Volume 55, Number 7, 2001 APPLIED SPECTROSCOPY 8090003-7028 / 01 / 5507-0809$2.00 / 0q 2001 Society for Applied Spectroscopy

submitted papers

Experimental Investigation of Sodium Emission FollowingLaser Photofragmentation of DifferentSodium-Containing Aerosols*

M. HIDALGO NUNEZ† and NICOLO OMENETTO‡European Commission, Joint Research Centre, Environment Institute, T.P. 290, 21020 Ispra (Varese), Italy

Several time-integrated and time-resolved experiments have beenperformed on different sodium-containing dry aerosols in air to in-vestigate the behavior of atomic sodium emission at the yellow dou-blet after photofragmentation with 193 nm photons provided by anArF excimer laser. It is shown that sodium chloride, sodium hy-droxide, and especially sodium sulfate require different thresholdsfor the overall process of vaporization, dissociation, and excitationof atomic emission. This can be clearly seen by the experimentalfact that (1) the onset of the time-integrated emission signal startsat different laser energies, and (2) the time-resolved shapes of theemission signal during the laser pulse (13 ns) are markedly differentin the case of chloride and sulfate aerosols, the former rising fasterthan the latter. From an analytical point of view, the detection limitfor sodium in airborne particles is 0.3 ng of sodium per gram ofair (;0.017 ppbv), which is about one order of magnitude betterthan the detection limit reported by early laser-induced plasma ex-periments.

Index Headings: Time-resolved emission; Laser-induced photofrag-mentation; Sodium chloride aerosols; Sodium hydroxide aerosols;Sodium sulfate aerosols.

INTRODUCTION

In a recent work,1 the possibility of an indirect methodof detecting sulfuric acid aerosols by monitoring theatomic emission of sodium resulting from the photofrag-mentation of sodium chloride with an ArF excimer laserwas investigated. The method is based upon the observeddecrease of the yellow sodium emission when increasingquantities of acid aerosols are mixed with sodium chlo-ride aerosols, a decrease which is attributed to the for-mation of sodium sulfate aerosols which are not frag-mented and excited as ef� ciently as the chloride.

The exploitation of laser photofragmentation for thedetection of alkali compounds under excimer laser irra-diation (mostly at 193 nm) is well known and docu-mented.2–6 In a thorough investigation of photofragment

Received 23 August 2000; accepted 9 March 2001.* This paper is dedicated to Paolo Papoff on the occasion of his 80th

birthday, as an appreciation of his many seminal contributions to the� eld of instrumental analytical chemistry.

† Present address: Facultad de Ciencias, Universidad de Malaga, De-partamento de Quõ mica Analõ tica, Malaga, Spain.

‡ Author to whom correspondence should be sent.

� uorescence as an analytical technique, Oldenborg andBaughcum2 reported the detection of trace amounts ofalkali chlorides with parts per billion by volume (ppbv)sensitivities. Hartinger et al.3 measured sodium concen-trations in the � ue gas of a � uidized bed reactor andinvestigated several different coals, with sensitivities alsoin the ppbv range. Finally, in a series of papers, Chad-wick et al.4–6 used both 193 and 355 nm photons to pho-tolyze NaCl and NaOH and excite sodium atomic � uo-rescence. The � uorescence response under the high-pres-sure conditions typical of coal gasi� cation was modeled,taking into account radiation trapping and quenching.5

All the above studies were performed with the alkalicompounds present in the gas phase. In our case, thecompound is present in the dry aerosol phase. As a con-sequence, in addition to the parameters already discussedand characterized in the above literature, the concentra-tion of sodium in the gas phase will depend upon theef� ciency of the primary interaction process between thelaser and the particle, i.e., on the energy deposition lead-ing to particle vaporization.

The aim of this paper is to gain some insight into thisinteraction process by investigating the spectral and tem-poral characteristics of the sodium emission following in-teraction of different sodium-containing aerosols with193 nm photons. Time-integrated measurements, wherethe laser-induced sodium emission is compared with thethermal emission signal obtained by nebulizing aqueoussolutions in an air–hydrogen � ame, are complementedwith time-resolved measurements, where the temporalpro� le of the laser-induced emission signal during thelaser pulse (13 ns) is resolved.

