Absorptionspectroscopyofatmosphericspecies(ozone)Lab:MolecularSpectroscopy,U4150,restrictedarea,pleasecontactviamailorphonebeforecoming.

Responsiblestaff:VictorGorshelev

Phone:62120,62121,email:gorshelev@iup.physik.uni-bremen.de

Lastchanged:30.04.2016

ContentsIntroduction............................................................................................................................................2

Objectives.................................................................................................................................................2

Globalmonitoringofozone.....................................................................................................................2

Ozonemeasurementsinthelaboratory..................................................................................................3

Ozoneabsorptioncross-sections............................................................................................................3

1. Electronic-vibrational-rotationalspectra........................................................................................3

2. Beer-Lambertlaw............................................................................................................................6

3. Spectralinstrumentswithcross-dispersion....................................................................................8

4. Uncertaintyestimation.................................................................................................................10

Experiment............................................................................................................................................11

1. Challenges.....................................................................................................................................11

a) Experimentalsetupscheme......................................................................................................12

b) Spectrometer............................................................................................................................13

c) Dataacquisitionandprocessingsoftware................................................................................14

2. Safetyinstructions.........................................................................................................................16

3. Tasks..............................................................................................................................................17

a) Spectraacquisition....................................................................................................................17

b) Dataevaluation.........................................................................................................................18

c) Conclusions...............................................................................................................................18

Introduction

In the molecular spectroscopy laboratory we maintain set-ups for high and medium resolutionspectroscopyofgasesandmixturesofgases.Thelaboratorywascreatedtosupplythecommunitywithaccuratereferencedataonabsorptionspectraandratecoefficients,importantforatmosphericphysicsandchemistryinvestigations.

Objectives

Inthispracticalyouhavethepossibilitytolearnthetechniqueoftheclassicalabsorptionspectroscopyontheexampleoftheozoneabsorptionspectrumintheultravioletandvisible(UV-VIS)spectralrange.

You will learn about optical beam path alignment, broad-band light sources operating in the UV-VISspectral regions, andmodern two-dimensional detectors (CCD, ‘charge coupled device’) allowing fastrecording of broad spectra. You will work with an Echelle spectrometer using the cross-dispersionprinciple,whichisoperatedviaspecialsoftware.YouwillobtainspectraandcalculateopticaldensitiesusingtheBeer-Lambertlaw.

This practical guide will introduce you to the general ideas about the processes involved in theexperiment. There are several suggestions for reading and you are very welcome to check internetpagesusingrelevantkeywords.Itisinyourowninteresttocollectthenecessaryamountofinformationusingexternalsources,sinceitisnotpossibletodescribeeverythinginthispracticalguide.

Questionsandexamplesaregiventhroughoutthetexttogiveyoutheopportunitytocheckifyouhaveunderstoodthematerial.Pleasetrytoanswerthem!

Globalmonitoringofozone

Ozone(O3)isanimportantatmosphericcomponent,responsibleforUVabsorptionintheupperlayersof the atmosphere (stratosphere). Long-termmonitoringof theozonedistribution in the atmosphereand its trendsareperformednowadayswithanaccuracyofbetter than5%atmany locations.Globalcoverageisprovidedbyseveralsatellitescarryinginstrumentsforremotesensingofozone,whichwerelaunchedinthepastdecades.Newinstrumentsaretobelaunchedwithinthenextyears,sincethelifetimeofeveryinstrumentis limited.Alsothereisanetworkofground-basedstationsmeasuringozoneconcentrationlocally.

Referencedataon theozoneabsorptioncross-sectionsareneededto retrieve theatmosphericozoneamountsfromspectradeliveredbysatellitesorground-basedinstruments.MostoftheinstrumentsusethestrongozoneabsorptionintheUVregion(250nmornear330nm),whichhasaverycharacteristicstructure. Therefore knowledge of the absorption cross-sections at these wavelengths is of specialinterest.

Reading:

1. Please read the general information on the physical and chemical properties of ozone. There arenumeroussourcesintheinternet,youcansimplytrythehttp://en.wikipedia.org/wiki/Ozone

2. You can find a lot of informationon the satellite instruments at thehomepageof the European SpaceAgency.Thisisanoptionalreading.http://www.esa.int/Our_Activities/Observing_the_Earth

3. YoucanfindalotofinformationontheGOMEandSCIAMACHYspectrometersonthehomepagesoftheIUP. This is an optional reading. http://www.iup.uni-bremen.de/sciamachy/index.html,http://www.iup.uni-bremen.de/gome/

Questions1.

