused by some unknown third effect. Thepecifies of these correlations are discussed, and aodel for dual-pulse enhancement in femtosecond�

anosecond dual-pulse LIBS is presented.

. Experiment

. Equipment

igure 1 shows the basic equipment setup in thesexperiments. Briefly, a 10-mJ, 100-fs, 800-nm pulserom a Ti:sapphire laser �more fully described in theiterature�35 operating at five shots per second wasocused with a 100-mm-focal-length quartz lens toenerate a short-lived laser-induced plasma �LIP� inir. Anywhere from 0 to 500 �s later, depending onhe particular interpulse delay �t , the time between

d

244 APPLIED OPTICS � Vol. 43, No. 27 � 20 September 2004

ir spark formation and ablation by the second pulse�etting chosen for a given experiment, a 150-mJ, 5-ns,064-nm ablative pulse from a commercially avail-ble Nd:YAG laser �Surelite III, Continuum, Santalara, California� was fired and focused to a prede-

ermined point in or near the femtosecond pulselasma with a 100-mm-focal-length quartz lens torobe interspark interactions �Fig. 1, inset�. Atomicmission from the nanosecond ablative plasma wasollected along the axis of the nanosecond pulse, fo-used onto a 2-mm-diameter light guide �0.51 NA,dmund Scientific; Tonawonda, New York� with two00-mm-focal-length quartz lenses, and spectrally re-olved with a 0.25-m spectrograph �Chromex 250IS�F, Chromex, Bille, Massachusetts� with a 1200-roove grating blazed at 500 nm. Spectrallyeparated emission intensity was quantified with aate width of 200 nanosecond and a gate delay of 5 �sith use of an intensified gated CCD �I-MAX 1024E8�G�II P43, Princeton Instruments, Monmonthunction, New Jersey�.

. Spatial Mapping

hen we spatially mapped the femtosecond–anosecond dual-pulse interactions in both the pres-nce and the absence of solid samples �Figs. 2–4�, theanosecond ablative pulse focus was held constanthile the femtosecond air spark position was moved

tepwise throughout a predefined three-dimensionalolume to examine the effect of interspark crossingpon atomic emission. In experiments examining

nterspark interactions in the absence of a solid sam-le, td was 10 �s �an interpulse delay at whichemtosecond–nanosecond dual-pulse LIBS atomicmission and signal-to-noise improvements haveeen previously observed�, the gate width was 200 ns,nd the spatial resolution was 0.5 mm between dataoints in the x, y, and z directions �x being along the

ig. 1. Typical dual-pulse LIBS setup. A pulse from one laserA� produces an air spark parallel to and millimeters above theample surface prior to an ablative pulse from a second laser �B�,ith the interpulse delay controlled by a timing generator �C�.se of a dichroic mirror �D� allows ablative pulse focusing andlasma emission collection with a light guide �E� along the sameptical axis, making focusing of collection optics a trivial task.lasma emission is then spectrally resolved with a spectrograph

F� and analyte emission is temporally separated from continuummission by adjusting the gate delay for the ICCD �G�. In thesearticular experiments, the preablation air spark was generatedith a 10-mJ, 100-fs, 800-nm Ti:sapphire pulse, and ablation wasith a 150-mJ, 5-ns, 1064-nm Nd:YAG pulse.

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xis of the air spark, y being perpendicular to thatxis but at the same height above the sample surface,nd z being the height above the sample surface,long the axis of the ablative pulse�. When we char-cterized the effect of interspark crossing above arass or aluminum sample, data resolution was 0.2m in all three dimensions.