The experiments were performed with NaCl, NaOH,and Na2SO 4 aerosols.

EXPERIMENTAL

An excimer laser (Lambda Physik, Model EMG 102),operated with ArF (193 nm), was directed into an aerosolsample. The beam was mildly focused with a cylindricallens, resulting in an elliptical excitation area (about 13mm 2). The temporal shape of the pulse was monitoredwith a fast photodiode or with a microchannel plate pho-

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FIG. 1. Experimental setup used for time-resolved emission measure-ments. PD, photodiode; MCP, microchannel plate photomultiplier.

tomultiplier (MCP) (H amamatsu Photonics , ModelH3376). The MCP has a rise time of 128 ps and wasoperated at 22800 V. Linearity of operation was ensuredby � lters placed in front of the detector or before theaerosol sample when the energy of the laser was varied.The laser scatter and sodium emission were transferred,without the use of optics, into a high-luminosity mono-chromator (Jobin-Yvon, Model H-10) and detected withthe MCP. The monochromator was placed at 908 withrespect to the laser beam and the aerosol sample locatedclose to the entrance slit of the monochromator, at a dis-tance calculated so that the laser-induced emission geo-metrically � lled the entire collimator (an area of about 9mm2 was observed). The detector signal was fed into afast (1 GHz bandwidth) digitizing signal analyzer (Tek-tronix, Model DSA 602A). For time-integrated measure-ments, the MCP was replaced by a high-gain photomul-tiplier (Hamamatsu, Model R3788) operated at 21200 V.This experimental setup, essentially used for the time-resolved experiments, is schematized in Fig. 1.

For the time-integrated emission experiments, the sameapparatus described in the previous paper1 was used, withthe addition of a simple, home-made, circular burner, sup-porting a small hydrogen-entrained air � ame, using argonas the nebulizer gas. This arrangement allowed the com-parison of the different behavior of a purely thermalemission signal with that induced by the laser photofrag-mentation/excitation process. Brie� y, the measurementswere performed with a high-resolution monochromator(Jobin-Yvon, Model THR 1000), equipped with inter-changeable gratings (100 lines/mm and 1200 lines/mm,corresponding to reciprocal linear dispersions of 10 nm/mm and 0.8 nm/mm, respectively), coupled with an in-tensi� ed diode array (Jobin Yvon, Model SIDA-700).The � ame and/or the laser-interaction area was imaged(1:1) on the entrance slit of the monochromator. The tim-ing and data acquisition program were the same as thosedescribed in the previous paper.1 The laser energy wasmonitored w ith a calibrated pyroelectric Joulemeter(Lambda Physik, Model ED-500) and varied with � lters.Since no calibrated neutral-density � lters were available,several combinations of thin plates made of quartz andmethacrilate material were used. For each combination,both the laser pulse shape and energy were measured.

Sodium-containing aerosols are generated by nebuliz-

ing aqueous solutions, prepared at different concentra-tions, with an ultrasonic nebulizer (CETAC Tech., Inc.,Model U-5000 AT), followed by desolvation at 140 8C.The dry aerosol produced is characterized, in terms ofnumber density and size distribution, with a differentialmobility particle sizing (DMPS) system (TSI, Models3071 and 3022).

RESULTS AND DISCUSSION

Time-Integrated Measurements. A time-integratedexpression relating the functional dependence of the sig-nal upon the laser, instrumental, and spectroscopic param-eters has been derived and reported in the literature cit-ed.2–5 In the case of NaCl, it can be shown that the signal,S, due to the radiative deactivation of excited sodiumatoms, (Na*), depends upon the following parameters:

S } (NaCl)g(sabsI`Dt

`)F(Na*)[A/(A 1 Q)] (1)

In the above expression, (NaCl)g represents the numberdensity of NaCl in the gaseous state (cm23). The term(sabsI`

Dt`) represents the number of absorption transitions

during the laser interaction time (Dt`), sabs being the ab-

sorption cross section at 193 nm of gaseous NaCl (cm2)and I` the laser photon irradiance at 193 nm (photons s21

cm22). The term F(Na*) represents the yield of the frag-mentation process resulting in the production of excitedsodium atoms, Na*. The � nal ratio is the quantum ef� -ciency of sodium emission, A being the Einstein spon-taneous emission rate coef� cient (s21) and Q the quench-ing rate coef� cient (s21).

