AFD-A094 343 INDIANA UNIV AT BLOOMINGTON DEPT OF CHEMISTRY F/6 20/9OERATIONAL CHARACTERISTICS OF A HELIUM MICROWAVE-INDUCED PLASM--ETC(U)JAN 81 A T ZANDE R, S M HIEFTJE N0001476-C-838

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4. TITLE (and Subtitle) S. TYPE OF REPORT & PERIOD COVERED

Operational Characteristics of a Helium - rnMicrowave-Induced Plasma at Atmospheric ,InterimTechnicalORG R ePR,.

Pressure ' PERFORMIN ORO. REPORT NUMBER

7. AUTNOR(a) 8. CONTRACT OR GRANT NUMBER(e)

Andrew T./ Zander aRd Gary M' Hie f tje N. N- 14-C-0838

9. PERFORMING ORGANIZATION NAME AND ADDRESS t0. PROGRAM ELEMENT. PROJECT. TASK

Department of Chemistry AREA 6 WORK UNIT NUMBERS :

Indiana University NR 51-622Bloomington, Indiana 47405

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Approved for public release; distribution unlimited

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MP1 W!3 Ti~ O WDC OD=TLEW AS 10*X11 CAY M= . V pM o =Ui W

IS. SUPPLEMENTARY NOTES

Prepared for publication in ICP Newsletter

IS. KEY WORDS (Continue on revere. ide It neceesary and Identify by block number)

LI-1 microwave plasmaemission spectroscopy

La. multielement analysis

expitat ion source' (Continue on reveree side It noceesary and Identify by block number)

The microwave-induced helium .plasma generated at atmospheric pressure in theTMoIo resonant cavity is. attractive as an excitation source for emission spec-trometry. The efficiency of the cavity produces an extremely high energy-density plasma and its short longitudinal dimension permits close spatial andphotometric monitoring system proximity between the plasma and different fypesof sampling devices. Examination of the electron density, excitation tempera-titrt .ld tI)tlol ion:1 I I'tlntoit tiro in this tni crowave rli mu reval s i ts unusualcharactci' ald situw ts cert i ,lcchanisms of' oorat ion

DO JN 1473 EOITION OF I NOV 8S IS OBSOLETE U

4A S/N 0 10 2 04 6 I 3DO ~ 0 LLN 17 / ~ 04 b 1 / " lP z

OFFICE OF NAVAL RESEARCH

Contract N14-76-C-0838

Task No. NR 051-622

TECHNICAL REPORT NO. 30

OPERATIONAL CHARACTERISTICS OF A HELIUM MICROWAVE-

INDUCED PLASMA AT ATMOSPHERIC PRESSURE

by

Andrew T. Zander and Gary M. Hieftje

Prepared for Publication

in

ICP NEWSLETTER

Indiana University

Department of Chemistry

Bloomington, Indiana 47405

January 26, 1981

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AIBSTRu\cT

'['e ii c owve inucd e Ii rnp1asiua gene rat cd at, atmiiospheic

pres---ure inl tilhe TH~ rouitCavity is attractive as an ex-

citatio" sour1ce for- cmissioii spoctirointry. The efficiency of

thle ca-vity p~roduces ain extremely hi gh energy -ecens ity .1 asmia

and its short longi tudj nal. (lilenSi onl permuits close spatial

and photollicti-ic monitiorii ng systcma proxiui ty between tile pl asuma

and di fferenti types of samp] jug devices. Exanminati on of the

ci cci ron dii ; ity , ('xc iat ion t empera ture and rotational tomI-'

peralre in this; 1microwave p1 asmna i rve~als i ts unusual char-

acter and suggests certa in mecchanisms of operation.

I N'P.O DI JC' ON

Mi jC roWkvC i ud uc ed l a 1SlW!a (Hl 1) i ye be en of C oh t ilai np.iliterest as atoli 031C issionl C)XC) tat)ol 011 00 Jcv! fol. spec I fo-

chemiical analysis (1 2). )In accord %i lh~Iorae iii c rest. in

induction pl asmas in general , undoubtedly) spur red by advance-

ment s wi thi RF 1 CP' s, the MI P has recently receiv ed moi-e attention

than it has had in the past. However, the resurgence of interest

has been mnore directly the result of two independent. factors.

