Measurement of Polystyrene Nanospheres Using Excimer Laser Fragmentation Fluorescence Spectroscopy

JONG HYUN CHOI,* CATHERINE P. KOSHLAND, ROBERT F. SAWYER, and DONALD LUCAS Mechanical Engineering Department, University of California, Berkeley, California 94720 (J.H.C., R.F.S.); School of Public Health, University of California, Berkeley, California 94720 (C.P.K.); and Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (D.L.)

Monodisperse polystyrene nanospheres with a mean diameter of lasers is used to generate neutral atoms, molecules, and 102 nm are photofragmented with 193 nm light in N2 at laser ¯u- ions for chemical analysis, such as aerosol time-of-¯ight ences from 1 to 20 J/cm2. Carbon atom ¯uorescence at 248 nm from mass spectrometry (ATOFMS).8 Excimer laser fragmen- the disintegration of the particles is used as a signature of the poly- tation ¯uorescence spectroscopy (ELFFS) uses mildly fo- styrene. The normalized ¯uorescence signals are self-similar with cused UV light to photolytically decompose non-¯uo- an exponential decay lifetime of ϳ10 ns. At ¯uences above 17 J/cm2, optical breakdown occurs and a strong continuum emission is gen- rescing large molecules or particles into smaller, ¯uores- erated that lasts signi®cantly longer. A non-dimensional parameter, cent species without signi®cantly heating the particles/ the photon-to-atom ratio (PAR), is used to interpret the laser-par- molecules or creating a plasma. The ¯uorescence is ticle interaction energetics. Carbon ¯uorescence from polystyrene measured as a signature of the parent molecules or par- particles is compared with that from soot, and a similarity between ticles, providing information on the chemical composi- the two particles is observed when normalized with PAR. Carbon tion and concentration. We and other researchers previ- emission from bulk polystyrene was also measured. Similar emis- ously used ELFFS to measure combustion-generated spe- sion signals were observed, but the breakdown threshold of the sur- cies,9±11 particles,12±15 and toxic metals in soil.16 The in- face is signi®cantly lower at 0.2 J/cm2. teraction of UV photons with particles was also used to Index Headings: Nanosphere; Nanoparticle; Photofragmentation; disintegrate the particles and produce nanoparticles with Fluorescence; Disintegration; Breakdown; Plasma; Excimer laser a controlled size and morphology.17±20 However, questions fragmentation ¯uorescence spectroscopy; ELFFS. with regard to the extent of particle disintegration and the behavior of the ejected species that in¯uence the measured emission have not been fully answered. INTRODUCTION Laser interactions with combustion-generated soot, the Combustion processes are a dominant source of small most commonly encountered atmospheric particles, are particles in the atmosphere, and these particles adversely complex; the interaction strongly depends on the wave- affect environmental and human health.1 Evidence sug- length and energy of the light, the particle size, and the gests that many adverse impacts may be associated with chemical composition. In addition, soot particles are frac- nanoscale particles. Nanoparticles may be more effective tal agglomerates of primary particles. To simplify the in transporting toxic materials during exposure as a result study of the basic behavior of nanoparticles with 193 nm of their high surface area per unit volume.2 The unique light, we irradiate monodisperse polystyrene nanospheres properties of many nanoscale particles, while posing po- with 193 nm photons. Unlike soot, polystyrene particles tential risks, are the assets leading to applications in drug possess a uniform size, are spherical, and have well-char- delivery,3 bio-molecule detection,4 and microelectronics,5 acterized material and optical properties. A better under- to name a few. The widespread potential as well as con- standing of the nanoparticle interaction with UV light, cerns make it desirable to develop accurate analytical which has features distinct from gas or solid interactions, techniques to characterize basic particle properties, in- helps to improve not only laser-based measurements of cluding chemical composition, size, morphology, and soot and other aerosol particles, but also novel nanopar- concentration. ticle production techniques using UV light. Various in situ, real-time spectroscopic methods have The speci®c aims of the present study are to (1) ana- been employed to monitor airborne nanoparticles. Laser- lyze the characteristics of carbon atom ¯uorescence pro- induced incandescence (LII) utilizes visible/infrared (IR) duced by 193 nm laser irradiation of polystyrene nano- laser beams with intensities lower than 107 W/cm2 to heat spheres, (2) measure the emission formed by the optical the particles without signi®cant vaporization.6 The broad- breakdown of small particles, (3) develop a parameter band incandescence is used to deduce information on pri- (the photon/atom ratio or PAR) to estimate the extent of mary particle size and volume fraction. Laser-induced particle disintegration, and (4) assess how laser-particle breakdown spectroscopy (LIBS) employs higher intensity interactions differ from laser interactions with a gas or a visible/IR lasers to create a plasma, thermally disintegrate bulk solid. the particles, and measure atomic emission in the plume as the gases cool.7 Direct ablation with ultraviolet (UV) EXPERIMENTAL

Received 23 March 2005; accepted 8 July 2005. Nearly monodisperse polystyrene (C8H8)n nanospheres * Author to whom correspondence should be sent. E-mail: jhyunc@ in water (Duke Scienti®c) have a size distribution of 102 me.berkeley.edu. Ϯ 3 nm. Nitrogen gas at 6 standard liters per minute

0003-7028 / 05 / 5910-1203$2.00 / 0 Volume 59, Number 10, 2005᭧ 2005 Society for Applied Spectroscopy APPLIED SPECTROSCOPY 1203 FIG. 1. Schematic of the laser and optical emission measurement system.

