Pressure dependence of emission intensity in femtosecond

laser-induced breakdown spectroscopy

-Serife Yalcın, Ying Y. Tsui and Robert Fedosejevs*

Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta,Canada T6G 2V4. E-mail: rfed@ee.ualberta.ca; Fax: (780) 492-1811

Received 18th March 2004, Accepted 27th July 2004First published as an Advance Article on the web 8th September 2004

Femtosecond laser produced plasma emission has been characterized as a function of pressure forapplications in laser induced breakdown spectroscopy (LIBS). Experiments were performed with aTi:sapphire laser system (130 fs, 800 nm), from atmospheric pressures down to 10�3 Torr and at pulseenergies on the order of 1–50 mJ, (0.1–5 J cm�2). Characteristic emission lines from Al, Mg, Si and Cuelements exhibited significant enhancement in signal intensity at a few Torr background air pressure ascompared to atmospheric air pressure. Spatially and temporally resolved emission measurements indicateenhancement due to a longer lifetime of the plasma expanding to a larger size at lower backgroundpressures. Further reduction in pressure down to 10�3 Torr resulted in a decrease in signal intensity, as aresult of a reduction of collisional excitation of the emission lines which occurs when the plasma plumeexpands into the ambient atmosphere. It has been also observed that signal enhancement at low pressure isvery much dependent on the measurement delay time and on the transition being observed. With a delaytime of 200 ns the integrated intensity of the neutral Al I lines at 396 nm exhibited 67 times enhancement insignal intensity at 4 Torr of pressure with respect to atmospheric pressure, whereas signal enhancement isonly 4 times when no measurement delay time was used. The best signal to noise ratio of 850 was observedat 4 Torr pressure for an 85 ns delay time. Measurements of crater size showed no pressure dependentchanges in the ablated mass, indicating that little plasma shielding occurs due to the short pulse duration ofthe femtosecond laser pulses.

Introduction

High peak power densities of femtosecond laser pulses allowfor the breakdown and ionization of material surfaces at lowenergy fluences. Such breakdown plasmas are used in a varietyof techniques for spectrochemical analysis of solid samples, e.g.laser ablation inductively coupled plasma atomic emission/mass spectrometry (LA-ICP-AES/MS) and laser inducedbreakdown spectroscopy (LIBS). However, studies on the useof femtosecond lasers in LIBS1–5 and LA-ICPMS6,7 for analy-tical applications are limited. Femtosecond laser pulses are alsoemployed in other applications for laser micro-machining,8–11

investigation of fast chemical reactions,12 and surgical applica-tions.13 Thus the ablation and emission dynamics are impor-tant topics for a variety of applications of femtosecond laserpulses.

There are a few advantages to using low energy pulses in themJ energy range for such applications. One can achieve break-down and ionization with very low energies when using smallfocal spots and a few studies have been reported in theliterature14–17 that employ such mJ pulse energies. Plasmasproduced by high energy nanosecond laser pulses typicallyhave a significant plasma shielding effect18 which can reducethe efficiency of the analytical method. Using low energy laserpulses significantly reduces the plasma shielding effect mostlyencountered when using nanosecond laser plasmas. In addi-tion, plasmas produced by low energy pulses have relativelysmall background continuum emission, which in turn wouldpotentially allow the use of non-gated CCD cameras asdetectors as compared to expensive gated CCD cameras.

There are several factors that affect line intensity in chemicalanalysis of solids by LIBS, such as: laser pulse length, laserwavelength, laser pulse energy, laser pulse homogeneity/stabi-lity, target surface roughness and ambient conditions. Under-standing the influence of ambient conditions, primarilypressure and the type of the background gas on spectral

