Cavity Ring-Down Spectroscopy: Techniques and Applications

336 Pages·2009·24.065 MB·English

Cavity Ring-Down Spectroscopy
Cavity Ring-Down Spectroscopy: Techniques and Applications Edited by Giel Berden and Richard Engeln
© 2009 Blackwell Publishing Ltd. ISBN 978-1-405-17688-0
Cavity Ring-Down Spectroscopy
Techniques and Applications
Edited by
GIEL BERDEN
FOM Institute for Plasma Physics ‘Rijnhuizen’
Nieuwegein, The Netherlands
RICHARD ENGELN
Department of Applied Physics, Eindhoven University of Technology
Eindhoven, The Netherlands
Thiseditionfirstpublished2009
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LibraryofCongressCataloging-in-PublicationData
Cavityring-downspectroscopy:techniquesandapplications/editedbyGielBerden,RichardEngeln.
p.cm.
Includesbibliographicalreferencesandindex.
ISBN978-1-4051-7688-0
1.Cavity-ringdownspectroscopy.I.Berden,Giel.II.Engeln,Richard.
QD96.A2C3842009
543’.59–dc22
2009016230
AcataloguerecordforthisbookisavailablefromtheBritishLibrary.
Typesetin10/12ptTimesbyThomsonDigital,Noida,India.
PrintedandboundinGreatBritainbyCPIAntonyRowe,Chippenham,Wiltshire
CoverphotocourtesyofNandoHarmsen(www.nandoonline.com).
Contents
Preface xi
Contributors xv
Glossary xvii
1 An Introduction to Cavity Ring-Down Spectroscopy 1
Kevin K. Lehmann, Giel Berden and Richard Engeln
1.1 Introduction 1
1.2 Direct Absorption Spectroscopy 3
1.3 Basic Cavity Ring-Down Spectroscopy Set-Up 6
1.4 A More Refined Picture 10
1.5 Fitting of Cavity Ring-Down Transients 14
1.6 A Few Examples 16
1.7 Going Beyond the Standard Pulsed CRDS Experiment 19
1.8 Summary 23
References 24
2 Cavity Enhanced Techniques Using Continuous Wave Lasers 27
J.H. van Helden, R. Peverall and G.A.D. Ritchie
2.1 Introduction 27
2.2 Properties of Optical Cavities and CW Lasers Relevant
to Cavity Enhanced Spectroscopy 28
2.2.1 Properties of Optical Cavities 28
2.2.2 Laser Bandwidth, Noise, and Cavity Interactions 32
2.3 Experimental Methods for CW Laser Cavity Enhanced
Spectroscopy 34
2.3.1 CW-Cavity Ring-Down Spectroscopy (CW-CRDS) 34
2.3.2 Cavity Enhanced Absorption Spectroscopy (CEAS/ICOS) 39
2.3.3 Phase Shift Cavity Ring-Down Spectroscopy (PSCRDS) 42
2.4 Spectroscopy with Resonant Cavities 47
2.4.1 Frequency Locked CW-CRDS 47
2.4.2 Methods for Locking Cavities and Lasers 48
2.4.3 Optical Feedback CRDS and CEAS (OF-CRDS/OF-CEAS) 50
2.4.4 Other Locked-Cavity Techniques 53
2.4.5 Optical Heterodyne Cavity Ring-Down Spectroscopy 54
vi Contents
2.5 Summary 54
References 55
3 Broadband Cavity Ring-Down Spectroscopy 57
Stephen Ball and Roderic Jones
3.1 Introduction 57
3.2 The Time and Wavelength Evolution of a Single Ring-Down Event 58
3.3 Two-Dimensional Techniques: Resolving Broadband Cavity Output
in Time and Wavelength 61
3.4 One-Dimensional Techniques: Time or Wavelength 64
3.4.1 Wavelength Selection Methods 64
3.4.2 Fourier Transform Methods 65
3.4.3 Phase Shift Cavity Methods 66
3.4.4 Broadband Cavity Enhanced Absorption Spectroscopy 68
3.5 How to Extract Quantitative Information from Broadband
Spectra 70
3.5.1 Mirror Reflectivity Considerations 70
3.5.2 Differential Optical Absorption Spectroscopy 71
3.5.3 Multi-Exponential Decays 74
3.6 Optimising the Sensitivity of a Broadband Measurement 79
3.7 Applications of Broadband Cavity Methods 83
3.7.1 Atmospheric Measurements 83
3.7.2 Liquid Phase Spectroscopy 85
References 87
4 Cavity Ring-Down Spectroscopy in Analytical Chemistry 89
L. van der Sneppen, C. Gooijer, W. Ubachs and F. Ariese
4.1 Introduction 89
4.1.1 Absorbance Detection in Liquid Flow Systems 89
4.1.2 Requirements for Detection Cells for Analytical Purposes 91
4.2 Condensed Media CRDS 92
4.2.1 Studying Solid-Phase Samples with CRDS 92
4.2.2 Studying Liquid-Phase Samples With CRDS 93