1. Whatdoes‘ozone’meantranslatedfromtheancientGreek?2. Whyisozonedangerousforhumans?3. Whydoweneedtomeasureozonewithhighprecision?4. Whyweareinterestedintheabsorptionat330nm?

Ozonemeasurementsinthelaboratory

Ozone is an unstablemolecule. In the laboratory ozone is produced from oxygen, flowing through avessel with electrical discharge causing the dissociation of the oxygen molecules in collisions withacceleratedelectrons:

O2+e-→O+O+e-; O2+O+M→O3+M

WhereMisathirdbody,usuallyanairmolecule.Incollisionswithmoleculesozonedecays:

O3+M→O2+O+M

Thisprocessresultsinanexponentialdecayoftheozoneinabsenceofanyproducingmechanisms:

𝑛"# $ = 𝑛&𝑒𝑥𝑝 −𝑘𝑛,𝑡

Here t is time, n0 is an initial concentration, k the decay rate and nM is the number density of thesurroundingmolecules. Thedecay ratedependson the temperatureand the typeof the surroundingmolecules.

Please,notethatozoneisharmfulforhumansifbreathedinconcentrationsaslowas0.16mg/m3

Example1.

Calculatethetimeneededforanozonedecayof10%atroomtemperatureandapressureof50mbar,assumingthesurroundingmoleculestobeozoneandusingthedecayratecoefficient

k=7.1510-10exp(-11195/T)[cm3/molecule/sec],TstandsfortemperatureinKelvins

Usetheidealgaslaw(seeexample2formoredetails)andneglectthedecaywhencalculatingnM.Comparethistimewithatypicaldurationoftheexperiment(1hour).

Ozoneabsorptioncross-sections

1. Electronic-vibrational-rotationalspectra

SomeinformationinthischapteristakenfromtheFTSpracticaldescriptionhttp://www.msc-ep.uni-bremen.de/services/lectures/practicals/pr_fts_2015.pdfYouarewelcometoreadmoredetailsintheoriginaltext.Atomsandmoleculescanonlyhaveaseriesofdiscretevaluesofenergywithcorrespondingquantumnumbers. Energy levels (E) are divided into electronic (Eelec), rotational (Erot), vibrational (Evib) andtranslationalenergy(Etrans),whichisnotconsideredfurtheroninthisdescription.

𝐸 = 𝐸/0/1 + 𝐸345 + 𝐸67$ + 𝐸$689: (1.1).

Theabsorptionofradiationwithfrequencyf(measuredinHz)occurswhenaparticle(atomormolecule)makesatransitionfromonestatewithenergyEktoanotherstatewithhigherenergyEj:

𝐸; − 𝐸< = ℎ𝑓 (1.2).

HerehisthePlankconstant,whichisdependingontheunitsofenergy,is

h≈6.62610-34[J s]orh≈4.14×10−15[eV s].

TheWavelengthλistypicallymeasuredin[nm](10-9m)or[Å]‘angstrom’(10-10m)intheultravioletandvisiblespectralrangeandin[µk](10-6m)intheinfraredspectralrange.Wavelengthisconnectedwithfrequencybythesimplerelation:

𝑐 = 𝑓 ∙ 𝜆 (1.3).

Here c is the speed of light. Please, note that in spectroscopy sometimes the symbol ‘ν’ is used forwavenumberwhichisinverselyproportionaltothewavelengthandhasunitsof[cm-1].

Example2:

Oxygenhasanabsorptionbandatwavelengthsaround760nm,whichcorrespondstoanenergygapofabout1.63eV,wavenumberof13158cm-1andfrequency394.5THz.

Questions2:

Whydopeopleoftenuse[nm]intheultravioletregionand[µm]intheinfrared?

