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"x " PHOI"TOOHEMIGAL IGNITION STUDIES.Sr11. IGNITION BY EFFICIENT AND

RESONANT MULTIPHOTON PHOTOCHEMICALFORMATION OF -MICR-OPLASMAS-

XI - ...

BRAD E. FOtCHANDRZEJ W. MIZIOLEK

JUNE 1987

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PHOTOCHEMICAL IGNITION STUDIES. I11. IGNITION BY EFFICIENT AND RESONANT MULTIPHOTON

PHOTOCHEMICAL FORMATION OF MICROPLASMAS12 PERSONAL AUTHOR(S)

Brad E. Forch and Andrzej W. Mitiolek

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Published in Combustion Science and Technology

17. COSATI CODES 1. SUBJECT TERMS 'CO0ftut On =;v;1 "eety S--•"n Mir ALbbi )FIELD GROUP SUB.GROUP UV Laser Ignition, Oxygen Atom Resonances, Multiphoton21 02 Photochemistry, Multiphoton Excitation, Laser Microplasmas07 04ý

19. ABSTRACT (Coninu On 06 it= iP rwouiry and ikti n~jnilW)

;;is is the third of a series of reports concerning the activation (ignition) of reactivegases using focused ultraviolet lasers. The goal of this research is to ascertain the

potential of uv laser multiphoton photochemical ignition as a primary igniter or ignition*augmentation .ource for propellants or their pyrolysis products. In this report, ignitionproperties of premixed 112/02 and 112 /N,0 flows at atmospheric pressure have been studied.Tuning the laser in the 225.6 nm wavelength region has yielded three minima in the amount ofincident laaer energy (ILE) that is required to ignite either mixture. The minimacorrespond exactly to the two-photon resonant excitation wavelengtho for the three spin-

orbit split ground electronic states of oxygen atoms. A determination of the ILE necessaryto ignite both premixed Clove as a function of equivalence ratio shows a minimum far intothe fuel-lean region. Also, the minimum ILE value for the ignition of H 2/02 was found to bearound 0.3 mW, while ignition of the same mixture with the green beam from a frequency

doubled Nd:YAG laser (532 nm) required an ILE value near 13 mW. Additional time-resolved' 04 ,)

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19. Abstract (Cont'd):

p--spectral studies were carried out on 02 and N 0 flows alone. These indicateda resonant formation of a microplasma with a fifetime on the order of 100nsec. All of these results lead to the conclusion that multiphotan

G photochemical ignition is a phenomenon consisting of three major components:(1) the multiphoton photochemical formation of oxygen atoms; (2) multiphotonionization of these atoms to efficiently form free electrons in the laserfocal volume; (3) the formation of a laser microplasma using the electronsformed in the previous process as seed electrons.ý.As such, this new laserignition source appears to be more efficient and ore controllable than thewell-known laser-produced spark (gas breakdown) p ocess and it thus should beuseful for further ignition studies.

TABLE OF CONTENTS

Page

LIST OF FIGURES .................................................. 5

lo INTRODUCTION ... o..... . . . . . .. . . . . . . . . . . . . . . .7

II. EXPERIMENTAL .o..o......... .... ........... .......

III. RESULTS AND DISCUSSIONo...........................................8

A. Ignition. ....... 9...................9 ......... . 9... . ........... 8

B. Microplasma Formation9...................................... 12

IV. CONCLUSION. .................. ............... ....... .... 17

ACKNOWLEDGEMENT .. ...... ........... ...9..... 18

REFERENCES999 ... 99. 9 ......... 9999.99................ 9.. ....... 919DISTRIBUTION LIST.... ... o ..... ...... ............

nil ~1n For

trTIS CRA&MO011C TA[2

t Lzb;: . I,1 ,,y r

3

LIST OF FIGURES

Figure Page

I Experimental Schematic...........................................9

2 Incident Laser Energy Necessary to Ignite a Premixed Flowof H /N 0 as a Function of Laser Wavelength in the 225.6 nm(Dougled Dye +1.06 Micron) Region ............................... 10