The above expression contains the parameters relevantto the photochemical dissociation/excitation process ofthe molecular vapor leading to sodium emission. In ourcase, the aerosol is present in the solid phase (assumingcomplete desolvation in the nebulization process). Theobservation of sodium emission could then result fromtwo mechanisms: (1) a purely ‘‘thermal’’ process, inwhich the aerosol is heated and vaporized by the laserand the sodium is collisionally excited in the resultinghot vapor plume, and (2) a mixed thermal-photochemicalprocess. In the latter case, the energy deposited by thelaser in the primary interaction process results in particleheating and subsequent vaporization, while the formationof excited sodium atoms from the molecule in the gasphase can still be photochemical, i.e., due to phtofrag-mentation which leaves the sodium atom in its excited2P1/2, 2P3/2 states. If the process of particle heating leadingto vaporization is faster than the duration of the laserpulse, these two processes (thermal vaporization and pho-tofragmentation) can occur simultaneously during thesame laser pulse.

A correct analytical treatment should then take intoconsideration the time behavior of the various parametersduring the laser pulse. This issue is considered outsidethe scope of the present paper. To emphasize the differentadditional parameters that need to be considered in ourcase, it can be argued that the number density of NaClmolecules in the gas phase appearing in Eq. 1 will beproportional to.

(NaCl)g } (NaCl)s,`(aabs abs)(hvap) (2)

In the above expression, (NaCl)s,`[[ (NaCl)sVexc] is the

APPLIED SPECTROSCOPY 811

FIG. 2. Experimental behavior of sodium atomic thermal emission andlaser-induced emission as a function of � ame height.

FIG. 3. Time-integrated thermal emission and laser induced emissionof sodium as observed with the optical multichannel analyzer. The or-dinate values have been normalized to the same scale, taking into ac-count the different integration times used in the pulsed mode and in thecontinuous mode. Na concentration in the solution: 400 mg/mL. (a)Flame on/laser off; (b) � ame off/laser on; (c) � ame on/laser on. Laserenergy: 25 mJ/pulse. Laser � uence: 196 mJ/cm2.

number of molecules of solid NaCl in the laser excitationvolume Vexc (cm3); (NaCl)s is the number density of NaClmolecules in the aerosol phase (cm23); (aabs abs) representsthe product of the absorption coef� cient (cm21) for laserinteraction with the solid and the interaction length (cm);(hvap) is the vaporization ef� ciency (no units), whichshould be proportional to the expression [exp (2DHvap/kT`)], where DHvap is the enthalpy of vaporization (kcal/mol), k the Boltzmann constant (kcal/mol degree K), andT

` the temperature reached during laser interaction (K).This last parameter is a function of I`, the laser photonirradiance at 193 nm.

Several papers exist in the literature describing the in-teraction of laser beams with aerosols, and only a feware cited here.7–11 In a review article, Lushnikov and Ne-gin11 consider the overall energy balance during the pri-mary interaction process between the laser and the par-ticle, which includes energy absorption per unit of time,particle heating, contact heating of the surrounding gas,radiation heat transfer from the particle, and � nally par-ticle melting and evaporation. A related effect, known aslaser-induced incandescence (LII) is also well document-ed in the literature.12–16 This technique is based on thefact that absorption of laser radiation by soot particlesresults in particle heating at temperatures above 4000 K.12

Such temperature increases can occur very rapidly afterdeposition of the laser energy. Laser irradiances on theorder of 107 W cm22 are suf� cient for the observation ofLII.