Molecular spectra, especiallywhen thenumberof atomsexceeds two,havea complex structureandinvolveelectronic,vibrational-rotationalandpurerotationaltransitions.Theenergylevelsaredescribedbyasumofcorrespondingterms:

𝑇/0 + 𝐺3345 + 𝐹3,G67$ =HIJK1+ ℏM

K1𝑣 + O

P+ 𝐵 ∙ 𝐽 ∙ 𝐽 + 1 (1.4)

andthewavelengthofthetransition isgivenbythedifferenceoftermsoftheupperandlower levelsinvolvedinthetransition.

Inequation(Eq.1.4):

− Eel-electronicenergy,

- Tel,G,F–electronicvibrationalandrotationalenergiesintermsofwavenumbers(cm-1).

− ω–istheangularfrequencyω =2π f,historicallyconnectedwiththevibrationalmotionoftheharmonicoscillator;

− ℏ = ℎ 2𝜋;

− v (v = 0, 1, 2, …) – is the vibrational quantum number; this very common designation issomewhatconfusing,sincetheverysimilarsymbolνisusedforthewavenumber.

− B–istherotationalconstantofthemolecule,connectedwiththemomentofinertia.− J(J=0,1,2,…)-isthevibrationalquantumnumber.

Differencesofquantumnumbersforupperandlowerstatesofatransitionobeycertainselectionrules:

− vibrationaltransitions:Δ𝑣=±1,±2,±3,…withfastdecreasingintensities;

− rotational transitions:Δ𝐽 = 0, ±1 . Observation of transition with ΔJ=0 depends on thesymmetryofthemolecularstructure.

− Electronic transitions: the electronic energy of one electron is a result of the distance to thestatic atomic nuclei and the electronic system formed by further electrons. The transitionsbetween two electronic states are ruled by the Franc-Condon principle, which states that anelectronic transition occurswithout changes in the nucleus positions during a transition. Thesyntaxoftheelectronictransitionsisnotgiveninthedescriptionofthispractical.

Example3:

Thelowestenergylevelofamoleculeisdescribedbytheterm

HIJK1+ ℏM

PK1

Thenucleiinamoleculeareheldtogetherbytheelectrons.Dependingonthequantumnumberoftheelectrons therearedifferentelectronic statesof themolecule.Nuclei carryout their vibrationsunderinfluenceofthepotentialenergywhichisasumoftheelectronicenergyandtheCoulombenergy.

In the figure below, potential energy curves for a diatomic molecule are shown as function of thedistance between nuclei. The black and blue curves correspond to states with different electronicenergy:groundelectronicstate(black,S0)andfirstexcitedelectronicstate(blue,S1).Theenergylevelsfor thevibrationalmotionare shownasblackandblue lines inside thecurves.Red lines in the insertshowrotational levels for thezerovibrational levelofS0.Notice that thecharacteristicenergy for therotationalmotion ismuchsmaller thanthat for thevibrationalmotion,andthe latter ismuchsmallerthantheenergyassociatedwithelectronicmotion.Thereisanequilibriumdistancewithminimalenergywhichisnotzero.

Fig.1. Potential energy curves of a diatomicmolecule.Pictureadaptedfrom“PHOTOCHEMISTRYTheoreticalConceptsandReactionMechanisms”http://www.photobiology.info/Ilichev.html

In the ozonemoleculewhich is not linearwith 3 nuclei there are 3 x 3 - 6 = 3 relative coordinates;therefore the potential energy under which the nuclei move in a given electronic state can be

represented by a 3-dimensional surface in the in a 4-dimensional space. Every electronic state isrepresentedby suchapotential surface. Suchhypersurfacesunlike the simplepotential curvesof thediatomicmoleculesaredifficulttovisualize.

TransitionsbetweenelectronicstatesleadtospectraintheUVregion.Transitionsbetweenvibrational-rotational levels belonging to the same electronic state produce spectra in the visible and infraredregions. Pure rotational transition within the same vibrational state leads to the spectra in themicrowaveregion.

Reading:

A very detailed book on molecular spectra is the work of G. Herzberg “Molecular Spectra and MolecularStructure”,consistingofthreevolumes:

“I.SpectraofDiatomicMolecules”“II.InfraredandRamanSpectraofPolyatomicMolecules”“III.ElectronicSpectraandElectronicStructureofPolyatomicMolecules”.

Unfortunately, this book is mostly for advanced spectroscopy readers. As a possible alternative with someexperimentalaspectsthebook“Molecularphysics”byW.Demtrödercanberecommended.

