Dual-Pulse Laser Induced Breakdown Spectroscopy in the Vacuum Ultraviolet with Ambient Gas

182 Pages·2012·16.73 MB·English

by  Xi Jiang

Dual-Pulse Laser Induced Breakdown
Spectroscopy in the Vacuum Ultraviolet with
Ambient Gas: Spectroscopic Analysis and
Optimization of Limit of Detection of Carbon and
Sulfur in Steel
ATHESISSUBMITTEDFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
PRESENTEDTO:
THESCHOOLOFPHYSICALSCIENCESANDTECHNOLOGY,
FACULTYOFSCIENCEANDHEALTH,
DUBLINCITYUNIVERSITY
AUTHOR
XIJIANG
B.Sc.,M.Sc.
ResearchSupervisors
PROF.EUGENEKENNEDY,PROF.JOHNCOSTELLO
Declaration
I hereby certify that this material, which I now submit for assessment on the pro-
gramme of study leading to the award of Ph.D is entirely my own work, and that
I have exercised reasonable care to ensure that the work is original, and does not
to the best of my knowledge breach any law of copyright, and has not been taken
from the work of others save and to the extent that such work has been cited and
acknowledgedwithinthetextofmywork.
Signed: IDNo.:
Date:
i
Formymother
ii
Acknowledgements
Firstandforemost,sincerethankstoProf. EugeneKennedyandProf. JohnCostello,
both as my supervisors for giving me the opportunity to study and work in DCU
andforguidingmywork.
Many thanks to postdocs Paddy Hayden, Pat Yeates, Patrick Casey for kind
help and useful chat, and to senior/previous members of the physics group in
DCU:EoinO’Leary,Mohamed,John,Caroline,Phil,Padraig,Conor,Mairead,Patrick,
Brian,Ricky,Mahua.
ThankstoallthecurrentcolleaguesinDCUphysicsdepartment: Colm,Conor,
Ruth,Mossy,Eanna,Seamus,Nicky,Hooiling,Catherine,Jack,Joe,Darrick,Justin,
Yang,Barry,Rajani,Aurora,Barbaraandmanyothers.
Thanks to Ricarda and Nadia who were two short term visiting students here,
thankyoubothforco-workinginthelabandbringingjoytothegroup!
Thanks to the school secretaries Shiela and Lisa for taking care of the paper
works, and to Daz, Ray Murphy, Pat Wogan, Alan Hughes for technical support.
ThanksalsototheentirestaffandfacultyofphysicsschoolandNCPST.
Thanks also to all my friends including my old friends in China, my college
mates(mostly)intheUS,myChinesefriendsinIrelandandthelocalIrishfriends,
as well as friends in Europe and other places around the world. Thank you for
makingmylifesowonderful.
Finally, my most sincere thanks to my family, thank you for supporting my
adventuresofarbothspirituallyandfinancially.
iii
Abstract
Laser Induced Breakdown Spectroscopy (LIBS) in the vacuum ultraviolet (VUV)
spectralregionwasappliedtostandardsteelsamplesindual-pulseexcitationmode
with an ambient gas. Two lasers were employed in collinear geometry, one as
an ablation laser (Spectron: 200mJ/15ns) and the other a reheating laser (Surelite:
665mJ/6ns).
Adual-pulseschemewasappliedtothetraditionalsingle-pulseLIBSandledto
asignificantenhancement,increasingboththesignaltobackgroundratio(SBR)and
concomitantly the limit of detection (LOD). Three types of gases, nitrogen, argon
andheliumwereinvestigatedindividuallyasambientenvironment. Thevariation
insignalgainwithambientgaspressurewasmeasured.
Thefeasibilitystudyofdual-pulse(DP-)LIBS)inthedeepVUVspectralregion
wascarriedoutbyoptimizinganumberofparametersforlimit-of-detection(LOD)
calibration. The pulse energy choices of laser beam, the signal recording position
for the space-resolved detection system, the CCD exposure time and the number
of laser shots accumulated for a spectrum were among the first few parameters
optimizedforthecurrentLIBSsystem.