(slpm) is supplied to a 3-jet Collision nebulizer (BGI, CN-24), generating droplets from the polystyrene solu- tion diluted with distilled water (Fig. 1). The droplets pass through a diffusion dryer (TSI 3062) that removes FIG. 2. The time characteristics of the normalized optical emissions of water from the particle-laden ¯ow. The polystyrene nano- carbon atoms at 248 nm from polystyrene nanospheres in a N2 stream ϭ ϫ as a function of laser ¯uence at 4 Hz. Optical breakdown occurs at spheres in the N2 stream (number concentration 1.6 approximately 17 J/cm2. Below the breakdown threshold, the ¯uores- 5 Ϫ3 10 cm ) are introduced into the laser interrogation re- cence signals are self-similar with a decay lifetime of ϳ10 ns. Once gion at 0.3 slpm. Some of the nebulized particles ag- breakdown occurs, the emission become continuous and the signal lasts glomerate, but size distribution and morphology mea- signi®cantly longer. surements with a scanning mobility particle sizer and a transmission electron microscope reveal that individual normalized in magnitude and shifted in time to a common nanospheres dominate the distribution. For the solid mea- scale as the different laser conditions affect the signal surements, a 1.5 mm thick polystyrene sheet was ob- magnitude and time at which lasing actually occurs com- tained from Professional Plastics. A 3 ϫ 3cm2 piece of pared to the trigger pulse. The 193 nm photons convert the sheet is irradiated normal to the incident light in an a fraction of the particles into gas-phase atoms, mole- N environment. 2 cules, and ions, producing some carbon atoms in an elec- An ArF excimer laser (Lambda Physik, LPX 210i) tronically excited state (1P 0 ← 1D ) through a multipho- generates 193 nm pulses with a duration of 20 ns. Ac- 1 2 ton process. The C(1P 0) atoms subsequently ¯uoresce to cording to the manufacturer, the spatial pro®le of the laser 1 the 1S ground state at 248 nm. While photochemical beam is ¯at-top across the width and Gaussian across the 0 pathways to produce C(1D ) from the polystyrene parti- height. The laser beam is focused by either a 3.8 cm 2 cles are very complex, the processes from various gas- diameter, 25 cm focal length or a 2 cm diameter, 6 cm phase molecules are relatively well understood.11,21 The focal length UV-grade fused silica lens (CVI Corp.). The resonance between the laser output and the electronic ex- laser energy is measured with a Gentec joulemeter (ED- citation of carbon atoms makes the measurement more 500ϩ). The beam footprint is determined by placing a sensitive. Previous measurements of 248 nm ¯uorescence paper in the laser interrogation region and measuring the from carbon-based particles showed that the detection size of the hole made by the focused laser pulses using limit of carbon atoms using this ELFFS system is sub- a microscope. The ¯uence is varied by changing the laser ppb.12 The C atoms in the 1P 0 state also depopulate due charging voltage. The ¯uences from 0.1 to 20 J/cm2 are 1 to collisional quenching. The collision-free lifetime in the used at repetition rates of 1 and 4 Hz. The residence time excited state is approximately 29 ns.22 Exponential curve of the particles in the focal volume ranges from 20 to 70 ®tting of the signals in the ®gure shows that the decay ms, so the ¯ow ®eld is frozen compared to the laser± of the 248 nm emission at ¯uences from 2 to 13 J/cm2 particle interaction timescale (ϳ20 ns). Hundreds to sev- is approximately 10 ns. In this range, the normalized eral thousand nanospheres interact with a single laser emissions are self-similar. pulse. The optical emission is collected at a right angle Direct photochemical bond breaking is the dominant bya5cmdiameter bi-convex collection lens and is trans- mechanism in the laser±particle interactions. We observe ferred to a GCA/McPherson 0.3 m scanning monochro- no broadband incandescence associated with particle mator with a Hamamatsu R928 photomultiplier tube heating using 193 nm light, while the same nanoparticles (PMT). The PMT signal is recorded on a LeCroy LT342 