2. Beer-Lambertlaw

Measurements of the ozone absorption are based on the well-known Beer-Lambert law, whichdescribestheintensityI(λ)ofthelightatthewavelengthλtransmittedthoughtamedium,containingtheabsorbinggas:

( ) ( ) ( ) ( ){ }dllpTnIIL

∫ ⋅−=0

0 ,,exp λσλλ . (2.1)

Here

I0-istheinitiallightintensityinabsenceofabsorbingmolecules(background);

n[molecule/cm3]-absorbinggasdensitywhichisafunctionoftemperatureT,pressurepandthepositionl

L[cm]–lengthoftheabsorbingmedium;

σ[cm2/molecule]–absorptioncross-section.Themeaningofthisquantitywillbedescribedbelow.

In the typical laboratory experiment, the gas of interest is filled into the cell, which has windowstransparentforlightwithwavelengthλ, definedbytheaimsoftheexperiment.Inthecurrentpracticalweinvestigateabsorptionatwavelengthsaround330nm(UV)forthereasondescribedabove.

Iftheabsorbinggasisdistributeduniformlyalongtheopticalbeam,equation(Eq.2.1)canbesimplified:

( ) ( ) ( ) ( ){ }LpTnII ⋅⋅−⋅= λσλλ ,exp0 . (2.2)

Thereareseveralconditionstobefulfilledtousetheequations(Eq.2.2):

- Theabsorbingmediummustbehomogeneouslydistributedintheinteractionvolumeandmustnotscattertheradiation;

- Theincidentradiationmustconsistofparallelrays,eachtraversingthesamelengthintheabsorbingmedium;

- Theincidentlightfluxmustnotinfluencetheatomsormolecules.- Inexperimentstwoconsequentmeasurementsofintensityhavetobedone:onewithandone

withoutabsorbinggas,allotherparametersmustbeconstant.

Theproductofcross-section,gasconcentrationandabsorptionlengthiscalledopticaldensityODandcanbefoundfromtheratioofthelightintensitieswithandwithoutgas:

OD = lnI0 λ( )I λ( )

= n T, p( ) ⋅σ λ( ) ⋅L . (2.3)

Fromtheopticaldensitytheozoneconcentrationcanbefoundifbothcross-sectionσandabsorptionlengthL areknown.Therefore,precise informationon thecross-sections is important forhighqualityremotesensingozonemeasurementsintheatmosphere.

Inphysics,thecross-sectionisaquantitydescribingtheprobabilityofaprocess.Theabsorptioncross-section describes the ability of molecules to absorb radiation of a certain wavelength under certainconditions.

Theabsorptioncross-sectionisconnectedwiththeprobabilityofthetransitionbetweenenergylevelsofthemolecule.Thereforeitisafunctionofwavelengthanddependsonparameterssuchaspressureandtemperature.Theabsorptioncross-sectionofozonehasaverystrongdependenceonwavelengthintheUVregion.

The dependence of cross-sections on pressure and temperature can be very complicated. For simplemolecules cross-sections can be calculated using information on the energy levels of the molecule.However,evenformoleculesconsistingofthreeatomsthesetypesofcalculationsareverycomplicated,especially for transitions in theUV region,which involveselectronic, vibrational and rotational levels.Despiteallefforts,theaccuracyofcalculatedozoneabsorptioncross-sectionsisstillpoorcomparedtoexperimental values. Nowadays, accurate cross-sections can be obtained under laboratory conditionsonly.

Duringthecurrentpractical,wewillobtainrelativecross-sections:

( )( )

( )( )λλ

λσII

pTA0ln

,1

= (2.4)

HereAisascalingfactor,whichistheproductoftheozonedensityandtheabsorptionpathlength.Wewill not define this factor, sinceanadditional experiment isneeded to find theozonedensity,whichgoes beyond the goals of this practical. The main interest is on the qualitative dependence of theozonecross-sectiononthewavelength.

Notethat incaseofasmallcross-section,thedifferencebetweenthe light intensitywithandwithouttheabsorbinggas isverysmallandtheresultingerrorbarsforthecross-sectionare large.Anobvioussolutionistoincreasetheabsorbinglengthortheconcentrationoftheabsorbinggas.Therearecertainlimitsfortheconcentrationofozonefromsafetyrules.Therefore,reflectiveopticsisinstalledinthecellto increasepath lengthbysending lightmanytimesthroughthegas(socalledmultipasscellofWhitetype).