Thedependenceofemissionintensityonfocusingconditionswasinvestigated
for both single-pulse (SP) and dual-pulse (DP) mode. In DP mode, the lens-to-
targetdistanceofthereheatingpulsewasvariedwhiletheablationpulsewasheld
at a fixed focusing condition. The optimal focusing for signal enhancement was
foundtobeabout10mmunderthesamplesurfaceforsingle-pulselaserbeamand
5-10mmforthereheatingpulseindual-pulsemode.
The effect of inter-pulse delay on the emission intensity were studied in vac-
uumandinambientgasbackgroundinthedual-pulseconfiguration. Anintensity
peakatabout100nsandanintensityrevivalintheµsrangewereobservedonthe
intensity vs. inter-pulse delay curves. The optimal inter-pulse delay was consid-
eredtobe100ns. Theµsrangeintensityplateauledtoadiscussionontheplasma
expansiondynamicsindual-pulsemode.
Finallythelimitofdetection(LOD)ofCandSinsteelwasimprovedbyafactor
of2ormoreoversinglepulseLIBSwiththecombinationofoptimizedexperimen-
tal parameters. The initial measurements and results suggest that DP-LIBS in an
ambient gas environment for VUV spectroscopy is practicable and brings with it
evenfurtherimprovementoftheLIBSperformanceintheVUVregime.
iv
Thesis Organization
Thisthesisisdividedinto10chaptersasfollows:
Chapter1-Laserproducedplasmasinspectroscopy
Thefirstpartofthethesisprovidessomeofthetheoreticalaspectsoflaserproduced
plasmas including a fundamental overview of laser produced plasmas and the
atomic processes involved. The principles for using laser plasmas as light sources
forquantitativeanalyticalspectroscopyisexplainedattheendofthischapter.
Chapter2-ThefundamentalsofLIBS
This chapter is a general introduction to Laser Induced Breakdown Spectroscopy
(LIBS)andcontainsaliteraturereviewofLIBSdevelopment. Aparticularemphasis
isplacedonthebasicsofstudiesperformedondeterminingconcentrationsoflight
elementsinsteelintermsofthelimitofdetection(LOD).
Chapter3-Theexperimentalsetup
Thischaptercontainsadescriptionoftheequipmentusedintheexperimentsper-
formed for the present work including details of the laser system employed, the
spectrometers,operationalcharacteristicsandthemethodbywhichradiationfrom
thelaserplasmawascaptured.
Chapter4-ThebasicLIBSanalysis
ThischapterdemonstrateshowLIBSisusedfordirectdetectionoftraceelementsin
steels. Anumberofhardwareparametersincludingthelaserpulseenergychoices,
observing position for space-resolved detection system, CCD exposure time and
number of shots accumulated for a spectrum are discussed and optimized. Fol-
lowingthebasicsettings,thecomputingprocessesforlimitofdetection(LOD)cal-
ibration including wavelength calibration and the data handling process are also
introduced. ThecompleteprocedureofspectroscopicanalysisbyabasicLIBSsys-
temissummarizedattheendofthischapter.
Chapter5-Theimprovementtechniques
This chapter provides a comprehensive study of dual-pulse LIBS in ambient gas
environments. Firstly the dual-pulse or DP-LIBS and the ambient gases are intro-
ducedtothebasicLIBSsetup. ThentheDP-LIBSoperationandtheVUVemission
inseveralambientgasesarestudied. Aseriesofspectralcomparisonsareprovided.
Emphasis is placed on the combination of dual-pulse and ambient gas techniques
asafurtherimprovementtotheconventionalLIBSsystem.