As stressed before, the adaptation of these theoreticaltreatments to our case is considered outside the scope ofthe present paper, since the most relevant parameters inEq. 1 are either not known or not easily amenable toexperimental evaluation. It was therefore regarded as use-ful to gain some insight into the ef� ciency of the primaryinteraction process by comparing the signal levels ob-tained by irradiating NaCl in the gas phase with thoseresulting from the interaction with the solid aerosol. Forthe generation of gaseous NaCl, different aqueous solu-tions of NaCl were nebulized in the � ame. The � ow ratesof argon and hydrogen were adjusted to produce a small,nonluminous � ame. Its temperature was not measured,

but it is believed to be lower than that reported for atypical air–hydrogen � ame burning on a conventionalburner at atmospheric pressure, i.e., much lower than2000 8C. In these experiments, the laser was gently fo-cused into the � ame where the aerosols were formed, andthe � ame height was varied while keeping the laser po-sition � xed and centered on the entrance slit of the mono-chromator.

The difference between sodium thermal emission inthe � ame and the signal induced by the laser photofrag-mentation can be seen in Figs. 2–4. Here, the spectralresolution, due to the use of a 100 lines/mm grating, wasnot suf� cient to separate the doublet. As seen in Fig. 2,thermal atomic emission increases with a correspondingincrease in the height of observation in the � ame. Asexpected, this pattern indicates that an ef� cient dissoci-ation of sodium chloride into atoms does not occur closeto the primary reaction zone of the � ame. The contraryholds in the case of the laser-induced signal, which ismaximum at the � ame location where the concentrationof gaseous sodium chloride is maximum.

The observation of atomic sodium emission when thelaser is present in the � ame fed with the aerosols may bemore informative if the spectral resolution is increased

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FIG. 4. Laser-induced, time-integrated emission of sodium as a func-tion of decreasing sodium chloride concentrations in the nebulized so-lution: (a) 400; (b) 100; (c) 25; (d) 5; (e) 0.5. All concentrations areexpressed in mg/mL of Na. Laser energy: 25 mJ/pulse. Laser � uence:196 mJ/cm2.

so that both D lines (589.0 and 589.6 nm) are observed.The 1200 lines/mm grating was then installed. In thisway, one could easily see the effect of self-absorption onthe width of the lines. Figure 3a shows the spectral be-havior observed when an aqueous solution of NaCl (400mg/mL in Na) is aspirated in the � ame in the absence oflaser excitation. As shown in Fig. 3a, a well-resolveddoublet structure is observed. This observation indicatesthat the effect of self-absorption on the outgoing emissionis negligible both because the low � ame temperature fa-vors the formation of NaCl molecules and because the� ame emission length in the direction of the observationis very narrow, making the emitting vapor optically thin.

When the � ame is not lit and the laser is the onlysource of interaction with the aerosols (Fig. 3b), the in-tensity of the laser-induced emission signal is about 4times lower than the thermal emission. This result indi-cates that the overall process of laser-induced vaporiza-tion and excitation has an ef� ciency close to that of the� ame. However, the striking difference between the twospectra is that the laser interaction results in a broad peak,with a barely recognizable doublet structure. The laserprocess seems to produce more ground-state atoms that

are less con� ned in space, thus lengthening the obser-vation path towards the monochromator and therefore in-creasing the in� uence of self-absorption. Figure 3c,which shows an intermediate behavior between the lim-iting cases (a and b), is obtained with both the laser andthe � ame acting together on the aerosols.

In a further investigatation of the self-absorption effect,increasing aerosol concentrations were irradiated by thelaser in absence of the � ame. The results obtained areshown in Fig. 4. As seen, the doublet can be regarded ascompletely resolved for concentrations of Na below 25mg/mL. For higher concentrations, self-absorption has amore pronounced effect and dominates the shape of thesignal at higher concentrations (spectra c–a). In all thesespectra, the laser � uence was kept at 192 mJ cm22 (25mJ per pulse over an area of 13 mm2).