irradiated by 532 nm pulses at similar ¯uences emit in- digital oscilloscope without any time gating. The slit candescence. Other researchers observed that ArF laser width of the monochromator is 0.4 mm, corresponding irradiation of a solid polymer surface produced fragments to a 1.1 nm spectral bandwidth. ranging from diatomics to high molecular weight prod- ucts and concluded that thermal effects were insigni®- RESULTS AND DISCUSSION cant.23,24 A mass spectrometry study in vacuum showed Figure 2 presents the time-resolved atomic carbon that the major product from the photochemical decom- emissions (248 nm) from polystyrene nanospheres in N2 position of a bulk polystyrene surface is neutral styrene 2 25 for laser ¯uences from 2 to 20 J/cm . The signals are monomers (C8H8). In this work we do not observe other

1204 Volume 59, Number 10, 2005 2⌬ → 2⌸ lines, such as CH at 431 nm (A X ) and C2 at 468 3⌸ → 3⌸ nm (d g a u), which were observed with diesel soot12 and hydrocarbon-adsorbed soot particles at similar laser conditions in our laboratory. At 20.5 J/cm2, the emission lasts longer with an exponential decay lifetime of approximately 70 ns, and the peak of the signal is an order of magnitude larger than those at ¯uences from 2 to 13 J/cm2, owing to plasma formation. Recombination of species in a high-temperature (Ͼ10 000 K) plasma gas takes place as the hot gas cools. Therefore, emission from the plasma lasts signi®- cantly longer than atomic ¯uorescence in the absence of optical breakdown. The mechanisms that initiate breakdown have been discussed previously.26±29 As laser ¯u- ence increases, more atoms, molecules, ions, and electrons are ejected, and continuum absorption of intense UV photons by the electrons leads to cascade collision ionization and optical breakdown. Once optical breakdown occurs, a strong visible and audible emission is generated. The breakdown threshold in our system is approximately 17 J/cm2. LIBS generates this type of signal with laser intensity greater than 109 W/cm2 and monitors atomic emission lines with time gating. ELFFS typically utilizes laser intensities low enough to avoid optical breakdown (107ϳ109 W/cm2) and measures the ¯uorescence peaks of the emitting species fragmented from the irradiated particles where the temperature change is neg- ligible. The background signals are also measured at 255 nm (not shown), where there is no known transition, and similar phenomena are observed. The background signal at ¯uences from 2 to 13 J/cm2 lasts approximately 10 ns. The signal is 3 to 7 times smaller than carbon ¯uorescence at 248 nm. At 20.5 J/cm2 where the plasma is formed, the background signal is almost identical to the emission at 248 nm. This strong background emission is observed over a wide range of wavelengths. ELFFS ben- FIG. 3. The 100 shot average carbon ¯uorescence from polystyrene e®ts from an increased signal-to-noise ratio without time and soot particles for various laser ¯uences and photon/atom ratios at gating since there is no intense continuum emission that 4 Hz. The ¯uorescence by ELFFS shows linear, intermediate, and sat- interferes with analyte signals. uration regimes. (a) Normalized ¯uorescence from the particles as a function of laser ¯uence. (b) Normalized ¯uorescence from the particles The 100 shot average ¯uorescence at 248 nm with the as a function of PAR. averaged background signal at 255 nm subtracted is shown as a function of laser ¯uence in Fig. 3a. These results are compared to combustion-generated soot results affect the emission signals. In this study we did not de- from our previous study.15 The soot particles, generated termine the lower limit of laser ¯uence necessary to pro- by a well-controlled non-premixed ¯ame, are composed duce detectable carbon ¯uorescence from the nano- of mainly elemental carbon as con®rmed by ELFFS and spheres. However, we observed that laser irradiation of EC/OC measurements. The particles have a polydisperse different particles including soot,20 NaCl,18 polystyrene, size distribution with a mean diameter of 280 nm and a and