issn1330–0008fizika.hfd.hr/fizika_a/av98/a7p037.pdf · 38 fizika a 7 (1998) 1, 37–48. gogi´c...

ISSN1330–0008CODEN FIZAE4

SPECTROSCOPICPROPERTIESOFTHE Li, Zn METALS AND Li-Zn ALLOYPLASMASGENERATED BY XeCl-LASERABLATION

SRECKO GOGIC AND SLOBODAN MILOSEVIC

Institute of Physics, P.O. Box 304, HR-10000 Zagreb, CroatiaE-mail: [email protected], Fax: 00 385 1 4680 399

Received23December1997;revisedmanuscriptreceived30April 1998

Accepted25May 1998

Using 308 nm laserablation,we have producedlithium, zinc and lithium-zinc plasmasandhavestudiedtheirspectral,temporalandspatialcharacteristics.In purelithium plume,linear Starkeffect is observed in n

�D-2�P transitions(�� ) up to 0.3 �� after

the laserpulse. In pure zinc plume, both atomic and ionic spectrallines are observed.Time evolution of spectralline intensitiessuggeststhat the mechanismresponsibleforpopulationof atomiczincstatesis acascadeof ion-electronrecombinationprocesses.Theablationof Li-Zn alloy showsmuchfasterdecayof Zn spectralfeaturesthanof Li spectrallines.Therefore,thedecompositionof plumeinto shellsof differentcoloursis observable.

PACSnumber:52.50.Jm UDC 537.53

Keywords:laserablation,Li, Zn andLi-Zn plasmas,spectral,temporalandspatialcharacteristicsofplumes

1. Introduction

TheLi-Zn system,thesimplestexampleamongalkali-metal-group-IIBmolecules,hasbeenstudiedrecentlyby preparingexcitedmoleculeswithin heat-pipeovensthroughthephotochemicalreactions[1]. Thesestudieshave indicatedtheneedof preparingexcimersin molecularbeams[2, 3] in order to achieve state-specificexcitation. However, when

FIZIKA A 7 (1998)1, 37–48 37

GOGIC AND MILOSEVI C: SPECTROSCOPIC PROPERTIES OF . . .

lithium andzinc aremixedin theoven,high temperatureof thealloy is neededto obtaina sufficient vapourpressurefor anappropriateexpansionandcoolingof moleculesin thebeam[4]. The expansionof laser-generatedplasmathroughan orifice is a widely usedmethodto producecoldmoleculesfrom thealloys whichhavea highmeltingtemperatureanda low vapourpressureof oneof theelements[5, 6]. Sucha sourcecanalsoproduceexciteddimermoleculesdirectly in the laser-generatedplume,asobservedin thecaseofPb [7], Na [8] andCu [9].

Thepulsedlithium atomicbeamwasproducedby CO -laser-inducedablationof a Limetal target in the low-energy-densityregime whenno plasmaforms on the surface,inorderto studycollisionsof lithium Rydberg atomswith buffer gases[10, 11]. Theablationof lithium andzinc hasalsobeenusedin variousapplicationssuchasa matrix isolationspectroscopy (lithium) [12] or in basicstudiesof ablationprocessof metals(zinc)[13,14].Thelaserablationof multicomponenttargets[15] suggeststhatdifferentablationprocessescouldbeexpectedwhendealingwith alloyssuchasLiZn.

We have undertakenthepresentstudywith theaim to characterizeplumesformedby308 nm XeCl-laserablationof lithium, zinc and lithium-zinc targets. We observed thespectraof laser-generatedplume and performedtheir temporaland spatialanalysesforvariousbuffer gasesand laserfluences.Preliminaryresultshave beenreportedrecently[16].