Questions3.

1. Howcanlightinfluencethegas?2. Whyarecalculationsofcross-sectionscomplicated?3. WhatdoesopticaldensityOD=0physicallymean?AndOD=1?4. Whydoweusemultipassoptics?

Example4.

a) Let’scalculatetheopticaldensityforthefollowingcase:

Amixtureatroomtemperaturecontains10%ofozoneinoxygenatatotalpressureof100mbar,thelengthofthecellis1m,thecross-sectionis10-18cm2/molecule.

Theozoneconcentrationno3canbefoundfromtheidealgaslaw:

kTpn O

O3

3 =

Therepo3ispartialpressureofozoneinPa,kistheBoltzmannconstantk=1.38x10-23J/KandTisthetemperatureinKelvin.

InourcasepO3=103Pa(100mbarx10%=10mbar;1000mbar=105Pa)andnO3≈2.410

23molecule/m3.Opticaldensity(notetheunits!):OD=2.41017molecule/cm3x100cmx10-18cm2/molecule=24.

ThismeansthattheintensitywillbeI=I0e-24.Thisintensityistoolowcomparedtothebackground

intensityI0.Remember,thatallparametersshouldbeconstant!IfthesensitivityofthesystemwasadjustedtothelevelofthebackgroundintensityI0,thesignalIwillbeonthelevelofthenoise.

b) For a cross-section of 10-20 cm2/molecule the optical densityOD≈0.24 and I=I0 e-0,24≈ 0.8 I0. This

conditionbelongstothefavorableregion0.1<OD<1andprovidesgoodresults.c) Calculatetherangeoftheozonepartialpressureformeasurementsofcross-sectionsintherange

σ=10-20–10-17cm2/molecule,assumingOD=1.

Suggest conditions (L, total pressure, concentration of ozone in oxygen) to realize theseexperiments.Rememberthat:lengthcantakediscretevaluesof1m,2metcbutnotmorethan20m (moredetailswill begiven in theexperimentalpartof thedescription); totalpressure cannotexceed1000mbar;concentrationofozoneinoxygencannotexceed10%.

3. Spectralinstrumentswithcross-dispersion

ToobtainaspectrumintheUVregion,broad-bandradiationofaDeorXelampissentthoroughthecellandanalyzedusingaspectrometer.Inourexperimentweuseaso-called“echelle”gratingspectrometer(from French, échelle, meaning stairs or ladder), which uses the principle of cross-dispersion. In anechelle spectrometer there are two diffractive elements (a diffractive grating and a prism)mountedorthogonallyinsuchawaythatthehighlyilluminatedordersaretransversallyseparated.

A diffractive grating is a periodic structure, consisting of reflecting or transparent grooves with acharacteristic sizeof theorderof themagnitudeof thewavelength.A grating is characterizedby thegroove density, usually counted in grooves per mm. A diffractive grating splits the incident light inseveraldirectionsdependingonthewavelengthandtheperiodofthegrating.

dx(sinφ+sinθ)=mλ (3.1),

here

distheperiodofthegrating;φ–theangleoftheincidentlightrelativetotheperpendicularofthegrating;

θ–theangleofthereflected(transmitted)light;λ–wavelength;m–anintegernumber(“order”).

Thezerothorder(m=0)correspondstospecularreflectionandisatthesameangleforallwavelengths.Therelation(Eq.3.1)canbederivedfromthephasedifferencebetweenbeamsreflectedbyneighboringperiods.Forlightfallingperpendicularlyonthegrating:

sinφ=mλ/d (3.2)

Fromequation(Eq.3.2)weimmediatelygetthat

m1λ1=m2λ2=m3λ3=sinφxd=const (3.3).

Thereforewavelengthsbelonging todifferentorderswill be refractedby the sameangle. The seconddispersive element, for example a prism, oriented orthogonally to the spectrumwill shift the orders,formingatwo-dimensionalspectrum.Thisarrangementisveryadvantageous,whenhighresolutionofabroadspectralregionisneeded.

Fig.2. Simplified schematic illustration of the 2D spectrum formedfromthe“white”spectrumusingprismandgrating

Questions4.