gold at ¯uences below 0.3 J/cm2 partially disinte- number concentration of 9.2 ϫ 105 cmϪ3. The soot is an grated the particles and produced smaller particles with agglomerate composed of nearly 40 nm primary particles an order of magnitude higher concentration. The low ¯u- with a fractal dimension near 1.8. The ¯uorescence sig- ence photolysis generates gas-phase species that subse- nals from these two kinds of particles are normalized in quently nucleate to form a new mode of particles. magnitude since soot particles have a much higher con- The carbon signals from soot also have linear, inter- centration, yielding stronger signals. mediate, and saturation regimes. In the soot system, how- The ¯uorescence signal from polystyrene increases lin- ever, the ¯uorescence increases at a slower rate; the linear early with laser ¯uence from 1 to 3 J/cm2. The signal regime ends at approximately 6 J/cm2 and the saturation saturates above 6 J/cm2 and remains constant until break- begins around 12 J/cm2 owing to different chemical com- down occurs. This type of ¯uorescence saturation at high- position and larger size, indicating that the extent of dis- er ¯uence was also observed with gas-phase molecules, integration of these two particles is different. 9 such as NH fragmented from NH3, and makes ELFFS We use a non-dimensional parameter, called the pho- more robust for quantitative chemical analysis as shot-to- ton-to-atom ratio or PAR, to interpret these data. PAR is shot variations in the laser energy do not signi®cantly the ratio of the number of absorbed photons to the num-

APPLIED SPECTROSCOPY 1205 ber of atoms in the particle and has been discussed previously.15,18,20 The laser ¯uence that is conventionally used to describe the laser energy density is effective for homogeneous target media such as pure gases or bulk solids, but nanoparticles suspended in a gas are discrete objects in the laser probe volume. The photon energy density for these particles can be better described by con- sidering the energy balance for each particle. For laser± particle interactions, the photon/atom ratio is more rele- vant than the ¯uence, because the total number of atoms is a function of particle volume (ϳd3) while the number of photons striking a particle depends on the cross-sectional area (ϳd 2). For photons with the same energy as the average bond energy in a particle, the PAR must be unity or higher for complete fragmentation of the particle. The number of photons striking the particles is calculated using the laser ¯uence and the cross-sectional area of the FIG. 4. The time characteristics of the normalized emissions of carbon particles, while the number of atoms is obtained with the atoms from a solid polystyrene surface. The laser ¯uences range from particle density and volume. The PAR concept can be 0.08 to 0.3 J/cm2 at 1 Hz. The signals are similar to those from nano- expanded by accounting for the absorption ef®ciency of spheres. The plasma appears approximately at 0.2 J/cm2. the particle material and incident wavelength. The absorption process of incident photons depends on the particle material, diameter, and wavelength. Here, the PAR manually translated in the vertical direction so that each values are corrected for the 57% absorption of 193 nm shot fragments a fresh surface. The characteristic emis- light by 102 nm polystyrene nanospheres using Mie the- sion from the bulk solid is analogous to that from nano- ory30 with a complex refractive index of 1.6±1.23i.31 spheres. The self-similar emissions at 0.08 and 0.12 J/cm2 The normalized ¯uorescence signals from polystyrene have an exponential decay time of approximately 10 ns. and soot are presented as a function of PAR in Fig. 3b. The magnitude of the signal increases with the laser ¯u- These two signals appear similar; both polystyrene and ence from 0.08 to 0.2 J/cm2. At 0.2 J/cm2, optical break- soot have a linear regime up to a PAR ϭ 1.5 and saturate down occurs, creating intense white light, and the signal above a PAR ϭ 3. The good agreement in the ¯uores- lasts longer. Then, the