. Temporal Mapping

omplete characterization of the femtosecond–anosecond interspark temporal interactions at alloints in the three-dimensional map was well beyondhe scope of this work. However, the points of max-mum analyte atomic emission deviation from nano-econd single-pulse LIBS intensities seemedeasonable locations at which to examine the effect ofnterpulse timing upon femtosecond�nanosecondual-pulse LIBS emission enhancement. Accord-ngly, atomic emission intensity was recorded for ni-rogen �742.364, 744.229, and 746.831 nm� andxygen �unresolvable lines at 777.194, 777.417, and77.539 nm, heretofore referred to as “oxygen emis-ion at 777 nm”� in the femtosecond�nanosecond dualulse of air experiments and copper �510.554,15.324, and 521.820 nm�, aluminum �394.401 and96.152 nm�, nitrogen, and oxygen in the femtosec-nd�nanosecond dual pulse of solids experiments asnterpulse delay was varied from 0 to �500 �s �thats, the femtosecond pulse was fired 0–500 �s prior tohe nanosecond pulse�. Dual-pulse enhancementsere calculated as the ratio of the atomic emission

ntensity seen in the dual-pulse experiment at theisted conditions versus that observed for fully opti-

ized nanosecond single-pulse LIBS.

. Results and Discussion

. Femtosecond�Nanosecond Dual-Pulse Studies of Air

ased on previous experience while optimizing thenterspark crossing in nanosecond–nanosecond or-hogonal preablation spark dual-pulse LIBS, we ex-ected dual-pulse enhancements in the femtosecond�anosecond dual-pulse configuration to depend, at

east in part, upon the alignment of the femtosecondir spark and the nanosecond ablative plasma. Inig. 2, it is apparent that the oxygen atomic emission

ntensity in femtosecond�nanosecond dual-pulseIBS of air, as in the nanosecond–nanosecond dual-ulse case for solids, strongly depends upon in-erspark crossing. Rings of decreasing atomicmission appear to form a bullseye pattern in thewo-dimensional maps �in three dimensions, a seriesf shells� focused on a point at �5 mm, 5.25 mm� in therbitrarily defined coordinate system, at which thexygen atomic emission intensity is reduced by morehan 80% relative to that seen for fully optimizedanosecond single-pulse LIBS. The nitrogen atomicmission lines at 742, 744, and 746 nm �not shown�ere found to behave similarly.Although a full examination of the temporal behav-

or of these dual-pulse atomic emission enhance-ents was, as stated above, beyond the scope of this

20

tudy, nitrogen �not shown� and oxygen atomic emis-ion was monitored as interpulse delay was variedrom 0 to �500 �s at the interspark crossing coordi-ates corresponding to the center of the three-imensional shell �Fig. 2�. If the femtosecond airpark is introduced even 100 ns prior to nanosecondulse firing, oxygen and nitrogen emission in theanosecond plasma decline sharply. At an inter-ulse delay of 2 �s, air spark formation by the nano-econd pulse is no longer possible at the nanosecondulse power used in these experiments, and nanosec-nd air spark formation remains impossible until an

ig. 2. Spatial and temporal dependence of oxygen emission ins–ns dual-pulse LIBS of air. A clear ring structure �a� is gener-ted when mapping femtosecond–nanosecond dual-pulse spatialnteractions in two dimensions �values indicate atomic emissionntensity relative to that seen for fully optimized nanosecondingle-pulse LIBS�. Additional two-dimensional slices of the vol-me probed in this work �not shown� indicate that the rings notedbove correspond to a three-dimensional shell structure. �Notehat the x and y coordinates are arbitrary, and that the center ofhe ring structure corresponds to direct overlap of the femtosecondnd nanosecond plasmas.� �b� Examination of the temporal de-endence of the atomic oxygen lines at 777 nm upon td shows annitial rapid reduction upon introduction of the femtosecond airpark, followed by a period during which nanosecond spark forma-ion was not possible. Oxygen �and nitrogen, not shown� atomicmission intensity slowly begins to increase at td � 20 �s, finallyeturning to nanosecond single-pulse levels at td � 80 �s. Errorars represent two standard deviations.

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nterpulse delay of 20 �s has occurred. After thatoint in time, oxygen and nitrogen atomic emissionteadily increase toward their nanosecond single-ulse LIBS levels, finally reaching them at an inter-ulse delay of 80 �s. The cause of these emissioneductions will be discussed below.