1. Foragratingwithagroovedensityof100gr/mmcalculate themaximumdiffractivewavelength intheorderm=100andorderm=50(lightfallsperpendiculartothegrating).

4. Uncertaintyestimation

Therearetwomainsourcesofuncertaintiesofthecross-sectionsobtainedusing(Eq.2.4):

- Statistical,arisingfromthestabilityoftheintensitiesIandI0.Thisuncertaintycanbefoundintermsofstandarddeviationofthe(arithmetic)meanafteraveragingNspectra:

𝑠𝑑 = [\I]^_[` ab`c c_O

, (4.1)

HereImeanisthemeanvalue,Iitheintensityofasinglespectrum,andNthenumberofobtainedspectra.

- Systematic,comingfromtheuncertaintyofthescalingfactor.Thissourceofuncertaintiesisnotestimatedinthiswork.Intheseterms,thesystematicuncertaintycharacterizestheaccuracy(differencebetweenmeasuredandrealvalue)andthestatisticaluncertaintycharacterizestheprecision(reproducibility)ofthemeasurements.

a bFig.3. Illustrationofsystematicandstatisticalerrors:a) lowaccuracy,highprecision;b)highaccuracy,lowprecision

Thetotalerroriscalculatedusingtheusualequation

𝛿𝐹 𝑥4 =𝜕𝐹𝜕𝑥4

𝛿𝑥4P

Questions5.

Calculatethetotaluncertainty(inpercent)fortheopticaldensityODobtainedfromI=20000andI0=40000,bothmeasuredwith1%accuracy(δI=200andδI0=400)

Experiment

1. ChallengesOzonecross-sectionvaluesspanover6ordersofmagnitudewithin250-350nmrange:

Availablescientificdevicesarenotabletosimultaneouslycovertheentiredynamicandwavelengthrangenecessaryforthewholeabsorptionspectrum.Inordertoobtainfullcoverageofthespectrumitisnecessarytoperformmultiplemeasurementsunderdifferentconditions.

Inoursetupwecancontrol:

- gasflowrate(100-600sccm–standardcubiccentimetersperminute.“Standard”meansthatthetemperatureisassumedtobe293Kandpressureisatmospheric)

- ozonegeneratorperformance(80-300arbitraryunitsontheozonegeneratorcontroldial,withhighervaluegivingca.10%O3intheresultingO2+O3mixture)

- gasmixturepressure(5-950mbar)orpureozonepressure(1-50mbar)ofthesampleinthecell- temperatureofthecell(190-295K)- absorptionpath(from5cmtoabout30meters)- lightsource(Deuteriumlampforshort/longsinglepass,XenonorLaser-DrivenLightsourcefor

allconfigurations)- spectrometersettings

Eachpairofspectra(with/withoutabsorbingmolecules)forotherwisestabilizedexperimentalconditionsyieldsoneOpticalDensity(OD)spectrum.DuetospecificsofthespectrometeritisonlypossibletoobtainanODspectrumspanninglessthan3ordersofmagnitude.CalculatedODspectrawithvaluesoutsidethe2-0,02tendtobeeithersaturated(strongabsorption->bigdifferencebetweentwospectra->highODs->lossofinformationwhenalmostalllightisabsorbed)ornoisy(weakabsorption->smalldifferencebetweentwospectra->lowODs->highinfluenceofwhitenoise).Wealsoestimate“darkcurrent”backgroundvalue.Itrepresentsthe“background”originatingfromspontaneoussignalcausedbythermalquantumeffectsinthespectrometersensor.Examplebelowisademonstrationofcross-sectionspectrumderivation.

Fig.4.FromsinglespectratoODandcross-sections

a) Experimentalsetupscheme

Theexperimentalsetupconsistsofseveralmajorcomponents(Fig.5):

1) Experimentalcell(optionallycoolable),inwhichgassampleiscontained.Thecellhasseveralpossiblealignmentpaths:singleshort(5cm)pass,andalongmultipass,resultingin5-30metersofeffectiveabsorptionlength(dependingonalignment)

2) Externalcryostatforcellcooling/gasprecooling.Thecryostatcoolsdowntheliquidethanoltothedesiredtemperatureandcirculatesitthroughthejacketofthecell