emission intensities of the femtosecond laser produced plasmaswill be quite helpful in exploring the analytical performanceand the limitations of the femtosecond LIBS for chemicalanalysis. Several groups19–25 have investigated the effect ofambient conditions on line intensities and plasma parametersby using different types and pulse width of lasers. Kagawa andYokoi,20 more than two decades ago, have investigated theapplication of 5 ns, 337 nm N2 laser pulses with 5 mJ pulseenergy for spectrochemical analysis of solid samples. Theyhave observed that when the pressure is lowered to 1 Torr,the plasma consists of two distinct regions denoted primaryand secondary plasma. The primary plasma is small in size (1–2mm), appears right after the laser pulse (few ns), is short lived(20 ns) and emits a mostly continuous spectrum. The second-ary plasma is created after the primary plasma ceases andexpands much further than the primary plasma as the pressureof the surrounding gas is decreased. The secondary plasmaconsists mostly of narrow line emission with negligibly lowbackground signal which facilitates quantitative species analy-sis with reasonable precision. The optimum pressure for suchanalysis was given as 1 Torr.The effects of pressure, type of the background gas and

irradiation wavelength on the signal emission intensities andthe spatial extent of a Cu plasma have been studied by Leeet al.21,22 In their work,21 by reducing the pressure from 760 to10 Torr, a 7-fold increase for the peak intensity of the neutralcopper lines in air and 11-fold increase in line intensities inargon atmosphere have been observed from a plasma producedby a 10 ns 193 nm ArF excimer laser. The signals below 10 Torrwere not reported and it appears from the data presented thatthe signals were still rising with decreasing pressure at thelowest reported pressure of 10 Torr. Lu et al.23 have observedtheir best signal to background ratio (S/B) and signal enhance-ment of 11 times for aluminium lines at 200 Torr pressure froma plasma produced by 10 ns 248 nm KrF laser pulses. Theirreported enhancement factor relative to atmospheric pressure

A R T I C L E

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DOI:10.1039/b404132a

T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4 J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 2 9 5 – 1 3 0 1 1295

decreases slowly with pressure below 200 Torr remaining at 9times for a pressure of 10 Torr. Margetic et al.2 in theircomparative study of nanosecond and femtosecond plasmashave observed maximum signal intensity from neutral Cuplasma emission at 30 Torr pressure from femtosecond andat 60 Torr pressure from nanosecond laser pulses in an Aratmosphere. Signal enhancement of greater than 10 times wasobserved for femtosecond excitation when going from 300 to30 Torr pressure.

The feasibility of performing in-situ geochemical analysis ofMars rocks and soils by LIBS under Mars atmospheric condi-tions, 5–7 Torr CO2, has been studied by two groups.24,25

Knight et al.24 have investigated the effect of pressure on avariety of parameters: plasma size, signal intensity, mass abla-tion, electron density and plasma temperature by using 1064nm Nd:YAG laser pulses. They have observed increasedsignals with decreasing pressure increasing by a factor 10 timesfor pulse energies of 80–100 mJ and 4 times for 175 mJ relativeto atmospheric pressure. They also observed a primary andsecondary emission plasma. They concluded that the increasedsignal is related to the pressure dependence of mass ablationwhich in turn is controlled by plasma shielding. Brennetotet al.25 have also studied the expansion and lifetime of theplasma produced by nanosecond Nd:YAG lasers operating atfundamental, second and third harmonic wavelengths. Thesewere carried out under Martian atmospheric conditions usingpulse energies in the range of 6 to 40 mJ. They have measuredan increase in lifetime of the Cu plasma from 600 ns atatmospheric pressure compared to 2500 ns at 5 Torr CO2

background atmosphere. They also observed an enhancementof the order of 5 times or more in emission intensity for theemission lines from the Basalt minerals at 5 Torr CO2 ambientpressure vs. air at atmospheric pressure.

Here, we present results from an investigation of femtose-cond laser produced plasmas as a function of ambient airpressure at mJ pulse energies. Under low-pressure conditions,significant signal enhancements from characteristic emissionlines of Al, Mg, Si and Cu elements have been observed relativeto atmospheric pressure conditions. In order to understand thesignal enhancement under low-pressure conditions, the plasmaplume expansion, ablated mass and plasma emission lifetimewere investigated as a function of pressure.