. Examination of Ablated Analyte Emission above aolid Surface

s for dual-pulse LIBS of air, spatial mapping oftomic emission enhancements in the femtosecond�anosecond dual-pulse LIBS of solids provides valu-ble information regarding the source of atomicmission enhancements in this pulse configuration.

three-dimensional shell structure is again gener-ted as atomic emission enhancement for copper inrass is monitored while the femtosecond plasma iscanned throughout a predefined region above theample surface �Figs. 3 and 4�. Unlike oxygen anditrogen emission in the case of the femtosecond�anosecond air experiment, atomic emission by cop-er in this pulse configuration shows a substantialmission enhancement as the pulses approach theirptimal interspark crossing, increasing up to four-old with respect to the intensity seen for fully opti-

ized nanosecond single-pulse LIBS �Fig. 3�c��. Theituation is similar for aluminum, which shows up to

ig. 3. Spatial dependence of copper emission in femtosecond–namission intensity at 510 �not shown�, 515 �not shown� and 521 nmeights of �a� 0.2, �b� 0.4, �c� 0.6, �d� 0.8, and �e� 1.0 mm above theith respect to that seen for fully optimized nanosecond single-puIBS of air map �Fig. 2� and that of aluminum �Fig. 4�, “zero” in thhese contour plots is at �x,y� � �0, 0.75�, with the z coordinate eq

246 APPLIED OPTICS � Vol. 43, No. 27 � 20 September 2004

hree-fold atomic emission enhancement at its opti-al femtosecond�nanosecond interspark crossing

Fig. 4�c��. For comparison, oxygen atomic emissiont the same interspark crossing and interpulse delayn the femtosecond–nanosecond air experiment is re-uced by at least 90% �verified simply by removinghe sample and measuring oxygen emission at 777m� relative to that seen in the nanosecond single-ulse case.Examination of copper atomic emission depen-

ence upon interpulse delay at the optimal in-erspark crossing found in the spatial mappingxperiment above is shown in Fig. 5�a�. Emissionntensity at 521 nm �closely paralleled by the copperines at 510 and 515 nm, not shown� increases by aactor of 2 upon introduction of the femtosecond pre-blation air spark at a short interpulse delay. Afterhis initial enhancement, copper emission slowly fallsoward its nanosecond single-pulse level, finallyeaching it at an interpulse delay of 140 �s. Theemporal dependence of femtosecond�nanosecondual-pulse atomic emission enhancement for alumi-um displays a generally similar dependence upon

nterpulse delay �Fig. 5�b��. Atomic emission at 396m �and 394 nm, not displayed� shows a similar rapid

ncrease in intensity at a short interpulse delay, withfour-fold signal enhancement. However, although

ond dual-pulse LIBS of brass. Spatial mapping of copper atomicates a series of two-dimensional contours at femtosecond air sparkple surface, with values indicating atomic emission enhancementIBS. As in the case of the femtosecond–nanosecond dual-pulserdinate system is arbitrary. The center of interspark overlap in

o the femtosecond air spark height above the sample surface.

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he aluminum atomic emission enhancement decaysoward the ns single-pulse LIBS level with increasingnter-pulse delay, as in the case of copper in brassFig. 5�a��, the temporal dependence of the aluminummission decay diverges significantly and showseveral currently unexplained three- to fourfoldpikes in intensity at an interpulse delay greaterhan 150 �s before finally settling to nanosecondingle-pulse LIBS levels at an interpulse delay of50 �s. Future work will hopefully allow betterharacterization and understanding of this currentlynexplicable behavior.

