Plasma spectroscopy of H, Li, and Na in plumes resultingfrom laser-induced droplet explosion

Johannes H. Eickmans, Wen-Feng Hsieh, and Richard K. Chang

The plasma emission resulting from laser-induced breakdown of large transparent H20 droplets (with andwithout NaCl or LiCl) has been spectrally and spatially resolved along a strip which encompasses the dropletand two plasma plumes associated with materials streaming from the droplet. From the emission line shapes,relative emission intensity ratios, and absorption line reversals, estimates can be made of the electron density,plasma temperature, and spatial inhomogeneity of the plasma along a strip in the direction of the laser beam.Use of the emission lines of H, Li, and Na as atomic tracers for plasma diagnostics is discussed.

1. Introduction

Laser-induced explosion of absorbing droplets witha radius smaller than the incident wavelength (a < X,e.g., micrometer-size H20 droplets irradiated by 10.6-,um CO2 laser pulses) has been investigated experimen-tally with schlieren photographs to observe the shockwave and with regular photographs to record theplume shape resulting from the material streamingfrom the droplet.1 2 Computations have been at-tempted to solve the complex hydrodynamics problemduring and after laser heat has been deposited withinthe droplet.3 -6

The mechanisms leading to droplet explosion forlarge transparent droplets (with a > X) are quite differ-ent from those for small absorbing droplets. For largetransparent droplets irradiated by a Q-switched laserbeam with intensity Io, dielectric breakdown is causedby multiphoton ionization and avalanche multiplica-tion. A dense plasma is produced and absorbs thesubsequent portion of the laser pulse, which causesdroplet explosion.7 Depending on the liquid and sur-rounding gas, breakdown can be initiated either (1) inthe gas just in front of the droplet shadow face at adistance r _ 1.6a from the droplet center, where thenear-field intensity8 can be 103Io, or (2) in the liquidjust within the shadow face at r 0.8a, where theinternal intensity8 can be 102Io.

The authors are with Yale University, New Haven, Connecticut06520.

Received 11 April 1987.0003-6935/87/173721-05$02.00/0.© 1987 Optical Society of America.

Recent experiments have determined where laser-induced breakdown occurs when a high-intensity laserbeam in the visible wavelength region (e.g., X = 0.532,gm) is incident on a large transparent droplet. 7 9 10

For H20 droplets in Ar or Xe gas, laser-induced break-down is initiated in the gas.7"10 The plasma emission isconfined to a strip just in front of the shadow face, andthe spectra consist of a continuum and singly ionizedAr II lines.7 For H2 0 droplets in air, breakdown occursin the liquid.9 At lower Io, photographs reveal a lumi-nous droplet, a luminous forward plume in front of theshadow face, and a dark zone between this plume andthe droplet. The spectra consist of the plasma contin-uum and the H Balmer lines (Hae and Hg) resultingfrom the decomposition of H20. At higher Io, photo-graphs reveal a luminous backward plume (behind theilluminated face of the H2 0 droplet) in addition to thebright forward plume and droplet (similar to thoseshown Fig. 1). The spectra in the backward plumeconsist of the plasma continuum, Hg and Ho lines, andthe lines of singly ionized 0 II and N II which resultfrom the decomposition of air.

A report has recently been published on the time-resolved, spatially averaged spectra of ionized andneutral carbon (C II and C I) and of ionized nitrogen(N ii) from glycerine droplets (a = 9 + 2 jim) in airirradiated by a Q-switched second harmonic Nd:YAGlaser."1 The emission spectra from the plumes anddroplets were spectrally analyzed with a spectrometer,detected with a photomultiplier, and processed by atransient digital analyzer. The spatially averagedemission intensities of the various species (C II, C i, andN II) were noted to reach a maximum value at differenttime delays after the laser pulse. Furthermore, thepeak wavelength of the Stark-shifted C I peak wasnoted to change as a function of time. From these