As the popularity of microwave ovens has increasd, the avail-

ability of high quality, high power magnetrons to supply cws

mi-icrowave power at 2450 MHz has become wider. Thus, laboratory

and research versions of microwave power supplies are inex-

pensive and more readily obtainable. The second factor was thc

introduction of the TM 010resonant cavity to analytical spec--

troscopists by Bcenakker in 1976(3) . In that and following

publications(2-8) the TM 010 cavity was described in detail and

shown to posseSs some very useful operational charac e-ristics

and to have tlic capability to excite mnost nozimictals and all of

the halogens (9-14).

The featurcs of the TIM 00cavity w~hich make it advantageous

for use in atomic emi ssion experiments arc numer ous. T1 ie

dimensions of the cavity are such that the discharge is con-

strained to a very sm-iall volume. It is within this volume that

all of the input power is concentrated. The result is that the

energy density in the location of the discharge is greatly) in-

creased rel ative to oilher cavit)' types. This all ows the self-

igni tioni and trigyfrar*petinof all al.mosplicri prssr

li schiargei in be~illun. Thie advanftaiges a ccruinrg 01c. tli use of

hel i ur as; the support g'as are lower over-all bi ckgrourd11-. spectru,111

higher metastabl e species ener-gy, and a1 more di Cfise plasma

which i s less prolne to errati c movements and dmislocat ions duii-ni-,

sample injection. The coinfij,'urati on of the TM 1 cavi ty is such

that the MVP is within a Few 11lii imneters of eithier end of the

caivit y, which permlits siiipl er and more di rect. aiccess to the d is -

charge . Th is ir r anoement elimi inat es a mul It ile or sample t van-,.

port problems and] imi ove:;. t he eff icielicy of ra:d ial 10 I COllecticenfromi the pl asmia. The MTP in moi(1 ce(as i)' %'i ewed inl the ax i.a]

direction, el ii!iJ, :jtalng s i gnal delgradati Ion whi0 i-s encomiteredis I e (luti i" w ] 11 troig, whutdi radial Ob.ervatioll I ilast be

-Irld(, , dh., with i use. Thet lM0 3.0 cavil), seems; to a]low moreeffi.cient tramn;fcr of power to the discharge as cvi denced by

the Jack of required cavity cooling apparatus, Fi1nal)Iy, tle

MI1P operatcs with support gas flow rates which are econonical

of le usage.

In the present studies excitation and kinetic temnperatures

and the electron density were spatially profiled to, define some

of the pri ncipal operating characteristics of the helium MIP

at atmospheric pressure. Previously reported portions of this

work have ind:i cat(,d possible flow rate/power level trade-offs

for optimal si gnal generation, various signal gradients in the

lIP, and suggestcd reduction of the .IP excitation capabilityby material injected into the plasma from wall erosion(lS-17).

More extensive reports have appeared which addressed the char-

acter of various low and high pressure microwave discharges(18-

24). In a number of cases attempts to elucidate excitation

mechanisms and pathways in MIP's were presented. The material

p-esented here is meant to further characterize the atmospheric

pressure helium MIP without, at this time, proposing such mech-

anist ic schcmes.

EXPERt/I M'WJAF/LThe 'lol microwave resonant cavity, its tuning charac-

teristi c,;, and our inert gas mani fold system have been described

in detail e]<.whwre(8,l5 17). The optical systeui imagled the

axialy.',--viewed plasma, without magnification, on the entranceslit of an (iellt, mo no ch i-ona Ior(Spectrafle tri cs, Inc., Andover,

IA) v'hi (ii had been lmodifi cd for ,avCveloi ct ii 11ih(1a, iol and was

Iiio ortO i ztcl for scmingiiig. Ioil r nce and Cxit apertut'tes were S01i

wide al 20(01i1 hio . The moilnOeromator was iised in the single

chainl. imode u: I h a EMI 625(6B hotomul tipl ier 1iibe (EMI (;encomlI,

P 1i I Iv i cv: , NY) wiih an11 ,1 - 1 i3e:;o!lse. ihe photoanodic current

was inpull to a i icrovoll Iamnneter(Model iS(, Keithile), Co.

Cltcv(.1 and, (11) ;id recorded oil a strip Ci rt recorder (Heat h, Co.