3 Dependence of the Incident Laser Energy Required to Ignitea H2/02 Mixture as a Function of Equivalence Ratio (a) Forthe Laser Wavelength Set at the Peak of the O-Atom Two-PhotonExcitation, (b) for the Nd:YAG Second Harmonic (532 n).........11

4 Excitation Curve for O-Atom Emission at 777.5 nm from (a)an 02 Flow, (b) a Stoichiometric H2/02 Flame....................13

5 Incident Laser Energy Necessary to Ignite a Premixed Flowof H /02 Which is (a) Fuel-Lean (Equivalence Ratio - 0.6)and fb) Stoichiometric (Equivalence Ratio =

6 Time-Resolved Emission From (a) Scattered Laser Light,(b) Flame 0-Atom Emission at 777.5 nm, (c) O-Atom Emissionat 777.5 nm from Microplasmas Formed in 02 Flovs .... . .... .... 16

5

I. INTRODUCTION

There has been a growing interest recently in the application of lasers

"which operate in the ultraviolet region to the ignition of reactive

mixtures. One such study has described the use of the excimer lasers F

(157 nm) and ArF (193 nm) for the ignition of H2 /0 2 or H 2 /air mixtures

photolysis of 02 into 0 atoms.1 More recently, experiments have been

,undertaken in wtich a KrF (248 nm) excimer laser has been used to photolyze 03Sinto 0 + 02 in the presence of CH4 and H2 /0 2 . 2 Both of these experiments haveinvolved single photon photochemistry. Ignition of reactive gas mixtures which

used multipboton photochemistry was first demonstrated in our laboratory on

simple hydrocarbon/air or N2 0 mixtures using the ArF and KrF excimer lasers. 3

Interpretation of those results was based to a large extent on related

experimental work in our and other laboratories which showed that uv laser

(particularly the ArF excimer) multiphoton interaction with small carbon-

containing fuels can be very extensive and can lead to substantial

photofragmentation and fragment excitation. A particularly illustrative case

involves the CH molecule which upon irradiation by the ArF laser yields

Sground and excited state radicals such as C21, C 2 , CH, as well as the 11 and C

atoms and C+ ions. 5 On the basis of these results, it was not surprising to

"find that a C2 H2 /air mixture required only 0.25 mJ of incident laser radiation

to ignite.

In a recent preliminary study we have observed a strong wavelength

dependence in the amount of incident laser energy required •o ignite a H /02

flowing mixture using a tunable laser system near 225.6 nm. Specificaly, we

found that the most efficient ignition wavelength corresponded to the peak of

the two-photon resonance excitation process for oxygen atoms in the J=2 ground

spin-orbit state. The focused ultraviolet laser not only apparently caused

photodissociation of 02 into 0 atoms, but also, when on O-atom resonance,

required the least amount of energy to ignite the gases. Furthermore, a plot

of incident laser energy as a function of equivalence ratio yielded a minimum

at 0.61, far from stoichiometry. This result further reinforced the

"•4 .conclusion that the laser-oxidizer interaction is an important element in the

ignition process of H2 /0 2 mixtures at 225.6 nm.

These preliminary experiments, however, were not detailed enough to

identify the specific mechanism(s) involved in the ignition process. Ini d e nti fy~o x g e tha toii c m ch n s m s 3 p 3 pparticular, laser twy-photon resonant population of the oxygen atom

states at 88,630 cm-r (10.99 e.v.) can lead to a number of processes including

excited state chemistry, heat deposition at the focal volume due to quenching

collisions, and/or the absorption of a third photon leading to the formation

of 0+ ions and free electrons. It is the purpose of this report to describe

an experimental effort aimed at a much more comprehensive characterization of

the multiphoton photochemical ignition phenomenon. The results presented here

indicate that the ion formation channel is a key process since it represents

an efficient and direct route for the production of the initial free electrons

early enough in the laser pulse such that they become the seed material for

- .~the creation of a laser-produced spark, i.e., microplasma. The primary role

of this short-lived microplasma (ca. 100 nsec) apparently is to be a localized

"source of highly reactive chemical intermediates at a very high temperature.