1 September 1987 / Vol. 26, No. 17 / APPLIED OPTICS 3721

Backward0 -4 -8 r/a

-. Ir__r_ i i I I I I I-T--T--

Spectrograph -

Slit Laser DirectionFig. 1. Exploding H 20 droplets (a 41 m) taken through amicroscope and blue filter. Indicated are the spectrograph slit, laserbeam direction, and relative distance from the center of the droplet(in units of rna, where ra > 1 for the forward plume in front of theshadow face and r/a < -1 for the backward plume behind theilluminated face). Part of the circular liquid-air interface is observ-able. This photograph required an accumulation of twenty-five

laser shots at Io 28 GW/cm 2 .

results, the spatially averaged electron density ne(varying from 0.7 to 0.3 X 10l8/cm 3 ) can be deduced asa function of time, assuming the plasma temperatureto be in the 10-15 X 103 K range. The intrusion of airinto the plasma in unknown quantities renders theestimation of the plasma temperature through theSaha analysis meaningless.'1

We report here on the use of spatially resolved,temporally averaged emission spectra from atomictracers in the forward and backward plumes of explod-ed H20 droplets containing LiCl or NaCl as a means ofproviding information on the inhomogeneous ne andelectron temperature Te. Plasma diagnostics fromvarious portions of the forward and backward plumesare possible by measuring the spatially dependentatomic emission linewidths, absorption line reversals,and emission intensity ratios of lines from H, Na, or Li.

II. Experiment

In the experimental configuration, the axes of thelaser beam, linear stream of droplets, and emissioncollection optics are orthogonal. The second harmon-ic Q-switched Nd:YAG laser beam (X = 0.532 jim, 20-nsduration, 10 pulses/s) is focused by a 25-cm lens to aspot diameter of 100 jim. The monodisperse drop-lets (with a 45 jim) are generated by a vibratingorifice. The collection optics magnify the image 5times, a Dove prism rotates the image 900, and a spec-trograph, which preserves the image presented on itsentrance slit (see Fig. 1), disperses the emission. Avidicon camera is placed at the spectrograph exit planeand accumulates the spatially resolved spectra from 50to 150 laser pulses which are synchronized to the drop-let generation rate.

Figure 1 is a photograph of the two luminous forwardand backward plumes and the luminous H20 dropletin air with Io 28 GW/cm2. The various vidicontracks along the laser beam direction can be related tothe distance to droplet radius ratio (r/a). Spatial av-eraging along the width of the slit and along the path of

U)

.E

M

I-U,zI-z

675 670 665

675 670 665

WAVELENGTH (nm)Fig. 2. Spatially resolved spectra of the Li resonance line (2s-2p) ofexploding H2 0 droplets (a 45 um) containing 5-M LiCl. Thevidicon tracks pertaining to the droplet are shaded, and those per-taining to the plumes at distances from the droplet center are speci-fied by r/a. The narrow valley is caused by the Li absorption linereversal, and the Li emission peak is Stark broadened. These spec-tra required an accumulation of eighty laser shots at Io 1.1 GW/

cm2 (a) and Io 1.8 GW/cm 2 (b).

the collection optics is unavoidable and affects thespectral line shapes and intensity ratios, particularlysince the plasma is inhomogeneous and optically thin.Temporal averaging throughout the emission lifetime(-100 ns) 11,12 is also unavoidable as our vidicon camerais not temporally gated.

Ill. Results and Discussion

A. Li Doublet

Figure 2 shows the spatially resolved spectra at twoIo levels (1.1 and 1.8 GW/cm 2 ) with 5-M LiCl H2 0droplets in air. The emission peak and line reversalabsorption valley correspond to the Li resonance 2s-2p transitions. The Li atoms are transported into theforward plume (r/a > 1) by the laser-induced shockwave and into the backward plume (r/a < -1) by alaser-supported detonation wave. The intensity andline shapes of the Li emission in the forward and back-ward plumes are different for each Io.