Experimental

The experimental set-up, shown in Fig. 1, consists of a femto-second laser system, focusing and imaging optics, a vacuum

system and detection systems. The experiments were performedat a wavelength of 800 nm using a commercial Ti:sapphire laser(Spectra-Physics, Hurricane) with pulse duration of 130 fs. Thepulse duration of the femtosecond pulses was measured with asingle shot autocorrelator (Positive Light Model SSA). Experi-ments were conducted in single and multi shot mode. The laserpulse energy was varied with the use of both neutral densityfilters and a half-wave plate/polarizer pair and monitored byusing a calibrated photodiode (Hamamatsu R617). The fem-tosecond laser pulses were focused onto a target with a 25 cmfocal length lens through a fused silica window, at normalincidence. The focused beam diameter measured by a CCDcamera (Cohu, Spiricon Inc.) was 28 mm FWHM. The axialposition of the lens was controlled by a micrometer transla-tional stage in order to achieve precise focusing on the target.The emitted light from the plasma was collimated and imagedonto the entrance slit (200 mm) of the spectrometer at a 451viewing angle with respect to the laser beam using a pair of 10cm focal length lenses. A 257 mm focal length, f/4 spectro-meter, (Oriel MS260i), coupled to a gated, image intensifiedcharge coupled detector (iStar, ICCD-Andor), was used fordetection of the emitted spectrum. The detector was operatedin the spectroscopy mode by vertically binning the 256 verticalpixels together for each spectral channel. Temporal resolutionof the plasma was obtained by gating the intensifier on and offwith a digital delay generator. Two-dimensional plasma plumeimages were taken by tuning the spectrometer to zero wave-length, to employ the specular reflection from the grating, andfully opening the entrance slit of the spectrometer to give a 3mm field of view. A sequence of plasma images were obtainedat a 901 viewing angle, (side view), from the laser beam axis.Aluminium plasma emission was also monitored by a photo-multiplier (PMT) detector, (Hamamatsu 7518R), with the useof a 400 nm interference filter (20 nm FWHM) at a viewingangle (451) for comparison with the ICCD/spectrograph mea-surements.The samples used for the experiments were; aluminium,

silicon and copper. Aluminium plates were hand polished withsandpaper and finished with a commercial polishing paste(Met-all), to obtain a flat and smooth surface. The Cu targetsused were 500 nm Cu coated silicon wafers. Silicon targets wereused in polished wafer form without any pre-treatment. Thetargets were mounted on translational and rotational stages toprovide continuous fresh target spots during sampling.An eight sided, 20 cm diameter aluminium chamber back-

filled with ambient air and evacuated down to 10�4 Torrpressures was used throughout the experiments. The chamberpressure was monitored by a Pirani Gauge, (BOC EdwardsAPG).

Results and discussion

1 Temporal evolution of the femtosecond microplasmas

The temporal evolution of the femtosecond plasma producedfrom an aluminium target with 20 mJ pulse energy, (1.7 � 1013

W cm�2 average intensity within the 1/e intensity radius), atatmospheric pressure is presented in Fig. 2. A 5 ns gate timewas used for this experiment. Fast thermalization and lowbackground emission characteristics of low energy laserpulses15 are clearly observed. At the early stages, for the first10 ns after the laser beam, the plasma emission exhibitsrelatively quiet and low background with broadened ionicand neutral Al lines. The characteristic ionic Al II line at281.6 nm reaches its maximum intensity within this timeinterval and drops to the noise level at approximately 30 nsafter the laser pulse. Also, broad line emission observed in the266–272 nm region of the spectrum, which only appears withinthe first 15 ns time interval of the plasma emission can beassigned to ionic Al II lines. However, emission from neutral

Fig. 1 Schematic diagram of the experimental set-up. L1: 25 cm F.L.focusing lens; L2, L3: 10 cm F.L collimating and imaging lenses; S:spectrometer; T: target; C: ICCD, intensified charge coupled devicedetector; PMT: photomultiplier tube; IF: interference filter.

1296 J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 2 9 5 – 1 3 0 1

Al I lines at 308.21, 309.27, 394.6 and 396.15 nm lasts beyond40 ns after the laser pulse with a good S/N ratio. Low back-ground emission levels produced as a result of using mJ pulselaser energies allowed us to detect low concentration speciespresent in the matrix. Emission lines observed at 285.2 and383.8 nm can be assigned to neutral Mg I lines coming from the0.8–1.2% Mg content present in Al-6061 alloy.