lifetime of the emission further cence signals, regardless of particle material, diameter, increases with ¯uence as the plasma plume grows. The shape, dispersion in size, and concentration, indicates that decay lifetime of the signal at 0.3 J/cm2 is longer than PAR can be an effective tool for understanding laser± 100 ns. Once the plasma is formed, the signal magnitude particle interactions, and can be used to assess the amount remains at approximately 60 mV, similar to the value for of particle disintegration. When the 193 nm irradiation the nanospheres, even at higher laser ¯uences. The sat- of soot or carbon-based polymer particles meets the con- uration of the signal above the plasma threshold is likely dition of PAR ϭ 3 or higher, the particles can be consid- due to the probe volume being ®lled with the hot plasma ered to be disintegrated into atomic species. Note that the gas. A similar phenomenon is observed with the back- PAR is corrected for absorption ef®ciency of 193 nm ground emissions at 255 nm from the bulk surface (not photons by particles. If the irradiated particle material shown); the signals have a decay time of ϳ10 ns at 0.08 were not to ef®ciently absorb 193 nm light, the saturation and 0.12 J/cm2, which increases with the laser ¯uence as condition would not be achieved and a majority of par- the plasma plume develops. When the plasma forms, the ticles would not be disintegrated. The saturation near background signal becomes identical to the carbon signal PAR ϭ 3 also suggests that the photofragmentation pro- at 248 nm. The background, however, increases faster at cess is not perfectly ef®cient; at least 2 photons are nec- lower ¯uences, resulting in a decrease of the signal-to- essary for the liberation and electronic excitation of an noise ratio. Thus, low laser ¯uences should be used for atom. However, 193 nm laser photolysis is much more ELFFS detection of a bulk material to maintain a high ef®cient in particle disintegration than direct laser abla- signal-to-noise ratio. tion methods or LIBS with nanosecond laser pulses that The ELFFS measurements of Pb at 405.8 nm from a use orders of magnitude more photons to create a plasma solid Pb(NO3)2 surface in our previous study produced and thermally disintegrate the particles. Optical break- similar results.16 The Pb emission increases with ¯uence down occurs approximately at a PAR ϭ 8.7 (17 J/cm2), and saturates when the plasma forms. The emission pro- implying that a plasma is formed when approximately 9 duced by optical breakdown lasts signi®cantly longer and photons strike each atom in the nanospheres. Note that the decay lifetime further increases with the ¯uence. in this work the PAR is used as a comparative parameter However, the breakdown threshold for Pb(NO3)2 was ap- for different particle composition, size, morphology, and proximately 2 J/cm2, which is a factor of 10 higher than concentration, and there are assumptions and approxi- that for polystyrene. Lencioni32 showed that the break- mations that the particles are evenly distributed in the down threshold for bulk surface could vary more than an laser probe volume and the laser energy for the PAR order of magnitude for different materials. Feldmann et calculation is temporally and spatially averaged. al.25 also observed that 13 ns pulses of 193 nm light from Figure 4 shows the time-resolved carbon emissions 0.015 to 0.05 J/cm2 photochemically decomposed a solid from the surface of the polystyrene sheet. The sheet is polystyrene surface with no plasma formed. The 193 nm