3) Gasflowmeters/pressureregulators/pressuregauges4) Ozonegenerator5) Rotaryandturbomolecularpumpsforcellevacuation6) Lightsources–Deuterium(D2)andXenon(Xe)lamps,Laser-DrivenLightSource(LDLS)7) Spectrometerforspectraacquisition

Fig.5.Experimentalsetup

b) Spectrometer

Inoursetupweuseaspectrometerwithtwointernaldispersiveelements(prismandaspecialtypeofdiffractiongratingcalledechelette).Ittransformsthelightfedviaaquartzfiberintothetwo-dimensionalpatternwithspectraldistributionandprojectsituponanimageintensifier(specialquantum-electricaldeviceusedtoamplifylightintensity–microchannelplateMCP)andthenaCCDsensor.Theimageisreadout,digitizedandprocessedintoaspectrumautomatically.

Fig.6.Echellespectrometer

Thespectrometercoversthewavelengthrangefrom212to840nmwithaspectralresolutionofabout0.02nm,butduetosomeconstructiondetailsgapsbegintoappearinthespectrumstartingfromabout590nm.

c) Dataacquisitionandprocessingsoftware

ThespectrometeriscontrolledviatheESAWINpackageinstalledonthebuilt-inPC.Theusercanchangevariousparametersoftheinstrumentdependingonthecurrentexperimentaldemands.

Fig.7.ESAWINmainwindow

Beforeacquiringspectraitisrecommendedtofamiliarizeoneselfwiththesoftwareundersupervision.

Themostimportantsettingsare:

- Gatewidthwhichdefinestheduration(normallywithin20-2000ms)ofthephaseduringwhichtheintensifierisoperationalandtheCCDisaccumulatingsignal;

- AmplificationdefinesthedegreeofsignalintensificationbysettingthevoltageontheMCP(availablerange1200-4000arbitraryunits)

CAUTION!Alwaysstartameasurementwithacombinationoflowerlevelsettings.Highvaluescanresultinoversaturationoftheintensifierandcancauseirreversibledamage!

Asafestartingpointisa20msgatewidthand2000unitsforamplification.Ourexperiencewithavailablelampsandtypicalalignmentoptionsshowsthatgatewidthandamplificationshouldneverexceed200msand2800unitsrespectively.Theresultingspectraaredigitizedandhaveanintensityrangebetween0and65535(0-216)counts.Theregisteredpeakintensitygrowslinearlywithgatewidth.Theincrementintervalforamplificationis100units.

Toproducespectrawithreasonablesignal-to-noiseratio,wehavetoperformaveragingofmultipleindividualspectrarecordedbythespectrometer.Thereareseveralaveragingmodesavailable,butwewillrestrainourselvestothefollowingroutine:thespectrometermeasures10spectraandaveragesthembeforesavingtofile.Normally,weaccumulatea100ofsuchfiles.Thisresultsinthepossibilitytoeffectivelyaverageover1000singleacquisitions.

Fig.8.Measurementsettings.

ToextensivelyprocessandanalyzespectraweusetheOriginProsoftware.Youwillbefamiliarizedwithrequiredfeaturesandroutinesduringthepracticalwork,butfeelfreetodownloadandtryouttheevaluationversionhere:http://www.originlab.com/index.aspx?go=Downloads/OriginEvaluation

YoucaninstallitonyourPCanduseinfullyfunctionaldemomodefor21days.

2. Safetyinstructions

Manyaspectsofthispracticuminvolveeithertheoperationoffragileandsensitiveequipmentorhandlingofpoisonous/flammable/explosiveorotherwisehazardousmaterials.

Intheeventofanyaccidentsoccurringduringyourworkfirstthingyoushoulddoisleavethelabandnotifythepersonnel(althoughyouwillbesupervisedmostofthetimeinthelab).

Ozone:subjecttoexplosivedecomposition(exothermicdecayofO3toO2),irritant,strongoxidizer.Inourexperimentwewilldealwithozoneconcentrationsupto10%vol.Itisthusnecessarytowearsafetymasksduringoperationsofgaspipes,cellfacetsetc.

Ethanol:flammablebothasliquidandasvapor.Usedascoolantcirculatinginthecelljacket.

LiquidNitrogenLN2:acryogenicliquidat−195.79°C(77K),cancausefrostburnsandsuffocation(1literofliquidnitrogenwilloccupyabout650litersasagasonceithasallvaporized).