Chapter6-Theeffectsoflaserbeamdefocusing
The laser beam defocusing is one of the parameters which can affect the signal
intensitysignificantlyinbothsingle-anddual-pulseLIBS.Thischapterprovidesa
v
detailedinvestigationoftheeffectoflaserbeamdefocusing. Insingle-pulsemode,
thedefocusingeffectmeanstheinfluenceoftherelativedistancebetweenthefocal
point and the sample surface on the plasma emission behaviors. In dual-pulse
mode,thedefocusingeffectreferstothedefocusingeffectofthesecond/reheating
laserbeam. Theobservationsanddiscussionsinbothsingle-anddual-pulsemode
areprovidedinthischapter.
Chapter7-Theeffectofinter-pulsedelay
Anotherworth-notingeffectintheoptimizationprocessfordual-pulseLIBSisthe
effect of inter-pulse delay. At certain inter-pulse delay times, the emission inten-
sity was dramatically enhanced. The optimal inter-pulse delay of 100 ns has been
achieved during the optimization process in chapter 5. When the inter-pulse de-
laywasextended tothe µsrange, an intensityrevivalwasfound inthe caseswith
ambientgas. Effortshavebeenmadetoinvestigatetheseinterestingobservations.
Questionsareaskedandaddressedwithinthescopeofthecurrentwork.
Chapter8-Emissionenhancementmechanismsindual-pulseLIBS
Two emission enhancements have been clearly shown and proved in the previ-
ouschapter7andtheuniquecharacteristics(i.e. ambientgasandpressuredepen-
dence) of the second enhancement indicate a new enhancement mechanism dis-
tinct from the first one. Further investigations regarding the enhancement mech-
anismsindual-pulseLIBS,especiallywithlow-pressureambientgases,havebeen
performed. A process of “phased enhancement” is proposed along with a brief
literaturesurveyseekingmoreevidencesonmulti-phaseenhancement.
Chapter9-Limitofdetectioncalculationandcomparison
Having optimized all possible parameters in dual-pulse LIBS in ambient gas, the
limitof detection (LOD) of carbon (C) and sulphur (S) in steel areextracted under
optimizedconditions. Thecomparisonsarealsoprovidedwhichincludethegood-
ness of LOD calibration curves and the final LOD values obtained under different
conditions. Finally the best LOD results achieved under the optimal conditions in
thecurrentworkarecomparedtothoseintheliterature.
Chapter10-Conclusionsandfuturework
This final chapter provides a summary of the experimental work and the general
conclusions formed as a result of the above research. Some future interests are
outlinedinthecontextofthecurrentresearchdirection.
Tohelpthereaderwithaclearviewofthethesis,thethesisstructureissumma-
rizedinfigure1.
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Chapter 1
Laser plasmas
Theories of Laser induced plasma
s
n
o and
cti Chapter 2 introduction to LIBS
u
d LIBS introduction
o
r
t
n
I
Chapter 3
Experimental
Experimental
Chapter 4
Basic LIBS Basic LIBS procedure
and
dual-pulse (DP) LIBS in ambient gas(es)
Chapter 5
Improvements
Chapter 6
s
t Defocusing
ul Further discussions of some topics on LIBS
s
Re including
Chapter 7 Laser beam defocusing;
Inter-pulse delay
Inter-pulse delay;
Enhancement mechanisms of DP-LIBS
Chapter 8
Mechanism
Chapter 9
Final calibration of limit of detection (LOD)
L.O.D.
-
u
cls Chapter 10 Summary
nn
oo Conclusions
Csi
Figure1: Outlineofthethesis.