If the spark is intense enough, then the resultant ignition kernel is

lsufficiently strong to permit transition into full combustion.

* A 7

II. EXPERIMENTAL

The experimental schematic ig given in Figure 1. Since it has beendescribed in detail previously,6,' only the major points will be highlighted.Tunable uv laser radiation in the 225.6 nm region was focused with a 50 mmfocal length lens at a position 1-2 vm above the burner surface. ippicallaser energies up to I mJ/pulse yielded power densities around 10 W/cm inthe focal volume. The water-jacketed H2/0 2 burner was fabricated from astainless steel Swagelock 0.25 in. terminator fitting through which a 0.9 mmhole was drilled. Matheson (Model 620) flowmeters were calibrated by e GCAPrecision Scientific wet test meter for H2 , ?2, and N20 fiows up to 2 LPM.This resulted in orifice linear flow velocities in the 10 cm/sec range. Theincident laser energies were always measured just before the focusing lenswith a Scientech (Model 38-0103) disc calorimeter-power/energy meter. Theemission signals were detected, averaged, and processed as describedpreviously. The excitation wavelength scans were performed manually, andeach emissiun wavelength data point represented the average value for 512laser shots.

Time-resolved emissions were digitized with a Tektronix 7912AD digitizer(7A24 a4plifier and 7B9OP timebase) and accumulated in a PDP-11/04 computer.The response time is ca. 25 nsec FWHM (see Figure 6) due to the relativelyslow response of the EHI 9558QA photomultiplier detector tube. The powerdependence of the O-atom emission intensities at 777.5 nm for 02 and N,0 flowswas measured using a 200 mm focal length lens to avoid the formation o!microplasmas and only to measure the photon dependence for the photolysis ofthose two molecules. For these experimntcs, the Nd:YAG laser amplifierflashlamp energy was varied, as before, to change the output power at225.6 um.

111. RESULTS AND DISCUSSION

A. Ignition

Figure 2 shovw the wavelength dependenre of the amount of incident laserenergy necessary to ignite a premixed flow of H 2 /N 2 0. A aimilar type ofbehavior has been found for H /02 premixed flows. The curve clearly shows astrong dependence of the incident laser energy (ILE) on the laser wavelengthwith toree prominent features around 225.6, 226.0, and 226.2 nm. Thewavelengths of these three features are exactly the same as the fluorescencepeaks ut'hih result from oxygen atom two-photon excitation of the grmu~delectronic spin orbit split states Jv2, Jul, and JuO, respectively."- Thisresult unequivocally indicates that the electronic excitation of oxygen atomsis an important feature of the ignition mechanism. Also, the spectral widthsof these features are considerably broader than those observed during theflame 0-atom excitation scans. The reason for this difference will bediscussed in the next section.

The dependence of the ILE on the equivalence ratio for H 2/02 flows isgiven in Figure 3. The lover trace results from the laser wavelength set atthe peak of the 0-stom two-photon excitation, while the upper trace is for thegreen laser beam, i.e., the second harmonic of the Nd:YAG laser (532 nm). Twnpoints relevant to this figure should be discussed. The first is the

8

&

000

p Ml

a Z f4

166

z CWW 0U

. .1U

15 -li . . .4

I Z ý4M

L204

U La•S0 00

- !a

II

0rNN

o u,

00

44

w

0omo

0 w0

N0

0 0 0 0 o 0OCD f-0 N

a.4

10

14.0.-_ IGNITION

13.0 - oC c o o o p o p03 '-

b.

1.20-

~.1.00- -- IGNITION--

Lu 0.80

aa.