1206 Volume 59, Number 10, 2005 laser photolysis of other polymer surfaces, such as CONCLUSION poly(methyl methacrylate) (PMMA),23 produced detectable species at ¯uences as low as 0.014 J/cm2 and sig- Polystyrene nanospheres in N2 are measured with ni®cant photodecomposition starting at 0.07 J/cm2. The ELFFS to study the photochemical interactions between photochemical cleavage of PMMA continued up to 0.5 the nanoparticles and 193 nm light. The characteristic J/cm2, likely due to the lower absorption coef®cient of carbon emissions from the particles are analyzed and the material (2 ϫ 103 cmϪ1) compared to polystyrene (8 compared to the signals produced when a plasma is ϫ 105 cmϪ1). formed. In ELFFS, the emissions from the nanospheres are self-similar and have an exponential decay lifetime The breakdown threshold of bulk polystyrene (0.2 ϳ J/cm2 or 107 W/cm2) is two orders of magnitude lower of 10 ns. Once a plasma is formed by intense UV ir- than the threshold for the gas±particle mixture, an indi- radiation, a strong continuum emission is generated and cation that small particles interact differently with lasers the signal lasts signi®cantly longer. While ¯uorescence than pure gases or bulk solids. The presence of particles signals from polystyrene and soot particles have linear, in a gas reduces the gas breakdown threshold. In this intermediate, and saturation regimes, the signals have dif- study, the plasma in the particle±gas mixture is formed ferent slopes with laser ¯uence, indicating that the extent 2 of disintegration of the two particles is different. When near 17 J/cm , while the optical breakdown in pure N2 is not observed even at 20 J/cm2 (109 W/cm2). Pure nitrogen normalized using the photon/atom ratio, a similarity in has a very high breakdown threshold, 5 ϫ 1011 W/cm2 at the signals from the two particle types is established in- 350 nm.33 Even taking into account the decreasing trend dependent of different particle properties and laser con- of the threshold with wavelength in the UV/visible ditions, suggesting that the extent of particle disintegra- 34,35 tion can be estimated with PAR. The PAR for complete range, the plasma threshold of pure N2 gas would be far above 109 W/cm2. Pinnick et al.27 also reported that particle disintegration and electronic excitation of liber- ated carbon atoms is approximately 3. The plasma thresh- the breakdown threshold of gas±particle mixtures is more 2 than two orders of magnitude lower than that of the pure old of polystyrene particles in N2 is found to be 17 J/cm while the threshold for a bulk polystyrene surface de- gas and decreases with light in the UV/visible range. 2 When compared with carbon-containing gases, the break- creases to 0.2 J/cm , which implies that laser interactions with small particles are different than with a bulk solid. down thresholds of polystyrene particles in N2 is 50 times lower; e.g., the thresholds of CO and CO2 at 1 atm by 193 nm laser irradiation are 2 ϫ 1010 and 7 ϫ 1010 ACKNOWLEDGMENTS W/cm2, respectively.28 We thank Dr. Thomas Kirchstetter for providing EC/OC measure- The gas breakdown threshold induced by the particles ments. This work was supported by the Environmental Health Sciences is also a function of the particle diameter. As shown by Superfund Basic Research Program (Grant Number P42ESO47050-01) 36 32 from the National Institute of Environmental Health Sciences, NIH, Smith and Lencioni, the threshold monotonically de- with funding provided by the EPA. creases with increasing particle diameter. For instance, air breakdown by a 10.6 ␮m laser occurs near 4 ϫ 109 W/cm2 in the presence of 300 nm carbon particles, while 3 ␮m 1. C. P. Koshland and S. L. Fisher, ``Diagnostic Requirements for Tox- particles decrease the threshold to 3 ϫ 108 W/cm2.An ic Emission Control,'' in Applied Combustion Diagnostics,K. inverse dependence of breakdown threshold on particle di- Kohse-Hoinghaus and J. B. Jeffries, Eds. (Taylor and Francis, New ␮ York, 2002), Chap. 24, p. 606. ameter is observed up to 10 m. As the particles become 2. M. D. Cheng, B. Malone, and J. M. E. Storey, Chemosphere 53, larger, the threshold approaches the value corresponding 237 (2003). to the laser±bulk solid interaction, e.g., 107 W/cm2 for sol- 3. F. Yu, Y. Liu, and R. Zhuo, J. Appl. Polym. Sci. 91, 2594 (2004). id carbon. Depending on the particle diameter, material, 4. J. M. Nam, C. S. Thaxton, and C. A. Mirkin, Science (Washington, D.C.) 301, 1884 (2003). and wavelength, the breakdown threshold induced by the 5. H. Htoon, J. A. Hollingsworth, A. V. Malko, R. Dickerson, and V. particles is higher than the threshold from laser±solid in- I. Klimov, Appl. Phys. Lett. 82, 4776 (2003). teraction by a factor of 10 to 100, similar to what we 6. H. A. Michelsen, J. Chem. Phys. 118, 7012 (2003). observe with the polystyrene nanoparticles and solid. On 7. D. W. Hahn and M. M. Lunden, Aerosol Sci. Technol. 