. Examination of Atmospheric Analyte Emission above aolid Surface

lthough the experiment was initially performedolely to ensure that the effects observed in the fem-osecond�nanosecond dual pulse of air experimentre seen in the femtosecond�nanosecond dual pulse ofolids experiment, an examination of nitrogen andxygen atomic emission intensity in the latter caseielded rather significant temporal correlations �Fig.�. At short interpulse delay, as in the femtosecond�anosecond of air experiment, nitrogen and oxygentomic emission decreased significantly, with an al-ost complete loss of nitrogen and oxygen emission

ig. 4. Spatial dependence of aluminum emission in femtosecondluminum atomic emission intensity at 394 �not shown� and 396park heights of �a� 0.2, �b� 0.4, �c� 0.6, �d� 0.8, and �e� 1.0 mmnhancement with respect to that seen for fully optimized nanosecual-pulse LIBS of air map �Fig. 2� and that of copper in brassnterspark overlap in these contour plots is at �x,y� � �0, 0.75�, witample surface.

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t interpulse delays between 2 and 20 �s �Fig. 6�a��,ven though ablation from the sample surface re-ains possible �recall that in the femtosecond�nano-

econd of air experiments, plasma formation by theanosecond pulse is impossible for interpulse delaysetween 2 and 20 �s�. Eventually, nitrogen and ox-gen atomic emission intensity returns to its nano-econd single-pulse LIBS levels. This general trendor temporal dependence of atomic emission upond, given similar behavior in the femtosecond–anosecond of air experiment, is not surprising.The greater significance of these reductions in ni-

rogen and oxygen atomic emission in the nanosecondlasma in femtosecond–nanosecond dual-pulse LIBSecomes apparent in Fig. 6�b�, showing the oxygenmission reduction and the aluminum atomic emis-ion enhancement for femtosecond–nanosecond dual-ulse LIBS of a solid aluminum sample, with someorrelation apparent between the oxygen emissionecreases and the aluminum emission enhancement.his relationship between the signal from atmo-pheric species and that from ablated species is evenore clearly demonstrated in Fig. 6�c�, where the

opper atomic emission enhancement and the oxygentomic emission reduction are shown as a function ofnterpulse delay. At t � 2 �s, when the copper

osecond dual-pulse LIBS of bulk aluminum. Spatial mapping ofenerates a series of two-dimensional contours at femtosecond airve the sample surface, with values indicating atomic emissioningle-pulse LIBS. As in the case of the femtosecond–nanosecond3�, “zero” in the coordinate system is arbitrary. The center ofz coordinate equal to the femtosecond air spark height above the

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tomic emission rapidly increases, the oxygen atomicmission swiftly decreases. As the copper atomicmission enhancement subsides and slowly decaysoward its nanosecond single-pulse level, the oxygentomic emission intensity returns to its nanosecondingle-pulse level at a comparable rate. Each spe-ies’ atomic emission intensity reaches its respectiveanosecond single-pulse LIBS level at an interpulseelay of approximately 140 �s.Because the femtosecond air spark emissive life-

ime is only 6 �s at the 10-mJ pulse power used inhis work, the direct plasma–plasma energetic cou-ling suggested as the source of enhancements inollinear nanosecond–nanosecond dual-pulse LIBSannot be the source of the increased atomic emissionbserved for ablated species in the femtosecond–anosecond pulse configuration. This means thattomic emission enhancement at interpulse delaysetween 6 and 140 �s—the atomic emission enhance-ent that spatially and temporally correlates with

itrogen and oxygen atomic emission decreases in theemtosecond–nanosecond pulse configuration—muste explained through some other mechanism.

ig. 5. Temporal dependence of ablated analyte atomic emissionpon td. Atomic emission for both copper in brass ��a�, 521.820m� and aluminum in bulk aluminum ��b�, 396.152 nm� showsapid enhancement upon introduction of the femtosecond air sparkn femtosecond–nanosecond dual-pulse LIBS. Aluminum �b�hows a much less well-defined return to nanosecond single-pulsetomic emission levels than copper �a�, with a series of large spikesn emission intensity at interpulse delays between �100 and �500s.