2 Pressure scaling

A: Emission intensity. When a high power laser pulse isfocused on a solid target, hot plasma formed above the surfaceexpands at supersonic speed by driving a shock wave into thesurrounding atmosphere. During this interaction, energy istransferred into the ambient atmosphere through several pro-cesses26,27 such as, shock heating, thermal conduction, radia-tive emission and ion recombination. The extent of thisinteraction depends on the energy, pulse duration and thewavelength of the laser used as well as, composition and thecondition of this ambient atmosphere. The measured influenceof ambient pressure on femtosecond LIBS emission signalintensities over the range 2 � 10�3 Torr to atmosphericpressure is given in Fig. 3. Each spectrum in the figure is at adifferent pressure and was obtained with a 10 mJ pulse energy,10 ns delay time and 100 ns gate width from a single laser shot.It was observed that both neutral and ionic Al lines exhibitsignificant increase in signal intensity as the pressure decreasesfrom atmospheric pressure to the order of a Torr. The pressureat which the maximum line emission is observed from the Altarget under these conditions was 0.85 Torr. Further reductionin pressure down to 10�3 Torr resulted in a decrease in signalintensity. Similar trends have been observed for the principalneutral atomic lines from different targets, Cu and Si,with varying enhancement factors and optimum pressuresof around several Torr. Signal enhancement as a function ofambient pressure has also been investigated as a function ofgate delay time and different enhancement factors (relative toatmospheric pressure) have been obtained at different delaytimes. Enhancement factors for aluminium emission obtainedfrom the ratio of net signal intensities, (background subtractedpeak area in a 7 nm window), at various ambient pressuresrelative to atmospheric pressure intensity for three differentdelay times; 0, 85 and 200 ns are given in Fig. 4. Integrating thecombined signal from characteristic neutral Al I lines at 394.6

and 396.15 nm exhibited 3-times enhancement at 2 Torrpressure when 0 delay time was employed. However, whenthe same sets of measurements were done at 85 and 200 nsdelay times, enhancement factors of 55 and 67 times wereobtained, respectively.

B: Signal to noise ratio. In order to understand the variationin enhancement factors with delay time the same set of data hasbeen assessed for signal-to-noise ratio, S/N. It is well knownthat a large S/N is the key for precision and low detection limitfor spectrochemical analysis. Any factor that can either im-prove LIBS signal intensity or decrease the noise level will bebeneficial in extending the capabilities of LIBS. Another oftenused measure of the strength of the emission spectra is thesignal to background ratio, S/B. The peak height value of the396.15 nm Al I line minus the background is taken as the signaland the noise is defined as the three times the standard

Fig. 2 Temporal evolution of ionic Al II: 281.6 nm and neutral Al I:308.21, 309.27, 394.6 and 396.15 nm aluminium lines from Al 6061alloy. Measurements were performed with 20 mJ pulse laser energies atatmospheric pressure using a 5 ns gate width.

Fig. 3 The effect of air pressure on aluminium line emission intensitiesfrom Al-6061 alloy. Each spectrum has been obtained from a singlelaser pulse of 10 mJ energy, 10 ns delay time and 100 ns gate width.Neutral Al I lines at 308.21, 309.27, 394.6 and 396.15 nm showsignificant enhancement with pressure. Low concentration of Mg(383.8 nm), Fe (360 nm) and Cu (324.7 nm) lines present in the Al-6061 alloy are also enhanced under low-pressure conditions.

Fig. 4 The relative enhancement factors vs. the ambient pressure forAl emission produced by laser pulses of 10 mJ energy on Al-6061 alloyusing a 1 ms gate time. Enhancement factors were calculated from theratio of integrated area under 394.6 and 396.15 nm neutral lines at eachpressure to the area at atmospheric pressure.