2. Experiment

The experimentalsetupis shown in Fig. 1. A XeCl-excimer laserat 308 nm wasusedfor ablation.The laserenergy couldbevariedup to 150mJperpulseandthepulsehalfwidthwasabout20ns.Thelaserbeam,whichis initialy 2 cmx 1 cm,wasfocusedby aquartzlensof a30cmfocal length,producingawaistof about80 � m onthetargetsurface,with a maximumpower of about10� MW/cm . Calibratedquartzplateswereusedto re-ducethefluence.Thetargetwasplacedinsideasmallvacuumchamber(10 � ) whichcouldbeevacuatedby aRootspumpsystemdown to 10�� mbarand/orfilled with differentgases(Ar, He,H ) up to atmosphericpressure.Thelight from theplumewasprojectedontotheplaneof theopeningof a bundleopticalfiber (diameter3 mm)by a lens.Theopticalfiberwasconnectedto a0.6m scanningmonochromator. Spectrallydispersedfluorescencewasdetectedby a photomultiplier(HamamatsuR2949).Thesignalwassentto a box-carav-erager(StanfordResearchModel 250)whereit wasprocessedandtheoutputwassenttoanIBM compatiblePC.A fastphotodiode,collectingthescatteredlaserlight, wasusedtotriggerthegateof abox-caraverageratacertaintimedelayafterthebeginningof thelaserpulse.Signalsfrom the photomultiplierandfrom the gateof the box-caraveragerweremonitoredwith a 100MHz oscilloscope.Thetemporalselectionof themeasuredspectracould be achieved in the rangefrom about20 ns to 15 � s after the laserpulse.Spatial,temporalandspectralanalysesof theplumewereperformed.Thespatialanalysiswasper-formedby selectinga certainpositionof theopeningof theopticalfiber alongthe imageof theplume.Thetemporalparticularsetof delayandgateparameterswhile scanningthemonochromator, or by settingthemonochromatorat thewavelengthof thedesiredspectral

38 FIZIKA A 7 (1998)1, 37–48


featurewhile scanningthedelayof thebox-cargate.

Fig. 1. Experimentalsetupfor laserablationof lithium, zincandLiZn alloy targets.

The spectralresponseof the systemwasmeasuredwith a calibratedtungstenribbonlamp.Thespectralresponsecurve is almostflat from 400nm to 600nm, decreasinglin-earlyto 20%at 800nm. Thespectrashown in Figs.2, 5 and7 werenot correctedfor thespectralresponseof thesystem.

The targetswerepurelithium, zinc metalandtheir alloys. The alloys werepreparedin heat-pipeovens[1]. Thetargetswererotatedin orderto getuniform plumes.Thatwasespeciallyimportantfor lithium sincethe laserbeameasilycreatesa craterin the targetwhichcausesspatialinstabilityof theplume.

3. Results and discussion

Whenthelaserpulsehits thesurfaceof thetarget,it vaporizesandionizesthematerial.Looking with the naked eye (time-averagedpicture), the light-emitting plasmachangesshapewhenthebuffer gasand/orits pressurearechanged.Undervacuumconditions( ��

mbar),abrightwhitespotis observedat theplacewerethelaserhits thetarget.Verylow intensityemission(blueor green)extendsin almostthewholevacuumchamber. Whenabuffergasis introducedinto thechamber, theregionof thisweakemissionsuddenlystartsto shrinkandformsasmallbrightball of emittinglight (of 5 to 15mmdiam.at2 - 5 mbar).This ball of light canhave shellsof differentcolours(e.g.ablationof LiZn alloy createsgreeninnerandreddishoutershells).As thebuffer gaspressureis increased(atabout100mbar), the ball shrinksinto a small very bright white spoton the target surface.Smallevaporatedparticlesof a sizeof a few micrometersthatwereleaving thetarget,couldbeseenwith thenakedeye in theincominglaserbeam.

FIZIKA A 7 (1998)1, 37–48 39


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z { | } ~ } � � � � � � � �Fig. 2. Spectraof lithium plasmafor differenttime delaysin Ar buffer gasat thepressureof 5 mbar. The light from the completeplumeof about1 cm in diameterwasfed to themonochromator. G is thegateandD thedelaysettingof thebox-caraverager. a) �� ns, �� ns; b) �� ns, �� nsc) �� ns, �� ns. Laser-pulseenergy was25mJ.

3.1. Ablation of pure lithium metal

Theplumeof lithium plasmais observedfor a laser-pulseenergy above10mJ.In Fig.2 wepresentspectraof theexcimer-lasergeneratedlithium plasmain theregion from 390nmto690nmfor differenttimedelaysafterthebeginningof thelaserpulse,integratedover100ns.Thelight from theentirebright ball wasfed into thefiber. In thatspectralrange,only neutrallithium atomicspectralfeaturescouldbeseen.For shorttime intervalsafterthelaserpulse(up to 100ns),continuumemissionis observeddueto thespectraloverlapof highly broadenedatomiclines.Themostprominenteffect canbeseenasa broadeningof the n � D � 2 � P transitions( �� ), while the n � S � 2 � P transitions(�� ) exibit weakerbroadeningwith a slight redasymmetry. Theforbidden3,4 � P � 2 � P40 FIZIKA A 7 (1998)1, 37–48


transitionswerealsoobserved.Then � D � 2 � P transitionsaresymmetricandtheir full-width at half maximum(FWHM) reaches12 nm at a time delayof 30 ns.Thelinewidthswerefound to increasealmostlinearly whenincreasinglaserpulseenergy or the buffergaspressure,and decreaseexponentiallywith increasingtime delay. The observationsindicatea high electrondensitywithin the first few hundrednanosecondsafter the laserpulse. Similar plasmaconditionswereobtainedby Ya’akobi [17] by the methodof anexplodinglithium wire. ThemainbroadeningmechanismhasbeenexplainedasthelinearStarkeffect.Preliminarycomparisonshowsthattheelectrondensitiesof 10� –10� ¡ cm¢�£areobtainedin thepresent