248 APPLIED OPTICS � Vol. 43, No. 27 � 20 September 2004

herefore, with this evidence, and operating on theresumption that the atomic emission intensity for aiven species is related to that species’ concentrationn a plasma, we propose the following picture regard-ng the sources of atomic emission enhancement inemtosecond–nanosecond dual-pulse LIBS, with po-ential partial applicability to the nanosecond–anosecond dual-pulse configuration.

ig. 6. Temporal dependence of atmospheric and ablated analytetomic emission on td. The initial rapid reduction in oxygen �a�nd nitrogen �not shown� atomic emission in femtosecond–anosecond dual-pulse LIBS coincides with large increases in alu-inum �b� and copper �c� emission. Although the return of oxygen

tomic emission to its nanosecond single-pulse level correlates onlyeakly with the return of aluminum emission to its nanosecond

ingle-pulse level �b�, copper atomic emission enhancement �c� cor-elates very strongly with oxygen emission reductions �dashedurve� in the orthogonal femtosecond–nanosecond dual-pulse con-guration.

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Shortly after femtosecond air spark formationbove the sample surface, the ultrahigh electron den-ities �up to 1018 electrons per cubic centimeter�2

resent early in plasma evolution decay, whereas theell-documented shock wave generated by LIP for-ation in air36–41 drives the atmosphere above the

ample away at supersonic speeds,42 resulting in anudible “crack” and a formation of a large low-ressure region �or, at least, a region of low nitrogennd oxygen concentration� focused around the pointf air spark formation. Although the surroundingtmosphere will slowly refill this low-pressure�low-oncentration region through normal diffusion andhe pressure�air concentration will eventually returno its steady-state levels of �1 atm total pressure0.79 atm N2 and 0.21 atm O2�, there exists someindow in time during which material can be ablated

nto the short-lived low-pressure�low-concentrationegion, thereby allowing atomic emission enhance-ent in a manner similar to that observed in “bulk”

ow-pressure environments, such as a partially evac-ated gas cell.19–22 The optimal time for ablation,hen, will be late enough that the air spark electronensity has decayed sufficiently to allow the effectiveransmission of the ablative pulse through the airpark region, yet early enough to minimize diffusionf atmospheric species into the low-pressure�low-oncentration region formed by the air spark.

. Conclusions

lthough previous reports regarding the source oftomic emission enhancement in nanosecond–anosecond collinear dual-pulse LIBS have focusedlmost entirely on the potential for energetic couplingetween the first and second LIPs, the close spatialnd temporal correlation between the reduction oftomic emission by gaseous species �nitrogen and ox-gen� and the enhancement of atomic emission byblated species �copper and aluminum� in this papertrongly suggests a role for atomic emission enhance-ents due to a localized region of decreased atmo-

pheric pressure�concentration following LIPormation in air. Note that although this researchppears to confirm a role for pressure effects in theemtosecond–nanosecond dual-pulse configurationnd, by extension, in nanosecond–nanosecond dual-ulse LIBS, it does not guarantee or eliminate a roleor a sample heating by the first plasma or an ener-etic coupling between the first and second LIPs inither the collinear or orthogonal-pulse configura-ion. In all probability, all three effects play a role inhe up to 33-fold neutral atomic and 100-fold ionicmission enhancements noted in nanosecond–anosecond dual-pulse LIBS, and their individualontributions to dual-pulse emission enhancementears investigation with the goal of making LIBSven more suitable for on-site and in situ analysis.uture work with this end goal, clearly, must includemore thorough examination of the spatial and tem-oral interspark interactions in nanosecond–anosecond LIBS �both collinear and orthogonalonfigurations� with a full characterization of the

20

oles of temperature, pressure, and interspark effectshrough the use of highly sensitive temperature andressure measurement techniques and high-esolution temporal analysis of plasma–atmosphere,lasma–sample, and plasma–plasma interactions.

We acknowledge the National Science Foundationor support of this research under grant CHE-316069. We also gratefully acknowledge Dwightrice, Klaus Widmann, and John Boyett of the Ul-rashort Pulse Laser Facility at Lawrence Livermoreational Laboratory for providing laser facilities and

echnical assistance.

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