XenonandDeuteriumlamps,LDLS:sourcesofintenseUVradiation.Itisnotrecommendedtolookdirectlyatthelamp;safetyglassesarerequiredtoavoidretinadamage.

Spectrometer:mightbedamagedwhenwrongsettingsareapplied.Seeabove.

Questions6.

1.Whyisitdesirabletomaintainsub-atmosphericpressureinthecell?

2.Whatmighthappenifthecellcooleddownto203Kisleftwithagasmixtureatapressureof900mbartowarmupto293K?

3. Tasks

Inthisworkyouwillneedtoobtaintheopticaldensityspectra(whichcanbefurtherscaledtotheozonecross-sectionspectra)aroundthe290-350nmwavelengthregion.Itistheregionwherespectralfeaturesaremostnoticeableandcross-sectiontemperaturedependenceismostpronounced.Themeasurementwillbedoneatroomtemperatureofabout296K.

Onthedayofthepracticalworkyouwillfindthelabequipmentin“warmedup”condition.Lightsource,spectrometer,andpumpswillalreadybeturnedon.Youwillalsobefamiliarizedwithothernecessarylabdevices.YouwillbeassistedonperformingsomedataprocessingoperationsinOriginPro.Equipment-criticalanddangeroustasks(spectrometersettingsadjustment,pureozonecollection/samplepreparation)willbeperformedbythesupervisor.

Belowisthetypicalsequenceofactionsnecessarytoobtaintheozoneabsorptionspectrum.

a) Spectraacquisition

• Stabilizationofthelightsourceo Lightsourcehastobeonforatleasthalfanhourpriorto

startingthemeasurements.• CollectionofpureOzone

o Usingthecondensationofozonefromtheozone-oxygenmixtureflowfromtheozonegenerator,asufficientamountofpureozonewillbecollectedinaliquidnitrogen-cooled“coldtrap”.

• Determinationofacquisitionparameterso Establishthelevelofamplificationandgatewidthtoobtain

lampspectrawithreasonablepeakcounts(35000–50000)• Spectraacquisition-emptycell

o Recordasetofspectraofthelightsourcepassingthroughtheevacuatedabsorptioncell.

• Samplepreparation-1o Releaseasmallamountofozonefillingthecellupto3-5mbar

pressure.• Spectraacquisition-lowconcentration(pressure)oftheOzonesample

o Recordasetofspectraofthelightsourcelightpassingthroughthecellfilledwithozonegassampleatlowpressure.

• Samplepreparation-2o Releaseanadditionalamountofozonefillingthecelltoabout

20-25mbarpressure.• Spectraacquisition-highconcentration(pressure)oftheOzonesample

o Recordasetofspectraofthelightsourcelightpassingthroughthecellfilledwithozonegassampleathigherpressure.

b) Dataevaluation

i. ConversionoftheobtainedspectratoASCIIfilesisperformedbyESAWIN.

ii. ImporttoOrigin.UsingOriginPro,importconvertedASCIIfilesintoOriginandperformaveragingofspectraobtainedduringmeasurementsseries.OneoftheproductsoftheaveragingroutineinOriginisthe“StandardDeviationoftheMean”value.Itmightbelaterusedtoestimatetheerrorofthemeasurements.

iii. Calculationoftheopticaldensities.UsingBeer-Lambertlaw,calculateopticaldensitiesfortheobtainedpairsofspectra:“empty”-“lowO3pressure”and“empty”-“highO3pressure”(backgroundshouldbeindividuallyestimatedandsubtractedfromallthreeaveragedsetsofspectra).

iv. Concatenation(usingthe“manual”and“LeastSquareFit”approaches).AfteryougettwoODspectra,performaconcatenation(up-ordownscaling)basedonthedataavailableforbothpiecesinthewavelengthregion,whereODvaluesliewithinthe0.1-1range.Routineswillbeexplainedon-site.

c) Conclusions

AsaresultofthecompletedtasksyouwillpresenttwoODspectra,theresultsoftheirconcatenationfromtwodifferentapproaches,andasetofuncertaintycurvesformeanspectraandcalculatedODs.Majorpartofitwillbeperformedon-site,withremainingprocessingandreportcompilationtobedoneathome.