AFD-A094 343 INDIANA UNIV AT BLOOMINGTON DEPT OF CHEMISTRY F/6 20/9 OERATIONAL CHARACTERISTICS OF A HELIUM MICROWAVE-INDUCED PLASM--ETC(U) JAN 81 A T ZANDE R, S M HIEFTJE N0001476-C-838 UtCLASSIFIED TR-30 NL * EhEE~hhhhh
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
UtCLASSIFIED TR-30 NL
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REPRT DOUMENTA,, PAGE REIM INSTRUCTIONSREPRET NUMjiR P BEFORE COMPLETING FORMNU1R. Y R 2. GOVT ACCESSION NO. 3. RECIPIENTS CATALOG NUMBER
THIRTY / .. ' ".
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
IL. CONTROLLING OFFICE NAME AND ADDRESS 3RU@At
Office of Naval Research j " '-Jan-.......W..98l
Washington, D.C. 13. NUMBER OF PAGESI0 is
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IS*. DECL ASSI FIC ATION/DOWNGRADING
A SCHEDULE
16. DISTRIBUTION STATEMENT (ol this Report)
Approved for public release; distribution unlimited
17. DISTRIBUTION STATEMENT (of the abetract entered In Block 20, It different howt Report)
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
Reproduction in whole or in part is permitted forany purpose of the United States Government
Approved for Public Release; Distribution Unlimited
DISCLAIMER NOTICE
THIS DOCUMENT IS BEST QUALITYPRACTICABLE. THE COPY FURNISHEDTO DTIC CONTAINED A SIGNIFICANTNUMBER OF PAGES WHICH DO NOTREPRODUCE LEGIBLY.
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
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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
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