0.60-z

0.20-

0.00 jr r0.2 1.6 1.0 1.4 1.8 2.2

EOUIVALENCE RATIO

Figure 3. Dependence of the Incident Laser Energy Required to ignitea U2/02 Mixture as a Function of Equivalence Ratio (a) For

the La-er Wavelength Set at the Peak of the O-Atowa Two-PhotonExcitatioa, (b) For the tNd:YAG Second Harmonic (532 acm)

It

observation that the minimum in the lower trace is far into the fuel-lean

region. A similar type of behavior was noted for H2 /N 20 flows. The reason

for this appears to be the same as before, i.e., the uv laser is clearly

interacting significantly with the oxidizer (02) component of the reactive

flow. Observation of the minimum so far from the stoichiometric point is, of

course, in sharp contrast with the usual behavior found for spark ignition in

a closed bomb. However, the recent report on the excimer laser ignition of

H2 !/ 2 mixtures also noted the most efficient ignition to be in the fuel-lean

region.

The second point relates to the upper trace of Figure 3 where ignition

was caused by the green laser. Not only is the actual value of the ILE much

higher at 532 nm than at 225.6 nm, but also the 532 nm dependence is virtually

flat across a very wide range of equivalence ratios. This type of behavior

has been obsIrved previously in our initial ignition studies involving

hydrocarbons and can be explained by the inherent properties of the laser"spark" (gas breakdown) process. Specifically, due to the sharp threshold

Q associated with the onset of absorption of laser energy, the spark (plasma)

when produced, ii typically much more energetic than the required ccitical

ignition energy. Furthermore, there is usually a sizable blast wave

associated with the spark.9 As indicated in Figure 3b, the spark intensity is

sufficient to ignite mixtures at either extreme of equivalence ratios, and is

clearly much greater than necessary for near stoichiometric mixtures.

Implicit in our discussions of multiphoton photochemical ignition of

H 2/02 and 112/N 2 0 is the fact that the process firsS has to start with the

photoproductiou of the oxygen atoms in the ground P atate. In order to study

this process, we measured the laser power dependence for the production of the

two-photon excited oxygen 4t=s whose fluorescence was detected at 777.5 mý

The reason for doing this is that frequently such a power dependence study

will indica.tte how many photons are iovolved in the process. Experiments were

undertaken on flows of 01 and N 0 roopectively using a 200 Mm focal length

lens to avoid problems ot microplasms formation. Tlies me asurements indicated

that the photochomita:l formation of ground state oxygen atoms was 0

n ultiphoton process for both 0a anid ?20 requiring tuet photons in each case,

i.e.. fot both cag-s we ra.ured a four photon dependence for the 0-atom

omniss ion, This is consistent with our provious moeasurement for ',iOQ whereas

-k for 02 this quantity had not been measured previously. The imti-ication ofthose findings to our ignition studies will bg 4iscussed in tl', next section.

B. Microplasa F.ormation

Duritng the course of our ignition expriments %e began to take note of a

*f-aint source of white light that omani;ted frtom the laser focal volumeregion. The intinsity of this light was cleary wavelength dependent with the

brightest enission occuring at the wavelengths corresponding to the peaks of

the 0-atoam two-photon excitation, In order to study this behavior in greater

detail, te initiated both spectral and temporal studies of these •aic€oilasaa

for 02 and N20 flows using the 50 c= focal length lens.

1Figure 4.a shows the excitation curve for ( atOM eMissiOn 4t 777.5 n%

ther. the 0 atoms w.re thesAelve!s generated by the same laser focused into the

0, flow. A similar plot (Figure 4b) is also include. for nattcent 0 ato" two-photon excitation in a stoichiometric 112/02 flame. Furthermore., twn ignition

12

1.0-

- 0.8

z

z 0.60

0

0.4-CM(J

2 0.20.