J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 2 9 5 – 1 3 0 1 1297

deviation, 3s, of the nearby background intensity.15 The S/Nratio and the S/B ratio for the characteristic aluminium line at396.15 nm for three different delay times; 0, 85 and 200 ns, atatmospheric pressure and 4 Torr pressure are tabulated inTable 1. Line emission spectra for this set of data are given inFig. 5. It is observed that the signal significantly improves fromatmospheric to 4 Torr pressure as is shown in Fig. 5(a,d), (b,e)and (c,f) , for each delay time used, whereas noise levels almoststay constant with the change in pressure. Therefore, enhance-ments in S/N of 7, 85 and 157 times were obtained for 0, 85 and200 ns delay times, respectively, when changing from atmo-spheric pressure to 4 Torr pressure when peak intensities areconsidered. It can be seen that the S/B ratio follows a verysimilar trend to the S/N ratio.

When we look at the data vs. change in delay time, we seethat increasing delay time resulted in a decrease in both lineemission and noise levels, at both pressures. It is a commonpractice to make LIBS measurements at a fixed delay time afterthe laser pulse because the background signal and the asso-ciated noise decreases faster than the line emission signal.Therefore, the largest S/N is achieved at some optimum delaytime. However, at atmospheric pressure in our case, the signaldecays rapidly leading to S/N ratios of 35, 10 and 4 at 0, 85 and200 ns, delay times, respectively. This fast drop in both signalintensity and S/N wrt delay time at atmospheric pressure canbe explained by the fact that the cooling of a relatively lowenergy plasma, produced by 10 mJ laser pulses, by the ambient

air is very rapid at atmospheric pressure. However, at 4 Torrpressure, Fig. 5(d,e), the signal has decreased by only 30% inintensity from 0 to 85 ns delay time, whereas the noise level hasdecreased by 5 times. Therefore, the S/N increases from 240 to850 with increase in delay time. Also, at 4 Torr pressure, thesignal intensity decreases only by half going to a 200 ns delaytime and exhibits relatively strong emission with a S/N of 629,Fig. 5(f). It is seen that the optimum delay time for maximumsignal to noise ratio is close to 0 ns at atmospheric pressure butincreases to the order of 85 ns at 4 Torr pressure. This peak S/N ratio at 85 ns delay time for 4 Torr pressure is 24 times largerthan the peak value at zero delay time for atmosphericpressure.We have also investigated the pressure enhancement of

signal intensities for another element present in the sameplasma. For this purpose we have utilized the Mg I unresolvedtriplet lines at 383.2–383.8 nm which is present in the Al-6061alloy in the range of 0.8–1.2% by weight. Plasma was createdwith 10 mJ laser pulses and a 1 ms gate width was used toobserve the emission. The presence and magnitude of Mg linesalong with Al lines can be seen in Fig. 5. At atmosphericpressure, Fig. 5(a–c), Mg line emission appears only at zerodelay time with a weak signal emerging from the backgroundemission. As the delay time increases no emission signal fromthe Mg line was detected at 85 or 200 ns delay times. However,at 4 Torr pressure, Fig. 5(d–f), the Mg line was clearlyobservable for all the delay times investigated. This shows an

Table 1 S/N ratio for Al I 396.15 nm line with respect to pressure and delay time. Noise values are 3s of the background signal obtained from the

analysis of 50 pixels in the quiet region of each spectrum, whereas signals are peak height values of the line of interest minus background. The peak

values of the signal and 3s noise are given in parentheses

Delay

time/ns

S/N

(Atm. press.)

S/N

(4 Torr)

Enhancement

in S/N

S/B

(Atm. press.)

S/B

(4 Torr)

Enhancement

in S/B

0 35 (21500/600) 240 (120000/500) 7 11.92 (23065/1934) 63.51 (122831/1934) 5.3

85 10 (1000/100) 850 (85000/100) 85 12.8 (1076/84) 1038 (87240/84) 81.1

200 4 (475/120) 629 (62900/100) 157 8.11 (682/84) 750 (63044/84) 92.5

Fig. 5 Al I 394.6 and 396.15 nm lines at three different delay times (0, 85 and 200 ns) and two different pressures: (a–c) for atmospheric pressure,(d–f) for 4 Torr pressure. The same experimental conditions as in Fig. 4 were used.