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Contents
1 IntroductiontoLaserPlasmasandVacuumUltravioletSpectroscopy 5
1.1 Plasmaformationandpost-breakdownphenomenaonsolidtargets . 6
1.2 Atomicprocessesinlaserinducedplasmas . . . . . . . . . . . . . . . 7
1.2.1 Free-freetransitions . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.2 Free-boundtransitions . . . . . . . . . . . . . . . . . . . . . . . 8
1.2.3 Bound-boundtransitions . . . . . . . . . . . . . . . . . . . . . 8
1.3 Generalcharacteristicsofplasmas . . . . . . . . . . . . . . . . . . . . 9
1.3.1 Debyesheathandplasmafrequency . . . . . . . . . . . . . . . 9
1.3.2 Thermodynamicequilibriuminaplasma . . . . . . . . . . . . 11
1.4 QuantitativespectroscopyintheVUV . . . . . . . . . . . . . . . . . . 12
1.5 Analyticalfiguresofmerit . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2 IntroductiontoLaserInducedBreakdownSpectroscopy 18
2.1 AnoverviewofLIBS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.1 LIBSfundamentals . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.1.2 BriefoutlineofLIBSinstrumentation . . . . . . . . . . . . . . 20
2.1.3 LIBSApplications . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2 LiteraturesurveyofLIBS . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.1 AbriefhistoryofLIBS . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.2 LIBSinrecentyears. . . . . . . . . . . . . . . . . . . . . . . . . 24
2.3 Aimsandobjectivesofthepresentwork . . . . . . . . . . . . . . . . . 26
2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3 EquipmentalSystem 28
3.1 Thelasersystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2 Targetchamberandtargets . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3 GCAandthespectrometer . . . . . . . . . . . . . . . . . . . . . . . . . 35
1
3.4 TheRowlandcircleandoff-Rowlandmount . . . . . . . . . . . . . . 37
3.5 Resolvingpowerofthespectrometrysystem . . . . . . . . . . . . . . 40
3.6 Signaldetectionandrecordingsystem . . . . . . . . . . . . . . . . . . 42
3.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4 BasicLIBS:TypicalLIBSAnalysisProcess 45
4.1 Choiceoflaserpulseenergies . . . . . . . . . . . . . . . . . . . . . . . 46
4.2 Time-integratedspace-resolvedLIBSsystem . . . . . . . . . . . . . . 47
4.3 Theoptimalobservingposition . . . . . . . . . . . . . . . . . . . . . . 49
4.4 CCDexposuretimeandnumberofshotsaccumulated . . . . . . . . 51
4.5 Wavelengthcalibration . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.6 Datahandling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.7 LODcalculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5 Dual-pulseLIBSintheVUVinAmbientGases: PreliminaryResults 61
5.1 IntroductionoftwomajortechnicalimprovementstobasicLIBS . . . 62
5.2 Experimentalsettingsfordual-pulseLIBSinambientgases . . . . . . 63
5.3 Dual-pulseLIBSoperation . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.3.1 Inter-pulsedelaygenerator . . . . . . . . . . . . . . . . . . . . 65
5.3.2 Dual-pulseLIBSspectrum . . . . . . . . . . . . . . . . . . . . . 66
5.3.3 Optimalinter-pulsedelaytime . . . . . . . . . . . . . . . . . . 69
5.4 Ambientgases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.4.1 Emissioninambientgases . . . . . . . . . . . . . . . . . . . . . 72
5.4.2 VUVlighttransmissioninambientgases . . . . . . . . . . . . 77
5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
6 EffectsofLaserBeamDefocusing 83
6.1 Defocusingeffectinsingle-pulsemode . . . . . . . . . . . . . . . . . . 84
6.1.1 Observationsofthelaserbeamdefocusingeffect . . . . . . . . 84
6.1.2 Discussionofdefocusingeffectinsingle-pulsemode . . . . . 85
6.1.3 Summary of on-focus, before- and beyond-focus defocusing
conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
6.2 Defocusingeffectindual-pulsemode . . . . . . . . . . . . . . . . . . 89
6.2.1 Observationsofthereheatingpulsedefocusingeffect . . . . . 89
6.2.2 Discussionsofthereheatingpulsedefocusingeffect . . . . . . 90
6.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
2