1hentoil Harbor, MI) Conditioii:; empl oyed here ucrc Sii~lar -or

* c~y-~l seto those rout me] y used for MI P-AES. The chamber

used, to comtin the M11 i ws a approxi mat ely 3cm long sect ion,

of quartz~ t ubinIg of 3mui i d. mid Simin o (1. The tubi ng (lid niot

ext end outs ide t he body oi the cavity.

App]. i d powe.,r l evel s which are rep~o-t ed were taken as thle

di ffcreiic beti.een readings of the forward and reflected nowcr,bot h ii lca ted o~i the generalo p0'Tower meterI. Cl early, *tiui s

v'al 110 s nlot 110cessfri 1) the actual power di ssi nat ed inl the

dischairge; somei is lest,; throughi radiation and impedance mis-

matching between the transmission line and the chaniging rcac-

taiice of the plasma. However, the determinat ion of the power

expendced inl the cihagcwould requi rc substantially more

comp~lex cxpcIrimentation. An examinati-on of recent literature

showed that nio reports thus far have att-cmnted to defiine the

l)ow. ri t ransferrcd so]. ely to the MIP.

Data were 1al-en at power levels fromn 601V to 35011. Previous

reports haver( exami nejCl th)e e~ffct, Of app1 jed no,,r level onl HIP1

tempera tu res andi densities(I 5-I7)) IAlta. reported here are for

1001 1 Pasmus on] y. The parmuci ri c pro filIes anid tr 1ods at this

po%:er level- are typ)ical of data at other power level.s up to

about 50.It should be n-oted that sjince the TNI0 10 caiv ity

affords such highi energy density inl the HIP1 that the 10014 level

repor-ted here is equival ent to miuch hipgher levels inl other

type)s of :av'ities. More-over, pivc]iminary studies inudi cate

that appl ied pov.er above 3001q (ocs not lecad to increased analyte

emission inl ensitI), nal~vs thle MIP more di ffi cull, to control,

and Fevolrely shlort enis thle lifetimen of repl a ccab] e compone nts(lS).

Thie cavity w.. iounl ed on n two- (Ii riensi onal posit ioiiing

St anid 1-1ulic.1 all owe.d foculs s1 iu, of anly point Onl thle 1,11 face Onil N

I iic' 101 mooh fOwil i 1'.11 T icce cAtllre. ThesptiaI slIio

.'a' ap1ioY.i matei. 70011. 11c Iativ oL mission 1.-inc( intenisities ailtz

lne mro 'i 1c 0sCons werec obta i :d across t he face of the axial ly-viowed plasma. Only at lower- pwcy levels (below about 70W) is 'thle kPh' cyl i rdr cal ly symc' trical wiv11h the p1 asin Chamber. In1

al1l ol htj a!e the plaslwa mosot 1rf; ide in thle (eie f tile

chamber along the axial di men!.ion, and is radial1>' asyimmetric

ill intenisity. Al.though the p1 a sma is re) at i'vely short com-

pared to other types of liP's it is not so short as to assume

hocnogeneity in temperature and clectron dens i ty along the

longitudinal coordinate. Since this study did not address

the more difficult task of accurately measuzi rig in-tensI..-I-es-

--the --aadial-..ir~e..tion ,--~-a .to.ob-a in 1 ongi t id i na I pro fi l es,

it must be kept in mind that the axially measured iniensities

are averaged reaidings along the line of sight. The excita-

tion temperatures were obtained from two thermometric species:Fe analyte atojiis and le support gas atoms. Measuremcnit s of

the relative jintensities of well-documented ) ines of these 1/

species afforded tenuirature determinations by the slope method.

The thermometric species used for rotational temperature

determinations was the C2 molecui.c, obtained from injection of

a continuous flow of acetone vapor. The electron density was

calculated from Ahe width of the Stark-broadened 486.1nm IB

line. Hydrogen is present in the MIP from organic impurity

components of the high purity fe used, from entrained atmos-

pheric water vapor, and fro.mr material eroded from the chamber

wall s.

For temperal ure measurements it is clear that the values

obtained can only be as reliable as the values of the atomic

transition probabiliti.es which are used in the det.erniiIi.nations.

These values are reliably known only to about. 20-25%. It

should be kept in mind that when working in plasmas suspected

to be far from thermodynamic equilibrium, as this tliP is,

relative line int ensi ty temperature del erminatj oils are at best

approximate. Consequent.I ly, the absol ut. e vxa lues of the t em-

peratures reported are of less significance than "the profiles

and trunids of these variables.