At lower Io [see Fig. 2(a)], the spectra within thedroplet consist of the plasma continuum which mono-tonically increases toward shorter wavelengths. Nodiscrete emission lines are observed, implying that theplasma within the droplet is optically thick. In theforward plume (r/a > 1), the resonance Li emissionlines are Stark broadened and exhibit an absorption

3722 APPLIED OPTICS / Vol. 26, No. 17 / 1 September 1987

Forward4

5M LiCI 2s - 2p

r/a

1

V ~ 4~MAX7.3

-

zwZ 2

X2.6

-2X 1.0

Xl .0

-7X4.2

595 590 585

WAVELENGTH (nm)

Fig. 3. Emission spectra of Na from exploding H 2 0 droplets (a n

45 jim) containing 5-M NaCl at several distances (specified by r/a)selected from spectra similar to those in Fig. 2. The maximum ofeach curve (including the continuum background) is multiplied bythe indicated scaling factor so that each peak has the same arbitraryintensity. The resonance Na (3s-3p) absorption doublet reversal inthe forward plume (r/a > 1) is in contrast to the emission doublet inthe backward plume (r/a < -1). An accumulation of sixty laser

shots at 3.1 GW/cm2 was required.

line reversal. [Note the narrow valley in Fig. 2(a)which is a doublet separated by 0.2 A at 0.6708 jim.]The Li emission in the backward plume is weaker thanthat in the forward plume, and, in the plot shown inFig. 2(a), the backward plume is masked by the dropletcontinuum. However, the Li emission line shapes inboth plumes are similar.

At higher Io [see Fig. 2(b)], the Li line shapes in theforward plume are nearly unchanged from those de-tected at lower Io. However, in the backward plume(r/a < -1), the Li emission has become more intensethan that in the forward plume. Furthermore, in thebackward plume, a sharp Li emission peak is detectedinstead of exhibiting an absorption line reversal.Since Li emission in both the forward and backwardplumes is detected simultaneously and since the inten-sity in the backward plume is larger than that in theforward plume, the valley in the forward plume spectrais not a result of detector saturation.

For the region along r = -a to r = a in Fig. 2(b), someof the plume emission is superimposed upon the drop-let continuum since the spectrograph slit encompassesnot only the droplet but also part of the divergingbackward plume (see Fig. 1). Narrowing the spectro-graph slit decreases the Li line intensity from the back-ward plume relative to the continuum intensity fromthe droplet, consistent with the fact that more of thebackward plume is blocked off by the slit between r =

-a and r = a.

B. Na Doublet

The advantage of using NaCl instead of LiCl is thatthe equivalent P3 /2 and P1 /2 doublet for Na is separatedby 6 A. Thus Stark broadening of the Na doublet canbe readily detected in the emission and absorption linereversal spectra. Spatially resolved spectra (similar tothose shown in Fig. 2) were measured for 5-M NaClH20 droplets falling in air. The spectrograph wastuned to the Na doublet, H, and Hol wavelength re-gions. Specific spectra at several locations in the for-ward and backward plumes with Io = 3.1 GW/cm2 wereselected and plotted in Fig. 3.

In the forward plume (r/a = 8,6, and 2 shown in Fig.3), the Na emission line shapes are greatly Stark broad-ened and dependent on r. The much narrower Naabsorption doublet causes line reversals. (Note thetwo narrow valleys which are separated by 6 A in thetop three curves in Fig. 3.) Such spatially dependentline shapes and the line reversals suggest that in theforward plume ne is dense and/or Te is high (hence thelarge Stark broadening), inhomogeneous along the la-ser direction (hence the r dependence), and inhomo-geneous along the collection optics direction (hencethe doublet absorption reversal as the broadened Naemission from the denser and hotter central part of theplume propagates through a less dense and cooler out-er plasma layer). The cooler Na atoms in this lessdense outer layer absorb at the narrow doublet wave-lengths (at 0.5889 and 0.5895 m). The spectrographand vidicon resolution is 2 A, and the Na emissionintensity is well within the linear portion of the vidicondynamic range.