0.0

286.70 286.30 285.90DOUBLED DYE WAVELENGTH (nm)

Figure 4. Excitation Curve for O-Atom Emission at 777.5 r=m from (a)an 02 Flow, (b) . SLoichiometric H2/02 Flame

13

plots for fuel-lean and stoichiometric H12/02 mixtures ar, given in Figures 5aand b, respectively, gith the differences ia th( absolute value of the ILE

explained previously. A number of similarities and diffexmnces can be seenand discussed. In particular, the spectral width for 0-atoms produced in a 02flow is much greater than for the nascent flame 0-atoms. The substantialspectral width in the 02 flow is, however, quite comparable to the ignitionprofile in the fuel-lean case, while the sDectral profile for the ignition ofa stoichiometric mixture is somewhat narrower and less efficient than the

fuel-lean case. The explanation for these observations is that the spectral

profile in the ignition case as well as in the cave of the 02 flow, is notreally a representation of an atomic spectral property, but rather anindication of a much more complex process, i.e., the formation of a

microplasma, which is inherently a highly nonlinear phenomenon. It is well-established that the laser-produced microplasma (spark) needs seed electrons

in order to grow. In our case it finds them in the spectral wings of the two-

photon resonant, three-photon ionization of oxygen atoms. When these freeelectrons are formed in the early part of the laser pulse, then the cascadeplasma formation mechanism is initiated and the plasma is ultimately heated upto a very high temperature by the inverse brehmsstrahlung effect. 0-12 Thus,when this occurs, it is no longer valid to consider the 0-atom emission at777.5 nm as a simple two-photon laser induced fluorescence process. It istherefore not surprising that the spectral behavior of the ignition ofpremixed gases, a process sensitive to microplasma formation, should besimilar to that of the microplasma prodicing precursor alone. This is, infact, what is observed for the 02 flow and the fuel-lean reactive mixture inFigures 4a and 5a. The fact that the stoichiometric ignition curve shows anarrower width w- '- less efficient as well, it explained by the fact thatthe H2 hampers tK.: ;rowth of the microplasma, presumably due to its highionization potential (I.P. for 02 - 12.063 e.v. and for H2 - 15.427 e.v.), and':hus the plasma is relatively less intense. In search for other explanationsfor these wide spectral widths, a wavelength dependence of the initial step,i.e., multiphoton photoehemical production of atoms, should be considered. Itwould be most unlikely, houever, for such a dependence to yield the similartype of spectral profile which is found for all three spin-orbit components(Figure 2).

An additional parameter that was investigated is the temporal behavior ofthe 0-atom emission at 777.5 nm. Figure 6 shows the time-resolved emissionfor scattered laser light (Figure 6a), flame 0-atoms (Figure 6b), and 0-atomsproduced in the u2 flow under conditions of microplasma formation (Figure 6c).Clearly the lifetiste is much longer for the 02 flow case since it actually isrelated to the lifetime of the plasma with direct laser produced signals fromsimple multiphoton photolysis and the two-photon excitation being a factoronly in the leading edge of the trace.

The role of excited state 0-atom chemistry has not been expliritlyconsidered so far. The fact that a microplasma exists in multiphotonphotochemical ignition implies that laser-populated excited state chemistry isprobably not important sirce the kinetics of the microplasma process wouldappear to overtake other competing processes. Furthermore, we undertook aseries of ignition studies ot H2/O2 flows using longer focal length lenses(100 rmm, 150 tmm, 200 mm) and found that the ILE required to ignite increasedconsiderably and ignition did not occur without the formation of themicroplasmas. On the other hand, even though our experiments strongly

14

1.00-

- 0.90 7E "IGNITION

W 0.80]

W

-' 0.70-

z 0A .00W

z 0.60-

0.50-

286.70 286.30 285.90

DOUBLED DYE WAVELENGTH (nm)

iigure 5. Irucident Laser Energy Necessary to Ignite a Premixed Flow

of 112 /0 Which is (W) Fu~l-Lcan (Equivalence Ratio - 0.6)

and (b) Stoichiometric (Equivalence Itatio m 1)

.15

1.0

0 0.8-z

zo 0.6

0,4-

- 0 .4

0 I a.

0 .0 J•. . .T, .. i-

0 100 200 300 430 500 600 700

TIME (ns)