33, 30 the other hand, the threshold approaches that of a pure gas (2000). 8. D. T. Suess and K. A. Prather, Chem. Rev. 99, 3007 (1999). as the particles become smaller. Another point to consider 9. S. G. Buckley, C. J. Damm, W. M. Vitovec, L. A. Sgro, R. F. in particle-induced breakdown is that the threshold tends Sawyer, C. P. Koshland, and D. Lucas, Appl. Opt. 37, 8382 (1998). to decrease with increasing particle concentration. The 10. C. S. McEnally, D. Lucas, C. P. Koshland, and R. F. Sawyer, Appl. threshold is determined by the largest particle in the focal Opt. 33, 3977 (1994). 11. R. C. Sausa, A. J. Alfano, and A. W. Miziolek, Appl. Opt. 26, 3588 volume, and larger particles likely exist at higher concen- (1987). 32 trations due to agglomeration. In the breakdown thresh- 12. C. J. Damm, D. Lucas, R. F. Sawyer, and C. P. Koshland, in 29th old measurements, accurate determination of the particle Symposium (International) on Combustion (The Combustion Insti- size is the most dif®cult task and the main source of ex- tute, 2002). perimental uncertainty. Note that the absorption process, 13. M. H. Nunez, P. Cavalli, G. A. Petrucci, and N. Omenetto, Appl. Spectrosc. 54, 1805 (2000). the particle size, and the chemical composition determine 14. M. H. Nunez and N. Omenetto, Appl. Spectrosc. 55, 809 (2001). the mode of photodecomposition of the particle surface 15. C. B. Stipe, D. Lucas, C. P. Koshland, and R. F. Sawyer, ``Soot and the behavior of the ejected species. Therefore, the pho- Particle Disintegration and Detection Using Two Laser ELFFS,'' ton-to-atom ratio can be useful in understanding particle Appl. Opt., paper accepted (2005). 16. J. H. Choi, C. J. Damm, N. J. O'Donovan, R. F. Sawyer, C. P. disintegration by UV photons and gives an insight into Koshland, and D. Lucas, Appl. Spectrosc. 59, 258 (2005). laser±particle interactions, which are different than the in- 17. H. Cai, N. Chauhary, J. Lee, M. F. Becker, J. R. Brock, and J. W. teractions with gases or solids. Keto, J. Aerosol Sci. 29, 627 (1998).

APPLIED SPECTROSCOPY 1207 18. J. H. Choi, C. B. Stipe, C. P. Koshland, R. F. Sawyer, and D. Lucas, 27. R. G. Pinnick, P. Chylek, M. A. Jarzembski, E. Creegan, V. Sri- J. Appl. Phys. 97, 124315 (2005). vastava, G. Fernandez, J. D. Pendleton, and A. Biswas, Appl. Opt. 19. P. V. Kamat, M. Flumiani, and G. V. Hartland, J. Phys. Chem. B 27, 987 (1988). 102, 3123 (1998). 28. J. B. Simeonsson and A. W. Miziolek, Appl. Opt. 32, 939 (1993). 20. C. B. Stipe, J. H. Choi, D. Lucas, C. P. Koshland, and R. F. Sawyer, 29. D. Smith and R. Brown, J. Appl. Phys. 46, 1146 (1975). J. Nanopart. Res. 6, 467 (2004). 30. C. F. Bohren and D. R. Huffman, Absorption and Scattering of 21. J. Bokor, J. Zavelovich, and C. K. Rhodes, J. Chem. Phys. 72, 965 Light by Small Particles (Wiley Interscience, New York, 1983). (1980). 31. J. Meyer, J. Kutzner, D. Feldmann, and K. H. Welge, Appl. Phys. 22. http://physics.nist.gov/cgi-bin/AtData/lines࿞form. B 45, 7 (1988). 23. R. Srinivasan, B. Braren, D. E. Seeger, and R. W. Dreyfus, Ma- 32. D. E. Lencioni, Appl. Phys. Lett. 23, 12 (1973). cromol. 19, 916 (1986). 33. D. I. Rosen and G. Weyl, J. Phys. D: Appl. Phys. 20, 1264 (1987). 24. E. Sutcliffe and R. Srinivasan, J. Appl. Phys. 60, 3315 (1986). 34. A. J. Alcock, K. Kato, and M. C. Richardson, Opt. Commun. 6, 25. D. Feldmann, J. Kutzner, J. Laukemper, S. MacRobert, and K. H. 342 (1972). Welge, Appl. Phys. B 44, 81 (1987). 35. H. T. Buscher, R. G. Tomlinson, and E. K. Damon, Phys. Rev. Lett. 26. P. Chylek, M. A. Jarzembski, V. Srivastava, R. G. Pinnick, J. D. 15, 847 (1965). Pendleton, and J. P. Cruncleton, Appl. Opt. 26, 760 (1987). 36. D. Smith, J. Appl. Phys. 48, 2217 (1977).

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