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Fig. 3. Temporalevolution of theemissionat 670.8nm (2P� 2Stransition)for differentargonbuffer-gaspressures.Gatewidth was6 nsandlaser-pulseenergy was25mJ.

case.We notethat in experimentswith anexplodinglithium wire, theself-reversalon theline shapes[18] is readily observed. The light trappingis definitely presentalso in thepresentexperiment,however, theself-reversalin the2P � 2Sline shapeis observedonlyabove500mbarbuffer-gaspressure.

Figure3 shows typical time evolutionsof the670.8nm fluorescence(2P � 2Stransi-tion) for differentargonbuffer gaspressures.They wereobtainedwith themonochromatorsetin thespectralwing of the2P � 2Sresonancetransitionandby scanningthedelayof

FIZIKA A 7 (1998)1, 37–48 41


thebox-cargatewhich was6 nswide.We notethattheintensitiesshown in Figs.2 and3cannotbeeasilycompareddueto differentintegrationtimes.

In orderto analyzetheobserveddependenceof ñ ò ó ô , weassumeasimplemodelwhichdescribesthepumpinganddecayingmechanismof the2Pupperstate.Therateof changeof theupperstateis givenby õ öø÷ ùú óüû ö ò ó ô ýÿþ öø÷ ù ò ó ô ý ÷ ù�� (1)

whereö ò ó ô is thetotaldensityof excitedatomsdecayingto the2Pstatewith theeffective

decayconstantý . Assumingö ò ó ô û ö�� e� � � � � ,

öø÷ ù ò ô û andwith

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- . / 0 1 0 2 3 4 5 6 2 7 8Fig. 4. Thetime constants9 : and 9 ÷ for the460.3nm line in Ar buffer-gasat pressureof10mbar. Laser-pulseenergy was25mJ.

ý û :� � � ý ÷ ù û :� ; , thesolutionis

öø÷ ù ò ó ô=< 9 ÷9 ÷ þ>9 : ò ? �>@A ; þ>? �>@A � ô (2)

Thefull line in Fig. 3 showstheresultof thefit of thefunctionalform givenby Eq.(2). Inthisparticularcase,thesatisfactoryfit provestheassumptionof theabovemodel,namely,

42 FIZIKA A 7 (1998)1, 37–48


that onedominantpumpingprocessexists. The effective lifetime, B C D at zerobuffer gaspressureis foundto beabout100ns.It increasesto about1.5 EGF whenthebuffer gas(ar-gon)is introducedinto thechamberatpressurebetween5 and10mbar. At higherpressureof about30 mbar, it decreasesto about1 E s. It is interestingto notethat this dependenceappearsto becloselycorrelatedwith thesuddenformationof thelight ball andits shrinkingasthebuffer-gaspressureis increased.Sincethenaturallifetime of the2P H 2Stransitionis only 27ns[19], thelargedecaytimesareobviouslydueto thelongpumpingtimeof the2Plevel anddueto theradiationtrappingeffectathigherbuffer gaspressures.It is strikingthat the simplemodelpresentedabove givesa gooddescriptionof the observed intensi-ties, sincethe 2P level populationis obviously underinfluenceof cascadesfrom higherlithium atomiclevels.If dominant,cascadecontributionsor severalpumpingmechanismswith differentrateswould give rise to slower decayof fluorescencein its tail (seefor ex-ampleEq. (3) in Ref.20).We assumethatLi(2P) atomsaredominantlyproduceddirectlythroughelectron-ionrecombinationinto the2P state.However, lithium ionic linescouldnotbeobservedin thepresentexperimentsto supporttheexistenceof excitedlithium ions,becausemostprominentlinesarein thedeepUV region,anda few linesavailablein thevisiblepartof thespectrumaremaskedby theStarkbroadened4D H 2Ptransitionat460nm.