F~igure 6. Time-Resolved EFmis..ion From (a) Scatterec' Laser Light,(b) Flame 0-Atom Emission at 77"i.5 nm, (c) O-•rom Emission

at 777.5 nm from Nicroplasmas Foru..d in 02Flows

16

implicate the microplasma formati-on process as a key element in ignition, wecannot discount the possible importance of the photochemical formation ofradicals in the converging laser beam near the microplasma. These radicalscould very well be important in the early stages of ignition kernel growth,but after the microplasma had decayed. Clearly, future experiments with highspeed photography, as well as in a closed bomb and with other appropriateoptical diagnostic tools, would be very helpful in promoting furtherunderstanding of this phenomenon.

IV. CONCLUSION

Focused ultraviolet laser radiation is capable of activating reactive gasmixtures through a new, previously unreported mechanism involving multiphotonphotochemistry, ionization, and microplasma formation. The major differencebetween this work and previous work on laser spark formation is thatmultiphoton photochemical ignition provides a more efficient and controllablemeans for liberating the free electrons which then lead to the laser sparkformation process. Due to these virtues, this laser ignition phenomenonshould open up new opportunities in ignition studies. Also, since laserspossess certain attractive characteristics such as beam propagation throughgreat distances, as well as excellent time-resolution, there may be newopportunities for practical applications which require the activation ofreactive systems.

17

ACKNOWLEDGEMENT

This research is supported in part by the US Air Force Office ofScientific Research, Contract #86-0008.

18

REFERENCES

1. M. Lavid and J.G. Stevens, "Photochemical Ignition of PremixedHydrogen/Oxidizer Mixtures with Excimer Lasers," Comb. and Flame,Vol. 60, p. 195, 1985.

2. D. Lucas, Lawrence Berkeley Laboratory, private communication, 1986.

3. A.W. Miziolek and R.C. Sausa, "Photochemical Ignition Studies. I. LaserIgnition of Flowing Premixed Gases," BRL Technical Report BRL-TR-2644,1985. Manuscript also submitted to Comb. and Flame.

4. J.R. McDonald, A.P. Baranovski, and V.M. Donnely, "Multiphoton VacuumUltraviolet Laser Photodissociation of Acetylene: Emission fromElectronically Excited Fragments," Chem. Phys., Vol. 33, p. 161, 1978.

5. R.C. Sausa, A.J. Alfano, and A.W. Miziolek, "ArF Laser Photoproductionand Sensitive Detection of Carbon Atoms from Simple Fuel Molecules,"ARBRL-TR-in press.

6. B.E. Forch and A.W. Miziolek, "Oxygen-Atom Two-Photon Resonance Effectsin Multiphoton Photochemical Ignition of Premixed H2/0 2 Flows," Opt.Lett., Vol. 11, p. 129, 1986.

7. J.E.M. Goldsmith, "Resonant Multiphoton Optogalvanic Detection of AtomicOxygen in Flames," J. Chem. Phys., Vol. 78, p. 1610, 1983.

8. A.W. Miziolek and M.A. DeWilde, "Multiphoton Photochemical andCollisional Effects Diring Oxygen-Atom Flame Detection," Opt. Lett.,Vol. 9, p. 390, 1984.

9. F.J. Weinberg and J.R. Wilson, "A Preliminary Investigation of the Use ofFocused Laser Beams for Minimum Ignition Energy Studies," Proc. Roy. Soc.Lond. A, Vol. 321, p. 41, 1971.

10. C.G. Morgan, "Laser-Induced Breakdown of Gases," Rep. Prog. Phys.,Vol. 38, p. 621, 1975.

11. E. Yablonovich, "Similarity Principles for Laser-Induced Breakdown inGases," Appl. Phs. Lett., Vol. 23, p. 121, 1973.

12. D.R. Cohn, M.P. Hacker, B. Lax, and W. Halverson, "Effects of Pressureand Magnetic Field Upon Physical Processes in Laser-Induced Gas

Breakdown," J. Appl. Phys., Vol. 46, p. 668, 1975.

19

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