RE;IlTS AN'D 1l (1J ,SION

It was ob:ervecd in all experiments thai as the flow rate

increas~d he center f the pla sma moved closer to 111C chambel

wal 1. The chambei, which is coitiI nimall)' erodi hg at all po%,er

level.;~ anid f) ow rat vs , /kppa rn Y11dep incre-ased CrOSioUl

alt higher l1.ie npu111 rate(s , ej (C1 i ng mo re 11a teri.al inlto tile MI11.

Thi s effect is nuspected to occuri becaus.e of re I at iv&

Cr power absorpt.i on by) the chamber wrall at inlc ieased f1low rates.

Power absorptijon by) the walgenerally increases as thle cavity

becomes det-uned , so that a tuninig dependence on input flIow I-ate

might be iinfcrred. It is sugges;ted that this wouldc bc (lue to

an adverse change in thc reactive load nature of the- MIP as

the flow rate rises, for example, through increased flow tur-

bulence. Ii any case, the mterial ejected into the pl asma

is transported O~lt Of the chamber through the N1iP. The es-

tab] ishment of a. route for the flow of material may then be

the causc of MI1P stabilizatIion onl the wall. Al terniat ively,

the erosj on of the chamber wall may l ead notitL ju1ST1-Aat eria 1

injection into the MIP; but to increased production of charge-

carrying speci es, the entrance of which into the 1.1P causes it

to stabilize at the region of their prodluction. The observa-

tion of the attachmnent of the MJ1P ends to the wall, as evi-

denced by di stort ion of the normally symminetrical pl asina , re-

quires that species which 'rc extractable froi-i the chamber wall,

for exawpl c , Si , O1l, H-, 0, Al, 13, N (depending upon the chiamber

material used) alw-ays be considered inl the characterization

of the MIP.

In Figure 1 are plotted the Hie excitation temperature

profilesT(e" at different flow -rates of support gas forCa 10011 Ile I P. The T C (fie) profiles at all flow rates exhibit

the same overall shape. It is clear that the lowest tempera-

tures are at, the M11P cent cr (111 lie11 MPcenter I s de fined as

the visj i)' bright c'st spot. of the plasma; its location is noted

on1 echI1 f .i go'rV. ) There J S a 1-f'~l onExternal1 to t he core of

the 1,1IP at v'ich T eb (til) is hig~hest Thle values of 'r (lie') at

thme MIP center compare well wil~ tim])ev1oti 1 y reported val nes.

Tile l ower T C(lie) a 1 the p1 a sm1a center Iuii ght be the resul t of

enegyloss to the Crosi on pcis in thme plasma, or de - xci -

t alion of t he lie b~y these ;ame specieCs. Furt her s 111(1 es 1 o

cha ract er* ; the !spcci vs p resent11 are ne0cessar11y t o Suipport t his

\ri CW. The hi,1)gh VaIlueS Of TI (l0e) exterriai to the M11, core ten(i

to inud icat e t hat, a a a Y tC cCXCi t at ion Din igli t be p)t i lium.11 ouits id

the M11P. Ty (le) at a]l1 pointi; drops; as the flow r, t c i ncrei : s .

This I rend miay be due to a decr ease in species resi dence time

in the excitation volume as the flow rate increases. Al. thi'

occurs at the same time thc plasma beconios more clongatec a

higheor fl ow. The ci onpa1.i on of the p1lasma 1 ci ds to support

the vi ew that excitat1ion species are beinig swept out ofr the

* chamber. The totalI pl asma volume rema ins approximately tin-

* changed.

In Figure 2 are plotted the exci tat ion I emperatures at

diffcren: flow rates, determined using Fe as the thermometric

species, T e (Fe) . The jprofi-]es are nearly flat across the

visible plasma core. T C (17e) is lowest. external to the core,

diffe-rent in character to 1' (Ile) . As thle flow rate rises

the profiles rise at all1 points, also opposite the beliz~vior

of T e (Ile) . The temperature closest to the wall rises most

rapidly. There may be some correlation of this rise w.ith the

increase wall erosion observed, but it is unclear at this time3

what it might be. These profiles w..ould tend to indicate thatej

aniz,7yt e xci tat ioii might be opDt.ir,111 Fuit the laSma center.