On theoscilloscope,we observe a largechangeof time evolution of thefluorescencewhile scanningthe monochromatoraround460 nm. In orderto clarify this observation,the effective lifetimes weremeasuredwithin the line profile of the 4D H 2P transitionat 460.3nm. For eachsettingof the monochromatorin the linewing of the 4D H 2Ptransition,the time dependenceof emissionwasmeasured,andthe curvessuchasthoseshown in Fig. 3 wereobtained.In Fig. 4, we show theresultof thefit, namely, thevaluesof theparametersB I and B C , versuswavelengthin the region of the line profile. We notethatthenaturallifetime of the4D level is ( J KGLMK ) ns[21]. TheparameterB I shouldreflecta typical time of the processeswhich populate4D level. It is between0.5 to 1 E s. It ispossiblethata certainstructurein thedependenceof B I N OQP exists,but largeerrorbarsdonot allow a conclusion.Thedisplayof theparameterB C versuswavelength(lower partofFig. 4) shows almostLorentzianshapewith a halfwidth of about0.09nm. (The spectralresolutionof themonochromatoris about0.02nm). Typical lifetimesin thewingsof theline profilesareabout1 E s. For example,at 5 nm from the line center, the linear Starkbroadeningis dominant(seeFig. 2), hencewe canconcludethat the measuredeffectivelifetime B C at thatwavelengthreflectsa lifetime of a high-electron-densityplasma.In thecenterof the line, the effective lifetime amounts5–7 E s which is most likely dueto thelight trapping(high densityof atomsin 2Pstates)but alsoto theprolongedfilling of the4D level.

3.2. Ablation of pure zinc metal

With atargetof purezinc,similarspatialshapesof plumeswereobserved,however, ofdifferentcolours(predominantlyblue).Figure5 showstypicalspectrain thespectralrangefrom 400to 700nm.Thespectrumshown in Fig. 5awastakenat a smalltime delayafterthe laserpulse,with the narrow gateof the box-caraverager, selectingthroughthe fiberonly the light from the almostwhite nucleusof the plume. Both atomicand ionic lines

FIZIKA A 7 (1998)1, 37–48 43


of zinc wereobservedwithin 300nsafterthe laserpulse.Theionic transition4 R FS T R�UV R D W T R at492.3nmshowsapronouncedStarkbroadeningsuitablefor diagnostics.Figure5bshowsthespectrumwhenonly light from theouterball wasfed throughthefiber. Onlylinesof atomiczincstemmingfrommetastable4p X PY Z [ Z R levelswereobserved.Comparingthe observationsof temporalevolutionswith purezinc andpure lithium, it appearsthatrecombinationprocessesresponsiblefor thespectralemissionfrom theplasmaarefasterin zinc thanin thepurelithium.

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Fig. 5. Spectraof excimer-lasergeneratedzinc plasmain theH R buffer gasat 6 mbar. a)takenfrom thewhitespot,D = 100ns, G = 100ns; b) from theouterball, D = 100ns, G =3 ó s. Laser-pulseenergy was115mJ.

Figure6 showstemporalevolutionof atomiczincemissionat481.05nmfromdifferentspatialregionsin (a) H R and(b) Heatmosphere.Thetemporalevolutionof thewhitespotintensityis characterizedby astrongpeakat thetimedelayof about250nsanda long tailwhich canalsobeseenasa well developedsecondmaximumat 3 ó s (Fig. 6b).Temporalevolutionof theouterball exhibitsonly onemaximumshiftedto about800nsor 4000ns,dependingon thebuffer gas.Neitherthewhitespotnor theouterball temporalevolutionscanbedescribedby thesimplemodelgivenabove.Rather, thesecondmaximumappearsto be the result of a chain of reactionsprecedingthe populationof the relevant upper

44 FIZIKA A 7 (1998)1, 37–48


state.Most probably, Znô ions recombinewith electronsgiving rise to the populationof long-livedRydberg statesof Zn which radiatively fill the upperstateof the observedtransition.This long-lived intermediatestateseemsto causethe secondarymaximumofthefluorescencesignal.

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Fig. 6. Temporalevolution of characteristicemissionat 481.05nm from specificspatialregionsof excimer-lasergeneratedzinc plasmaa) with H x buffer gasat 60 mbarandb)with Hebuffer gasat60mbar. Laserpulseenergy was115mJ.

We note that with the helium buffer gas,larger light intensitieswereobtainedwitha considerablylarger characteristiclifetime of the emissioncomparedto the caseof H xbuffer gas.This wasobservedboth for thewhite spotandtheouterball. We believe thatthemolecularbuffer gasis moreefficient in quenchingtheplasmacomparedto theatomicbuffer gas[10]. We notethat we have observeda weakemissionfrom atomichydrogenwhich could be an indicationthat reactionprocessesarealsotaking place.However, noexcitedmolecularproductscouldbeobserved(for exampleZnH). Thatdoesnot excludecreationof productsin thegroundstate.

