Quantitative aspects of IR spectroscopy as applied to adsorbed species Edoardo Garrone
Absorption spectroscopy of the atmospheric species 2016Absorption spectroscopy of atmospheric...
Transcript of Absorption spectroscopy of the atmospheric species 2016Absorption spectroscopy of atmospheric...
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Absorptionspectroscopyofatmosphericspecies(ozone)Lab:MolecularSpectroscopy,U4150,restrictedarea,pleasecontactviamailorphonebeforecoming.
Responsiblestaff:VictorGorshelev
Phone:62120,62121,email:[email protected]
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
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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/
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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$689: (1.1).
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Theabsorptionofradiationwithfrequencyf(measuredinHz)occurswhenaparticle(atomormolecule)makesatransitionfromonestatewithenergyEktoanotherstatewithhigherenergyEj:
𝐸; − 𝐸< = ℎ𝑓 (1.2).
HerehisthePlankconstant,whichisdependingontheunitsofenergy,is
h≈6.62610-34[J s]orh≈4.14×10−15[eV s].
TheWavelengthλistypicallymeasuredin[nm](10-9m)or[Å]‘angstrom’(10-10m)intheultravioletandvisiblespectralrangeandin[µk](10-6m)intheinfraredspectralrange.Wavelengthisconnectedwithfrequencybythesimplerelation:
𝑐 = 𝑓 ∙ 𝜆 (1.3).
Here c is the speed of light. Please, note that in spectroscopy sometimes the symbol ‘ν’ is used forwavenumberwhichisinverselyproportionaltothewavelengthandhasunitsof[cm-1].
Example2:
Oxygenhasanabsorptionbandatwavelengthsaround760nm,whichcorrespondstoanenergygapofabout1.63eV,wavenumberof13158cm-1andfrequency394.5THz.
Questions2:
Whydopeopleoftenuse[nm]intheultravioletregionand[µm]intheinfrared?
Molecular spectra, especiallywhen thenumberof atomsexceeds two,havea complex structureandinvolveelectronic,vibrational-rotationalandpurerotationaltransitions.Theenergylevelsaredescribedbyasumofcorrespondingterms:
𝑇/0 + 𝐺3345 + 𝐹3,G67$ =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;
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− 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
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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;
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- 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?
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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;
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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).
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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)
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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.
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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
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b) Spectrometer
Inoursetupweuseaspectrometerwithtwointernaldispersiveelements(prismandaspecialtypeofdiffractiongratingcalledechelette).Ittransformsthelightfedviaaquartzfiberintothetwo-dimensionalpatternwithspectraldistributionandprojectsituponanimageintensifier(specialquantum-electricaldeviceusedtoamplifylightintensity–microchannelplateMCP)andthenaCCDsensor.Theimageisreadout,digitizedandprocessedintoaspectrumautomatically.
Fig.6.Echellespectrometer
Thespectrometercoversthewavelengthrangefrom212to840nmwithaspectralresolutionofabout0.02nm,butduetosomeconstructiondetailsgapsbegintoappearinthespectrumstartingfromabout590nm.
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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.
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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.
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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?
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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.
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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.