Electron deisi ty profi I es across the piasna face were

observed to be essentially flat. The valuecs of thle electrion

density obtained were between 1.0 x 10' 14 cm - and 1.6 x 10 14cml-

A qual it ative assessment of the ioni zation) temperature

through the use of the Saha equati on i ud ica ied T.i va Iuc's Were

als esC SsentialIIy tunchanged across the M1ii1. The valucs est i mated

were reaIsonably close to other reported values (25). 1.

study is warranited for a quantitativxe ass;essment of the ioii -

z t ion t em]Ip( i-atlire.

In FigAl zir- plotted the rel ativ'e line intenisity pro-

files for- the 1Fe 371 .991)11 grounid staite line and the 287. 234m

nonrcs!oiiiicc line (levels '12537. 76cm to 7728.071cmi ) . ITeFe was inje cted c:ontinuously inTto the IPI with a mlicroarlc

saimpl ing device (8, ?6) . It if sObs;erved tha1.t thle prof ile s of

tetoline., '1reC di stilictly, different- in shiape. Al. eachi flow:

TI

rat c the maximum of the resw; o iice line itevit'it y 1' cxl ernalI

Io the MIP center and corresponds roughl])y Ith the peal: exci -

tation tcinpe rature for lie (cf. Figure 2) In contr-ast, the

1l1aximumii of the nonresonalnce line inten.si ty is aplpro.xiiiately

at the NIP cent er. The minima for each profile are similarly

opposite in character. This behavior suggests that higher

energy levels of an anii'lyte atom are higlly Populated ill the

p1a sina core. The high er n iensities of tlie resonance li ne

found external to thc plasma core follow the Te (le), and ll this

fact seems to suggest different excitation mechanisms inside

and outside the plasma core. If it is assumed, for example,

1hat lie metastalies are principally responsible for analyte

excitation (from the ground state, at least), then the low

reson.::ncc ine intensity within the HIP mi ght be due to a

melastablc )qe 12cbJg lip by wall -erosi.on materia]; or there mi.ghl

be sonme suprathermal iuelbanism in effect. This behavior is

still in the preliminary stages of examination.

All of theprofiles decrease ill magnitude at all points

as the flow rate increases. The amount of decrease at any ,, ' : ' "

sp-:i pic oint is not directly correlated with the amount of t .,I

flow rate incrcasoe. flowever, a possible expliaiiation for this

overall trend is a reduction of the residence time for all

types of species involved in excitation as the flow rate in-

crea es.The relative values of the line intensities at various,

positions ill the cham1ber and under varying conditions have not -i "

been compleley examined at this time. More extensive study

in Li s di rect Jon :i..; pl annecd and is expected to provide fur- (.

ther insight into the working.; of this till ).

The marked diffc'renc ,; fi the valie!:; of the various lem-

perat, re typos at all points and aImong the profile; suggets

st )oinl)- thai th e atvlospler~ic pressure mi cro:ave discha '., is

nonthenanl I . The cxiSt en-C of a pealeccd T (lie) profile wi.t h a jmininum at the Mi11 ceiler in contrast to the relatively featre-

le;s 'l le) piofilc is. . :spected to be (lie, at Ieast in part,

to a grcat (.'" ef ' t of Ilh chatmber walk cro'.ion .p)ecies o (lie).

CI

"h j; 1opot, i1 i on .is sug e, stive of diffcrent. liechallisl ic palhs

for each spcjwcies in the plasma. flowever, furlher elucidation

of any pla.siaa exci.tat:ion wechili.s wi]] recluire species pop-

Illation studies and cui s.i;jon ]line ratio siidies Some progress

toward ca Iri fying such a liechian i sm has been report ed (22,23)

CON CIAIS I ONS

The atmospheric pressure Ile NIP generated in the TM 0

cavil y has beel slown to posses; ratiher complex tempecrature

and emisslon jn:en;ity chara cter in a cLmber about three

times larger than the plasma volume. Mi ll effects on the MIP

probably quench the plasma core; however, other more complex

mechanism5; may be in effect.

Suppoartod in part: by the Office of Naval. Research and by the National

Science Foa wdtictii through grmnl s CIPli 77-22152 and C1I11 79-18073.

10

IiTiU~iiRATUR ClITED

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1972,22,553.