3.3. Ablation of lithium-zinc metal alloy

We performedablationof the lithium-zinc metalalloy thatwaspreparedin theheat-pipe oven [1]. Its composition(about50:50molar ratio) was found suitablefor photo-

FIZIKA A 7 (1998)1, 37–48 45


chemicalproductionof LiZn excimers.Figure7 showsspectraof lithium-zincmetalalloyplumein thespectralrangeof 450nm to 500nm, for differenttime delaysafter the laserpulse.This particularspectralrangewaschosenbecausetheblue-greencoloursdominatethemainportionof theplume,andalsobecauseLiZn excimeremissionis expectedat470nm [1]. The presenceof excited LiZn moleculeswasnot detected.The reasonfor thatcouldbethatmulticomponentplasmais

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Û Ü Ý Þ ß Þ à á â ã ä à å æFig. 7. Spectrafrom theexcimer-lasergeneratedplasmaof Li-Zn alloy in argonat67mbarfor differentsettingsof thedelayandgateof thebox-caraverager. Laser-pulseenergy was115mJ.

flying apartdueto a largemassdifferenceof Li andZn, andwhenit becomescoldenoughto allow formationof molecules,the densityof both componentsat the sametime andplaceis too low. This could be overcomeby preparingan alloy of specificcomposition.We notethat in thecaseof purelithium andpurezinc, we werenot ableto detectspectraof theexciteddimerLi ç or Znç molecules,in a widerangeof timedelaysandspace.

FromFig. 7, onecanseethat lithium spectralfeaturespersistfor a muchlongertime.This is the main reasonfor the spatialregionsof theplumewith differentcoloursin thecaseof laserablationof theLi-Zn alloys,namelyreddishoutershell(2P è 2Stransition,notshown in Fig. 7).

46 FIZIKA A 7 (1998)1, 37–48


4. Conclusions

Theablationof lithium metalwith the308nm beamlaserresultsin the formationofplasmafor laserpulseenergiesaboveabout10mJ.Theplasmais of low atomicdensitybutwith highelectrondensities,whichcausespronouncedbroadeningof thenD é 2Plithiumtransitionsby thelinearStarkeffect.An unusualtimeevolutionof thefluorescencewithinthe 4D é 2P line profile wasobservedwhich is consideredto bean interplayof effectsof radiationtrappingdueto largeLi(2P) populationandthe linearStarkbroadeningdueto a high electrondensity. Theablationof zinc targetsleadsto the formationof multiplyionizedatoms,which thenrecombineforming the highly excitedatomsandlessionizedatoms.This is reflectedin theformationof differentcolourballsandof layersaroundthepointwherethelaserbeamhits thesurface.

Acknowledgements

This work wasfinancially supportedby the Ministry of ScienceandTechnologyoftheRepublicof Croatia.We wish to thankDr. GoranPichlerfor valuablediscussionsandcontinuoussupportin thiswork.

References

1) S.Milosevic, X. Li, D. Azinovic, G. Pichler, M. C. vanHemert,A. StehouwerandR. Duren,J.Chem.Phys.96 (1992)7364;

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SPEKTORSKOPSKASVOJSTVA PLAZME METALA LITIJA I CINKA I Li–ZnLEGURA PROIZVEDENIH ABLACIJOMPOMOCU XeCl LASERA

Proucavali smo spektralna,vremenskai prostornasvojstva litijeve, cinkove i litij-cinkplazmestvoreneablacijompomocu laserana308nm.Kod cistoglitija opazenje pri nê D-2ê Pprijelazima( ëíìïî ð ñòð ó ð ô ) linearniStarkov efektuvremenudo0.3 õ÷ö nakonlaserskogpulsa.Kod cistogcinkaopazenesu i atomske i ionske spektralnelinije. Na temeljuvre-menske evolucije intenzitetaspektralnihlinija mozesezakljuciti da je mehanizamnasel-javanja pobud–enih stanjaatomacinka kaskadarekombinacijskihprocesaizmedu iona ielektrona.Pri ablaciji litij-cink slitine opazenoje mnogobrzegusenjezinkovih spektral-nih pojavanegolitijevih. To je glavni razlograzdvajanjaoblackalitij-cink plazmeu ljuskerazlicitih boja.

48 FIZIKA A 7 (1998)1, 37–48

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