Chadwick and Morrison5 reported that, in an experi-ment where the furnace temperature was maintained at490 8C, the sodium doublet was completely resolved afterphotofragmentation of 1.4 3 1011 cm23 molecules ofNaCl in a N2 atmosphere. The calibration curve shownby these authors showed a deviation from linearity atgaseous sodium chloride concentrations greater thanabout 20 ppmv, indicating that suf� cient atomic sodiumwas produced to enter the radiation trapping regime. Inorder to compare these � ndings with our results, the pro-portionality factor, relevant to our operating conditions,between the NaCl concentration in solution and its num-ber density in the gas phase needs to be established. Asolution of 5 mg/mL of Na, characterized with the DMPSsystem, was found to produce 1.44 ng of NaCl in theaerosol phase per cm3 of air. For a unity vaporizationef� ciency, this mass density would correspond to 1.5 31013 molecules/cm3 of air or about 0.6 ppmv. Self-ab-sorption becomes dominant (see Fig. 4) at concentrations20 times higher, i.e., directly extrapolating the data ob-tained at lower concentrations, at about 12 ppmv of NaCl.

From the signal intensities reported in Fig. 4, it can beseen that a calibration curve similar to that reported byChadwick and Morrison5 for gaseous sodium chlorideand with laser � uences of 150 mJ cm22 is expected.

Considering all the above experiments, even if onlyqualitative, the most signi� cant conclusion is that, in thecase of NaCl aerosols, the primary interaction processbetween the laser and the particle is very ef� cient.

Figure 5 illustrates the behavior of the laser-inducedatomic emission signal at increasing laser energies, in theabsence of the � ame. The values reported in the abscissarange from 2.5 to 26 mJ/pulse, corresponding to laser� uences and laser irradiances varying from 19 to 200 mJ/cm2 and from 1.5 3 106 to 1.5 3 107 W/cm2, respectively.As reported before,1 the Na2SO4 aerosols produce thelowest signal. However, the most interesting feature seenin this � gure is the well-de� ned laser energy thresholdexisting before sodium emission can be observed, suchthreshold being different for the three aerosol compoundsand increasing in the order NaCl , NaOH , Na2SO 4.Moreover, the behavior of the NaCl is the only one show-ing the tendency for the interaction process to saturate,with the sodium emission reaching a plateau above a cer-tain laser energy. Finally, different interaction regimesseem to be operative in different laser energy intervals,

APPLIED SPECTROSCOPY 813

FIG. 5. Experimental behavior of time-integrated sodium emission asa function of laser energy for three different sodium compounds: (a)Na2SO4; (b) NaOH; (c) NaCl. Concentrations for the three compounds:10 mg/mL of Na.

FIG. 6. (A) Time-resolved waveforms reproducing the laser pulseshape due to scatter by water droplets (a) and the laser-induced sodiumemission shape due to sodium chloride aerosols (b). In B, the time scalehas been enlarged 2.5 times with respect to A to emphasise the differentrise times of the signals. The signal amplitudes are normalized. Laserenergy: 25 mJ/pulse. Calculated peak irradiance (assuming a 13 ns pulsewidth over an area of 9 mm2) is 2.14 3 107 W/cm2. Solution concen-tration: 100 mg/mL of Na.

this effect being more pronounced close to threshold, es-pecially in the case of sulfate aerosols.

The above considerations have to remain at a specu-lative level. In fact, even if the solution concentrationsused for the different salts to obtain the curves of Fig. 5are the same, the size distribution of the aerosol generatedmight be different and will therefore affect the interac-tion. In the case of laser-induced incandescence, for ex-ample, it is known that the size of soot particles affectsboth the heating and the cooling rate of the particles,leading to different temperatures and emission character-istics.15 In any case, the conclusive evidence resultingfrom the curves of Fig. 5 is that, compared to NaCl, morelaser energy (� uence) is needed for NaOH and Na2SO 4

to satisfy the energetic requirements of the overall inter-action process leading to sodium emission.

Since the laser wavelength (193 nm) is too high tomeet the combined requirements of dissociation ofNa2SO4 in the gas phase and excitation of sodium emis-sion, multiphoton interaction can play a role in explainingthe signal observed with this compound. However, purelythermal vaporization/excitation cannot be excluded.