In the backward plume, far from the droplet (r/a =-7 in Fig. 3), a sharp Na emission doublet is detected.This suggests that near this end of the backwardplume, ne and Te are much lower than at a comparabledistance in the forward plume (see spectra at r/a = 6and 8 in Fig. 3). At locations closer to the droplet (ra=-3 and -2), the Stark broadening has increased andis sufficient to cause the emission doublet to mergeinto one broad peak. The fact that the Na linewidth atr/a = -2 is wider than that at r/a = -3 indicates that neand Te are increasing toward the droplet. The lack ofthe absorption reversal in the spectra at r/a = -3 and-2 suggests that the outer layer (consisting of lessdense and cooler plasma) surrounding the backwardplume is thinner than that surrounding the forwardplume.

1 September 1987 / Vol. 26, No. 17 / APPLIED OPTICS 3723

C. H Balmer

The emission spectra of the H Balmer lines (result-ing from the decomposition of H20) can be similarlyplotted at various r/a for a fixed Io. The Balmeremission lines, not being resonance lines, do not havethe complication of absorption doublet reversal as inthe case of Na or Li resonance lines. However, withoutsuch doublet reversals, spatial inhomogeneity in neand/or Te (along the direction of the collection lens) isless apparent, and a detailed analysis of the emissionline shape is required. Furthermore, the H Balmerlines are broadened by the linear Stark effect, while theNa or Li resonance lines are broadened by the quadrat-ic Stark effect.13 The former is much less dependenton Te and is thus more useful in estimating ne when Teis unknown.' 3 -'5

Figure 4 shows the Ha emission line shape at variousr/a when Io 28 GW/cm 2 is incident on a pure H2 0droplet flowing in air. Several implications about necan be drawn from the halfwidths of these spectra: (1)in the backward plume ne increases monotonically to-ward the droplet; (2) in the forward plume ne reaches amaximum at the centroid of the forward plume, i.e., atr/a 3; and (3) the plasma inhomogeneity along thecollection optics axis can be noted by the complicatedHa line shape. The H line (as well as the other dis-crete lines of H, Na, Li, N II, and 0 II) is superimposedon the plasma continuum which monotonically in-creased in intensity toward the UV, suggesting that Teis high.

D. ne and Te

Even though the plasmas in the forward and back-ward plumes are spatially inhomogeneous and tempo-rally evolving during the plasma emission time, Te andne can still be estimated from the spatially resolved,temporally averaged spectra. The intensity ratio ofone discrete line and the nearby continuum cannot beused to deduce T as the continuum radiation in thisexperiment can arise from several species.'3 Assum-ing a local thermodynamic equilibrium (LTE), usingthe tabulated oscillator strengths, and knowing thedegeneracy factor for the lower states,'13 the time-aver-aged T can be estimated at any r/a value from theintensity ratio of two discrete emission lines at that r/avalue. The intensity ratio of the Ha and H lines (at668 and 492 nm) and of the Li lines (at 671, 610, and461 nm) has been used to estimate the time-averagedTe at various r/a in the plumes. The intensity of theH3 peak is about 10 times lower than that of the Hapeak. Based on the H intensity ratio and the Li inten-sity ratio at the two locations in the forward and back-ward plumes which correspond to the maximum emis-sion intensity (r/a -2 and 2), we estimate T 5 X103 K with 50% uncertainty. This value of T is threetimes lower than the norm temperature's for Ha (at 15X 103 K).