1298 J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 2 9 5 – 1 3 0 1

improved performance in making line assignment (qualitative),and measurement (quantitative), of low concentration speciesby LIBS under reduced pressure conditions. The variation ofthe Mg line wrt pressure for an 85 ns delay time is shown inFig. 6. It is clear that Mg line emission becomes detectable atpressures lower than 47 Torr and rapidly increases to itsmaximum value at 4 Torr pressure. Absence of a detectableemission signal of the Mg lines at 85 and 200 ns delay timesunder atmospheric pressure did not allow us to make a directcomparison of pressure enhancement of the Mg emission wrtdelay time. However, our measurements at zero delay timeshowed that Mg line intensity exhibited 10 times enhancementat 4 Torr pressures relative to atmospheric pressures as com-pared to 4 times enhancement for the Al line emission.

Similar behavior of enhancements has been observed byKnight et al.24 from nanosecond laser plasmas at much higherpulse energies (80–100 mJ) in terms of the signal enhancement

at low pressures. The authors in that reference conclude thatthe signal enhancement at lower pressures is due to decreasedplasma shielding and hence increased mass ablation. However,in our case, due to the very short pulse duration, plasmasproduced by femtosecond laser pulses are not expected toexpand enough during the laser pulse to shield the target fromthe laser pulses. Therefore, plasma shielding observed usingnanosecond laser pulses is expected to be greatly reduced whenusing femtosecond laser pulses. In addition, the low pulseenergies will create a much smaller plasma plume and alsoshould lead to a reduction in plasma shielding.Our pressure dependent measurements of ablated material

presented below show constant material ablation from silicontargets independent of ambient pressure. This supports theconclusion that plasma shielding has a minimal effect forfemtosecond laser pulses. Instead, we can explain the enhancedemission by the enhanced expansion of the plasma plume intothe ambient atmosphere. At high pressures, the presence of thesurrounding gas cools the expanding plasma quickly by colli-sional processes and therefore the plasma is short lived. How-ever, at lower pressures of around several Torr, the plasmaexpands much further into the ambient atmosphere withoutbeing cooled as rapidly by the surrounding species. Thereforethe lifetime of the emitting species is longer and the integratedemission volume becomes much larger. Decreasing the pressureeven lower to mTorr levels results in a decrease in signalintensities. This can be attributed to decreased collisionsbetween the expanding plasma and ambient medium leadingto reduced collisional excitation. In order to better understandthe interaction of the plasma with the ambient atmosphere, theplasma plume expansion, ablated mass and plasma emissionlifetime were investigated in detail as a function of pressure.

C: Plasma plume expansion. The expansion of the plasmaplume and hence the size wrt pressure was observed by takingtwo-dimensional time gated images. Details of the imagingsystem are given in the Experimental section. The plasma sizehas been estimated from the measurement of a calibrationobject located in place of the target at atmospheric pressure. Asequence of plasma images obtained from a Cu target at a 901viewing angle from the laser beam, (side view), are shown inFig. 7. Imaging measurements were performed by accumulat-ing 10 laser shots with 50 mJ pulse energy, with a 10 ns delaytime and 500 ns gate width. At high pressures, specifically atatmospheric pressure, Fig. 7(a), the plasma is small (aboutB0.5 mm in width), confined to a limited region close to thetarget surface and is expected to have a very high density. Asthe pressure is lowered the plasma expands further towards thelaser and at a few Torr of pressure, Fig. 7(b), the plasma looksmuch more spherical in shape reaching a size of 2.5 mm. As thepressure decreases further, Fig. 7(c), the plasma extends furtherand the density of the plasma is expected to decrease further.Finally at the low pressure of 0.167 Torr, Fig. 7(d), thebrightness decreases because of the increased expansion anddecreased collision excitation. In all cases there is a bright

Fig. 6 Variation of 383.8 nm Mg I line emission wrt. pressure at gatedelay time of 85 ns after the laser pulse. A 1 ms gate width was usedthroughout the measurements.

Fig. 7 Two-dimensional plasma images of Cu plasma produced by the accumulation of 10 laser shots with 50 mJ pulse energy, 10 ns delay time and500 ns gate width. The 10 laser shots were each taken on a fresh target surface. *The scale at atmospheric pressure is twice that at other pressures.