To further investigate the different behavior observedbetween NaCl and Na2SO 4 aerosols, we decided to tem-porally resolve the pro� le of the emission signal duringthe laser pulse. Time-resolved emission studies of a laser-induced plasma in sodium chloride have been reported inthe literature.17 However, to our knowledge, no informa-tion has yet been published on time-resolved emission ofsodium induced by laser photofragmentation of dry aero-sols during the duration of the laser pulse. Such experi-ments are described below.

Time-Resolved Measurements. As described before,these measurements were made with the high-luminositymonochromator without collection optics in front of it,observing about 9 mm 2 of emission area. The signalswere taken with the MCP and displayed on the fast dig-itizing scope. The results, shown in Figs. 6–8, are print-outs of the pro� les observed with the scope.

In order to compare the sodium signals with the laserexcitation pulse, some laser light was re� ected by waterdroplets into the monochromator, and the resulting tem-

poral pro� le recorded. To ensure that this procedure didnot result in any distortion of the pulse, we also measuredthis pro� le with a fast photodiode placed in front of thelaser. The two pro� les were identical.

Figures 6A and 6B show a direct comparison betweenthe laser shape and the sodium emission resulting fromNaCl aerosols. The emission peak is observed after about

814 Volume 55, Number 7, 2001

FIG. 7. Time-resolved waveforms showing the difference in the risetime of the laser-induced sodium emission at a given concentration ofNaCl aerosols for three different laser energies: (a) 25 mJ/pulse; (b) 11mJ/pulse; (c) 6 mJ/pulse. All signal amplitudes are normalized. Solutionconcentration: 100 mg/mL of Na.

FIG. 8. Time-resolved waveforms due to (a) laser scattering by waterdroplets; (b) NaCl aerosols; (c) Na2SO4 aerosols. All signal amplitudesare normalized. Solution concentrations: 100 mg/mL of Na. Laser en-ergy: 25 mJ/pulse.

12 ns from the onset of the laser excitation pulse andabout 4 ns after the time at which the laser pulse hasdeveloped its maximum.

The time necessary to reach the maximum emissionsignal increases as the laser energy per pulse decreases.This pattern is shown in Fig. 7 for three different laserenergies, i.e., 25, 11, and 6 mJ/pulse. The temporal be-havior shown in this � gure, which refers to one aerosolirradiated with different laser energies, is essentially thesame as that observed when different aerosols (NaCl andNa2SO 4) are irradiated with the same laser energy, asshown in Fig. 8. In this particular experiment, the time-integrated signal due to Na2SO 4 aerosols is about 60 timeslower than the corresponding signal due to NaCl aerosols.Clearly, the time necessary to reach the maximum emis-sion is longest for the sulfate aerosols.

Lushnikov and Negin11 have de� ned a characteristicparticle evaporation time, which is limited by the absorp-tion rate of energy necessary for the vaporization of theparticle. This so-called hydrodynamically controlledevaporation regime occurs at laser irradiances similar tothose used in our study, i.e., on the order of 107–108 Wcm22. The evaporation time decreases with increasing la-ser irradiance and particle photo-absorption coef� cient,and increases with increasing particle diameter and en-thalpy of evaporation.

As said before, the relevant parameters are not allknown or amenable to experimental evaluation. The sumof the enthalpies of melting and evaporation, calculatedwith the aid of thermochemical tables18 is 54.9 kcal/molefor NaCl and 84.6 kcal/mole in the case of Na2SO4. As-suming that the other parameters are constant or similar,this difference would be responsible for an increase inthe evaporation time, as is indeed observed experimen-tally.

Finally, the temporal behavior shown in Fig. 8 is log-ically consistent with the observation of different thresh-old interaction energies for the two sodium compounds,as seen in the time-integrated plots shown in Fig. 5.