Since Te is undoubtedly a function of time, the Tededuced from the present temporally averaged tech-nique may be considerably different from the maxi-mum T which should be reached soon after the laser

.6 ~~~~~~~~X 4.3CE

X3.2

U, ~~~~~~~~X 1.4Z- 2W 2

I- ~~~~~~~~X 1.0Z

1 X 1.5

X2.3

X7.2

690 660 630

WAVELENGTH (nm)Fig. 4. Emission spectra of the H Balmer line (Ha) from explodingH 20 droplets (a 35 um) at several ra (selected from spectrasimilar to those in Fig. 2) and with indicated scaling factor. H isStark broadened and exhibits a complicated line shape caused by theinhomogeneity of the plasma. An accumulation of 150 laser shots at

Io 28 GW/cm 2 was required.

pulse ends. Furthermore, the deduced T is not theelectron temperature but the temperature of the atom-ic species (H, Li, or Na). By the time these atomicspecies emit the H Balmer lines and the Li or Naresonance lines, the electrons and atoms at various r/aare cooled by the intrusion of air into the plasma. Thistemporally averaged value of Te deduced from the Hand Li intensity ratio should, therefore, be judiciouslyused when characterizing the plasma.

The estimation of ne from the linewidths of Ha andH,#, which are linearly Stark broadened, can be madeeven if T is not well determined.13-'5 At r/a, whichcorresponds to the maximum intensity in the forwardplume, we estimate ne 810' cm-3 by assuming LTEand using the tabulated Stark broadening profiles forHa (50 A) and H (200 A). At r/a, which corre-sponds to the maximum intensity in the backwardplume, ne 1016 cm- 3 is estimated.

Using the estimated Te of 5 X 103 K and the tabulat-ed Stark broadening coefficients for the Li and Nalines, ne at various r/a can also be estimated from thespectra shown in Figs. 2 and 3. However, a moreaccurate determination of Te and ne requires detailedanalyses of the line shapes, which were not attemptedhere. An estimation of ne from the Stark shift"l"l3-15

(<2 A for ne 1018 cm-3) is not possible with the

3724 APPLIED OPTICS / Vol. 26, No. 17 / 1 September 1987

H2 0

present spectrograph and vidicon resolution of 2 A.Furthermore, an estimate of the plume velocity fromthe detected spectra is not possible, since the Starkbroadening is many orders of magnitude larger thanthe Doppler broadening.

IV. Conclusion

Spatially resolved, temporally averaged spectrosco-py has been used to measure the spectral line shapes ofdiscrete resonance (Na or Li doublets) and nonreson-ance (H Balmer) emission lines. At both low and highIo, the Li and Na emissions in the forward plume (infront of the shadow face) exhibit large Stark broaden-ing and absorption line reversals. At high Io, a back-ward plume (behind the illuminated face) is produced,and the Li and Na emissions are noted to have smallerStark broadening and no absorption line reversals incontrast to the corresponding emissions in the forwardplume. From the linewidth and intensity ratio ofemission lines at various r/a, ne and Te can be estimat-ed at specific locations within the two plumes. Otherspecies (neutral or ionized) can also be used to deducene and Te within the plumes which are produced bylaser-induced breakdown of large transparent H2O ororganic liquid droplets. Both spatially resolved andtemporally resolved spectroscopy will be required inthe future to characterize fully the laser-induced plas-ma from droplets.

Helpful discussions with L. J. Radziemski aboutplasma diagnostics are gratefully acknowledged as wellas the partial support of this work by the Air ForceOffice of Scientific Research (contract F49620-85-K-0002) and the Army Research Office (contractDAAG29-85-K-0063).

References

1. P Kafalas and A. P. Ferdinand, Jr., "Fog Droplet Vaporizationof Fog Droplets by a 10.6-,um Laser Pulse," Appl. Opt. 12, 29

(1973).

2. P. Kafalas and H. Herrmann, "Dynamics and Energetics of theExplosive Vaporization of Fog Droplets by a 10.6-Jim Laser

Pulse," Appl. Opt. 12, 772 (1973).3. G. Sageev and J. H. Seinfeld, "Laser Heating of an Aqueous

Aerosol Particle," Appl. Opt. 23, 4368 (1984).