J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 2 9 5 – 1 3 0 1 1299

emission spot at the target surface due to the hot high densityplasma initially expanding from the surface. Previousauthors20,24 have also observed a primary plasma close to thetarget surface and a secondary plasma with scale size of severalmm at reduced pressures of 1–100 Torr. In these previousreports it was also determined that the emission from theprimary plasma was predominantly continuum radiation whilethat of the secondary plasma was predominantly line radiation.

It is seen that the plasma plume moves away from the targettowards the laser direction as it expands into lower ambientpressure gas. Therefore, observations of emission made byimaging the emission at right angles onto the narrow entranceslit of a spectrograph would show different enhancementcharacteristics vs. pressure depending on the exact positionbeing viewed by the spectrometer. Other studies2,21 showingdifferent enhancement factors may be influenced by this factor.Our spectral emission measurements have been performed at a45 degree viewing angle with respect to the laser beam in orderto reduce this effect.

D: Ablation depth vs. pressure. In order to establish whetherthe enhancement of line intensities at specific ambient pressuremay be due to variations in ablated mass, the depth of thecraters has been investigated with respect to pressure. For thispurpose, silicon targets were irradiated with single femtose-cond laser pulses with 14 and 35 mJ pulse energies, at differentambient pressures of air. Ablation depth profiles of the cratershave been measured by an optical profilometer (Zygo 5000).Fig. 8 shows no significant changes in ablation depth wrtpressure, for both laser energies used. Ablation craters atdifferent pressures exhibited 19 and 40 nm average depths for14 and 35 mJ pulse�1 laser energies, respectively. The craterdiameters were also similar in size for all ambient pressureswith an average diameters of 35 and 48 mm for 14 and 35 mJpulses, respectively. The error from those measurements wasestimated as �10% due to shot-to-shot energy variation,changes in target position relative to the best focal point dueto the target rotation between shots and measurement errors inthe optical profilometer. This constant material ablation vs.ambient pressure demonstrates the absence of plasma-shieldingeffects in plasmas produced by low energy femtosecond laserpulses. In contrast to nanosecond laser produced plasmas, theultra-short duration of femtosecond laser pulses prevents sig-nificant expansion of the plasma during the laser pulse and freeexpansion of the plasma into the ambient atmosphere primar-ily takes place after the laser pulse has finished. For nanose-cond plasmas, as has been observed by Knight et al.,24 massablation is a strong function of pressure, which they concludefor nanosecond 1064 nm laser pulses is a direct result of the

plasma shielding effect. The difference in wavelength betweenour results (800 nm) vs. that of Knight’s work (1064 nm) is notlarge enough that the enhanced wavelength dependent pene-tration of our laser radiation in an air-breakdown plasmawould be a significant factor alone.

E: Plasma lifetime. The lifetime of the plasma exhibitsstrong dependence on pressure. The plasma lifetime at differentpressures has been determined by measuring the spectral lineemission as a function of time. Line emission from plasmaproduced from Al-6061 alloy with 10 mJ pulse energies areshown in Fig. 9. The results are measured by a PMT using a 20nm FWHM line filter at 400 nm at three selected pressures: (1)atmospheric pressure, (2) the pressure at which the maximumsignal emission is observed (0.85 Torr) and (3) a lower pressureat which the signal intensity is reduced again (0.2 Torr). Curvefitting has been performed to the second peak as shown inFig. 9. At atmospheric pressure, the signal emission demon-strates a single exponential decay with an approximately 7 nstime constant (t). At lower pressures, for both 0.85 Torr and200 mTorr, as the plasma plume expands into the ambient air,two separate emission periods were observed. The two emis-sion peaks in time most likely corresponds to the primaryplasma formed at the target surface and the secondary plasmagenerated as the expansion plume expands into the surround-ing gas. Kagawa et al.20 have reported that the primary plasmaceases its emission after about 20 ns which would agree withthe first emission peak seen in Fig. 9. Exponential fits to thesecond emission curves resulted in decay times of 33 and 50 nsfor 0.85 Torr and 200 mTorr pressures, respectively. Rieger etal.17 have reported 10 to 100 ns emission decay lifetimes for thesame aluminium lines for 1–100 mJ 50 ps pulses at 248 nm froma KrF laser at atmosphere pressure. These are longer than the 7ns lifetime reported here. However, in the previous work17 amuch shorter, 20 mm, focal length lens with a much smallerfocal spot was used producing a hotter and smaller plasma.Also the enhanced absorption due to the utilization of theultraviolet wavelength, 248 nm, radiation would potentiallylead to a larger absorbed fraction of the incident energy alsocontributing to a hotter and more dense initial plasma. It isexpected that a hotter and denser plasma will cool more slowlyleading to the somewhat longer lifetimes than reported here.This can be seen in our own results where the emission decaylifetime of the Al I lines around 400 nm for the hotter plasmaproduced with a 20 mJ pulse is 14.3 ns as seen from Fig. 2.Other previous studies2,25 of emission lifetime as a function