2. S1logerboc, R. K. Colemain, G. N., Anal. Chinl., 1976,48,

611 A.

3. Beenakker, C. I. M., Speccrochim. Acla, 1976,31_B,483.

4. Beeilkker, C. I. H. , Spectrochim. Act a, 1977, 3211, 173.

S. Ben ahkicer, C. I . H. , Bootm, os , P. I. J. M., Spccl rochin. Acta,

1978,33B, ,53.

6. Beenaklker, C. I. M. , Bosman, B. , Boumans , P. W. .1. H., Spec-

trochim. Acta, 1978,33B,373.

7. Van Dlien, J. P. J., l)eLezenne Coulandcr, P. A., DeGalan , L.,

Spectrochim. Acta, 1.978,33B,545.

8. Zander, A. T., Hfieftjc, G. M., Anal. Chem., 1978,50,1257.

9. Quimby, B. 1)., ldein, P. C., Barnes, R. M., Anal. Chei ., 1978,

510, 21 12.

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Anal. Chem., 1979,51,875.

12. Pomnirs, P. J., Bouimans, P. IV. J. H., XXI CSI/8th ICAS,

C,-:mbriJge, U. 1., 1979.

13. Vaii Dalen, J. P. J., Kwcc, 1'. G., DeGalan, L., XXI CSI/

8th ICAS, Cambridge, 1. K., 1979.

14. Hulli gan , K. J., Ilahn, . II., Caruso, J. A., Fricke, F. L.,

Anal. Chem., 1979,51,1935.

15. Zander, A. T., Hieftje, G. M., 30th Pitts. Conf. on Anal.

Chem. and App]. Spec., Cleveland, 01, 1979.

16. Zande , A. 1'. , lIieftj c, G. h., XXI CSI/Sth ICAS, Cambridge,

11. K. , 1979.

17. Zander, A. T., liicfljc, G. h., llth Central Reg. MIg. ACS,

Co umliblls, O1, 1979.

1F). P.a ii, . , Malesnen F. J. M. J.' ])eGalan, L., Spectro-

chim. Acla, 1976,:11 ,B 37.

19. Serravall o, F. A., Ri.,hy, T. H., Ar,.il. Ch)eml)., 1975,47,2141.

20. Lmpe, F. W. , i.,by, T. II. , ;crriva lo, F. A. , Anal • Clicilli.

19'77,49, 560.

21. Van JDhce, J . P. J . DcuJezcrinc (.otithider, 11. A. , DkGilan, ,. ,.

Anal. Chiin. Actal 97 7, *94,1L

22. Schwii-, 1. P1. 0 Anal. Cheiii. , 1978, 'o 5,1006O.

23. lBra-ssevi, P. , Macoss'n , F. J. M. J. , flcGalo, , .*, Spectro-

c1him. Actt, 3978, 31,753.

24. Schiwarz., 1. 11. ,Anal. Chem. p 3.979fslI ,log.

25. Hubert, J. , Moisan, M'. , Ricard, A.,. Spcctrochim. -Acta,

26. Layman, L. 1., icftjo, G. M. , Anal . Chemt. 197S,47,,194.

1: 1GJRE CA I' ] 0 N

]:jgurc.1 Excit.zit ion I empOlature profi les at different flow

r, t cs . Thermoiorc I, Hc .;peci e,: lie

A. 3.95 ,/m

B. 3.(00 L/n

C. 4.11 1/,

Chamber region occupied by MJP at specified flow rates:

a. 1.95 L/

b. 3.00 L/

c. 4.11 LI/ni

Figure 2: Excitition temperature profiles at different fliow rates.

Thermonmetric species: 1-e

A. 1.9S L/m

B. 3.00 L/m

C. 4.11 L/m

Chamber region occupied by f11 at specified flow rates:

a. 1 .'95 L/ni

b. 3.00 b/m

c. 4.11 L/n

Figure 3: Relativc intensity profiles for Fe I:

A], I, Cl: 287. 734nmu

A2, P2, C2: 371. 99'lnm

A. I .0), ,/r

B. 3. 0 0 ,/

C. 4.11 Lira

16

14-

12- A

o-(co04

C B

E *

.24

-1.5 -1.0 -0.5 0 0.5 1.0 1.5

Radial Distance Fromn Tube

Center (m

16

14

12

V-) 100

BL -

4 A'4

-1.5 -1.0 -0.mi 0 0.5 1.0 1.5

( 3 ~H'Lur (~nil)

Mim

0E

S0

d)A_-A.

0 0 0:0o W LO

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