4. R. L. Armstrong, "Aerosol Heating and Vaporization by Pulsed

Light Beams," Appl. Opt. 23, 148 (1984); "Interactions of Ab-

sorbing Aerosols with Intense Light Beams," J. Appl. Phys. 56,2142 (1984); R. L. Armstrong, P. J. O'Rourke, and A. Zardecki,

"Vaporization of Irradiated Droplets," Phys. Fluids 29, 3573(1986).

5. S. M. Chitanvis, "Explosion of Water Droplets," Appl. Opt. 25,

1837 (1986).6. J. C. Carls and J. R. Brock, "Explosion of a Water Droplet by

Pulsed Laser Heating," Aerosol Sci. Technol. 7, 79 (1987); S. C.

Davies and J. R. Brock, "Laser Evaporation of Droplets," Appl.Opt. 26, 786 (1987).

7. W.-F. Hsieh, J. H. Eickmans, and R. K. Chang, "Internal and

External Laser-Induced Avalanche Breakdown of Single Drop-lets in an Argon Atmosphere," to be published in J. Opt. Soc.

Am. B.8. D. S. Benincasa, P. W. Barber, J.-Z. Zhang, W.-F. Hsieh, and R.

K. Chang, "Spatial Distribution of the Internal and Near-FieldIntensities of Large Cylindrical and Spherical Scatterers," Appl.Opt. 26, 1348 (1987).

9. J. H. Eickmans, W.-F. Hsieh, and R. K. Chang, "Laser-Induced

Explosion of H20 Droplets: Spatially Resolved Spectra," Opt.Lett. 12, 22 (1987).

10. P. Chylek, M. A. Jarzembski, V. Srivastava, R. G. Pinnick, J. D.

Pendleton, and J. P. Cruncleton, "Effect of Spherical Particleson Laser-Induced Breakdown of Gases," Appl. Opt. 26, 760

(1987).11. A. Biswas, H. Latifi, P. Shah, L. J. Radziemski, and R. L.

Armstrong, "Time-Resolved Spectroscopy of Plasmas Initiatedon Single, Levitated Aerosol Droplets," Opt. Lett. 12, 313

(1987).12. L. J. Radziemski, T. R. Loree, D. A. Cremers, and N. M. Hoff-

man, "Time-Resolved Laser-Induced Breakdown Spectrometryof Aerosols," Anal. Chem. 55, 1246 (1983).

13. H. R. Griem, Plasma Spectroscopy (McGraw-Hill, New York,

1964).14. W. L. Wiese, D. E. Kelleher, and D. R. Paquette, "Detailed

Study of the Stark Broadening of Balmer Lines in a High-Density Plasma," Phys. Rev. A 6, 1132 (1972).

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WORKSHOP ON THE ROLE OF OPTICAL SENSORS IN

POWER SYSTEMS VOLTAGE AND CURRENT MEASUREMENTSSeptember 16-18, 1987

Gaithersburg Marriott HotelGaithersburg, Maryland

This workshop will explore opportunities for improved measurement

methods that will satisfy the needs of evolving power systems in

the 1990's. Emphasis will be placed on the role of

optoelectronic techniques. Topics to be discussed include:

design and testing of optical systems for voltage or current

measurement, interfacing of electronic and optical components

with existing metering and control systems, opportunities for

application of new measurement hardware required for increased

automatic monitoring and control of future transmission and

distribution systems, and techniques for optical measurements of

electric fields in power systems and components. Sponsored by

Bonneville Power Administration, Electric Power Research

Institute, Empire State Electric Energy Research Corporation, and

the National Bureau of Standards. Contact: Raymond S. Turgel,

B162 Metrology Building, National Bureau of Standards,

Gaithersburg, MD 20899, 301/975-2420.

1 September 1987 / Vol. 26, No. 17 / APPLIED OPTICS 3725