of pressure have shown increased emission lifetime with de-creasing pressure down to the order of a few Torr.25 In thesecases the emission was from the long lived Cu I 515.32 nm or

Fig. 8 Ablation depth of the craters produced on silicon wafer wrtpressure by single laser pulses at 14 mJ (&), and 35 mJ (J), energies.

Fig. 9 Effect of air pressure on Al I 394.6 and 396.15 nm emissionsignal as a function of time. Measurements were performed with aPMT (�320 V) and a 400 nm (20 nm FHMW) interference filter. Eachtrace was obtained from the average of 10 laser shots with 10 mJ pulseenergy. The standard deviation errors are calculated from the spread inthe multiple measurements used to obtain the average results.

1300 J . A n a l . A t . S p e c t r o m . , 2 0 0 4 , 1 9 , 1 2 9 5 – 1 3 0 1

Cu I 521.8 nm lines and the lifetimes were on the order ofmicroseconds. The longer lifetimes are also due to the higherpulse energies used in those experiments of 400 mJ2 to 2 mJ.25

Conclusion

In this study, the effect of ambient pressure on size, shape andsignal emission intensities of femtosecond laser producedplasmas at mJ pulse laser energies has been studied. At lowpressures, the emission peaks are better defined and back-ground emission is reduced. In addition, signal intensity atlow pressures (1–4 Torr) increased significantly relative tosignal intensities at atmospheric pressure with the enhancementdependent on the element and the delay time used. This signalenhancement can be explained by the free expansion of theplasma plume into the ambient atmosphere at lower pressures.At high pressures the presence of surrounding gas helps theplasma cool faster by collisions and therefore plasma emissionis short lived. However at lower pressures of around severalTorr, the plasma expands freely into the ambient atmospherewithout cooling quickly, therefore, leading to longer lifetimesof the emission. Decreasing the pressure even more to mTorrlevels results in a decrease in signal intensities, as the collisionalexcitation of the emission transitions decreases. We did notobserve differences in ablation depth of the craters with respectto pressure, indicating an absence of significant plasma shield-ing effects in low energy femtosecond laser produced plasmas.

The degree of signal enhancement and optimum pressurewill depend on the viewing geometry and in particular whetherall the light from the expansion plume is measured or whetheronly one spatial location in the expansion plume is probed.Also the degree of enhancement will depend on the incidentenergy, the incident wavelength and the background gasemployed. All of these factors lead to a wide range of scatterin results reported in the literature to date.

With this study, we have shown that use of reduced pressuresin the case of femtosecond lasers can result in significantimprovements in the analytical performance of LIBS forchemical analysis, in terms of significantly enhanced signalintensities and signal to noise ratios under low-pressure con-ditions of a few Torr. It is important to note that the mostimportant parameter is the signal to noise ratio since thisultimately determines the limits of detection and precision ofthe measurements and this is clearly enhanced by reducedpressure and optimum choice of delay time. It is clear thatfurther study is required to sort out in detail the role of all thevarious factors in determining the enhancement of emissionintensity.

Acknowledgements

The authors wish to thank the Natural Sciences and Engineer-ing Council (NSERC) of Canada and MPB Technologies Inc.for their financial support. Also they wish to acknowledge the

excellent technical assistance of Blair Harwood and RickConrad.

References

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