Analytical Considerations. The characterization ofthe laser photofragmentation technique as an analyticaltool for the detection of sodium in aerosol particles wasnot the purpose of this work. Nevertheless, it was re-garded as interesting to assess the analytical capabilitiesof the technique, i.e., the detection limit obtainable forsodium in airborne particles. The following data refer tothe use of the high-luminosity monochromator and thehigh-gain photomultiplier. By nebulizing a 0.5 mg/mLaqueous solution of NaCl, we obtained a total of 0.034ng/cm3

air, as measured with the DMPS system (see Fig.9). With a laser energy of 25 mJ/pulse, and averaging100 laser pulses, the resulting sodium emission signalwas characterized by a signal-to-noise (S/N) ratio of 100.The noise here is de� ned as the root mean square noiseof the baseline under the sodium peak observed by neb-ulizing water. The detection limit, for an S/N 5 3, was0.015 mg/mL of NaCl, which extrapolates to 1 pg of NaClper cm3

air or to 0.4 mg of Na per m3air. The detection limit

reported for Na in one of the early studies with the tech-nique of laser-induced breakdown spectrometry (LIBS)19

is 6 ng/gair. When converted into these units, our valuebecomes 0.3 ng/gair, i.e., better by more than one orderof magnitude.

It is worth emphasizing that the calculated detectionlimit refers to the size distribution shown in Fig. 9. Thedependence of the vaporization ef� ciency upon the di-ameter of the particles has not been taken into consid-eration. As a consequence, a more valid comparison withthe LIBS method should be based upon results obtainedwith similar particle diameters. As said before, with the

APPLIED SPECTROSCOPY 815

FIG. 9. Measured size distribution of the dry aerosol obtained nebu-lizing a solution of 0.5 mg/mL of NaCl and used for the calculation ofthe limit of detection (see text for discussion).

photofragmentation technique, the vaporization time in-creases with increasing particle diameters. However, aslong as this variation is con� ned to times shorter than theduration of the laser pulse, the above considerationsshould be valid.

Laser-induced vaporization at � uences below break-down has been evaluated before and � rst reported as ananalytical technique for the direct detection of trace met-als in liquid samples.20 The sensitivities reported for dif-ferent elements, on a single-shot basis, were very prom-ising. To our knowledge, this technique has not been pur-sued further. Our result for sodium, if also extended withsimilar sensitivities to other elements, indicates that themethod is worth being revisited.

CONCLUSION

The interaction of laser photons at 193 nm with sodi-um-containing compounds in the dry aerosol phase hasbeen shown to be an ef� cient process, leading to strongatomic emission of sodium and potentially high detectionsensitivity of this element in airborne particles. In thistechnique the laser � uence is kept below the thresholdnecessary to produce a luminous plasma.

The behavior reported in the literature for gaseousNaCl was also observed in the case of dry NaCl aerosols.As shown by time-integrated measurements, self-absorp-tion effects become dominant when laser photons interactwith aerosol concentrations higher than about 10 ppmvof NaCl (assuming unity vaporization ef� ciency), a be-havior in agreement with similar observations made withgaseous NaCl.

Time-resolved measurements, performed during the la-

ser pulse, have shown that the rise time of the atomicemission signal is different for different sodium com-pounds, indicating that the interaction process leading tovaporization, dissociation, and excitation of atomic emis-sion is strongly dependent upon the primary energy de-position step.

The results presented in this paper and the associateddiscussion remain qualitative. Indeed, the complexity ofthe processes leading to particle disintegration has beenemphasized in a recent publication,21 indicating that notonly thermal and photochemical effects may play a role,but also photomechanical and electronic effects. In ad-dition, the overall interaction process must be clearly de-pendent upon the size distribution of the aerosol. How-ever, such dependence was not studied in this work.

Finally, the analytical sensitivity obtained for sodiumis indeed better than that reported in early LIBS studies,indicating that the technique might be advantageously ex-tended to other elements.

ACKNOWLEDGMENTS

M. Hidalgo Nunez is grateful to the Environment Institute of the JRCIspra for granting her a doctoral fellowship. Useful discussions with P.Cavalli and G. Petrucci are also acknowledged.

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