4.2.3 Incoherent Broad-Band Cavity-Enhanced Absorption
Spectroscopy: IBBCEAS 96
4.2.4 CRDS Absorption Detection in Liquid Chromatography 97
4.3 Evanescent-Wave CRDS 102
4.3.1 EW-CRDS Using Monolithic Resonators 102
4.3.2 Applications of EW-CRDS to Condensed Media 104
4.4 Future Trends and Perspectives 107
References 108
5 Cavity Ring-Down Spectroscopy Using Waveguides 113
Hans-Peter Loock
5.1 Introduction 113
Contents vii
5.2 The Basic Experiments 114
5.2.1 The Fiber-Loop Ring-Down Experiment 114
5.2.2 The FBG Cavity Ring-Down Experiment 116
5.3 Optics and Instrumentation 117
5.3.1 Waveguide Optics 117
5.3.2 Waveguide Materials 121
5.3.3 Fiber-Optic Components 124
5.4 Review of Waveguide CRD Literature 127
5.4.1 Measurement of Optical Loss of Connectors and Fibers 127
5.4.2 Mechanical Sensing with Waveguide CRD 127
5.4.3 Interfaces to Microfluidic Devices 132
5.4.4 Lensed Fiber Ends 132
5.4.5 Amplified Fiber CRD 134
5.4.6 Evanescent Field Absorption Spectroscopy
Using Waveguide CRD 136
5.4.7 Choice of Wavelength for Absorption Detection 137
5.4.8 Refractive Index Sensing Using LPGs in Fiber Cavities 138
5.5 Conclusion and Outlook 140
Acknowledgements 140
References 141
6 Cavity Ring-Down Spectroscopy of Molecular Transients
of Astrophysical Interest 145
Harold Linnartz
6.1 Introduction 145
6.1.1 Astrochemical Setting 145
6.1.2 Plasma Techniques: Cell Discharges and Plasma Expansions 147
6.1.3 Sensitive and Selective Detection Schemes 148
6.2 Experimental 149
6.2.1 High-Pressure Pulsed Planar Plasma Source 149
6.2.2 Pulsed Cavity Ring Down Spectroscopy – the Detection Scheme 151
6.2.3 Pulsed Cavity Ring-Down Spectroscopy – an Example 151
6.2.4 CW Cavity Ring-Down Spectroscopy – the Detection Scheme 155
6.2.5 CW Cavity Ring-Down Spectroscopy – an Example 156
6.2.6 Frequency Plasma Double Modulation Spectroscopy – the
Detection Scheme 157
6.2.7 Frequency Plasma Double Modulation Spectroscopy –
an Example 159
6.2.8 CW Electron Impact Source 160
6.2.9 Production Modulation Spectroscopy – Detection
Scheme and Example 162
6.2.10 CW Cavity Ring Down Spectroscopy – Detection
Scheme and Example 164
6.3 Astronomical Considerations 167
6.4 Results 168
viii Contents
6.5 Outlook 172
Acknowledgements 173
References 173
7 Applications of Cavity Ring-Down Spectroscopy in Atmospheric
Chemistry 181
Gus Hancock and Andrew J. Orr-Ewing
7.1 Brief Overview 181
7.2 Measurement of Trace Atmospheric Species by CRDS 184
7.2.1 An Example of a CRDS Apparatus for Atmospheric
Composition Measurements 185
7.2.2 Open-Path CRDS Measurements of Atmospheric Composition 188
7.2.3 Diode Laser CRDS Detection of Atmospheric VOCs
and Preconcentration of Air Samples 190
7.2.4 Considerations for the Sensitivity of Atmospheric
CRDS Instruments 192
7.3 Laboratory-Based Studies of Atmospheric Interest 193
7.3.1 Rate Constants 193
7.3.2 Quantum Yields 197
7.3.3 Absorption Cross-Sections 199
7.4 Optical Properties of Atmospheric Aerosol Particles 201
7.4.1 Light Scattering by Atmospheric Aerosol Particles 202
7.4.2 Effect of Aerosols on Radiative Forcing of the Atmosphere
and Climate Change 202
7.4.3 Some Fundamental Principles of CRDS of Aerosols 205
7.5 Future Developments 207
References 208
8 Cavity Ring-Down Spectroscopy for Medical Applications 213
Manfred Mu¨rtz and Peter Hering
8.1 Introduction 213
8.2 Trace Gases in Medicine and Biology 214
8.2.1 Composition of Exhaled Human Breath 214
8.2.2 Other Biological Sources of Volatile Markers 217
8.3 Instrumentation for Laser Analytics of Breath and Other
Biological Gas Samples 218
8.3.1 Sample Collection and Preparation 218
8.3.2 Laser Spectroscopic Approach 220
8.3.3 Comparison with Conventional Techniques 223
8.4 Applications to Life Sciences 223
8.4.1 Monitoring Exhaled Ethane 223
8.4.2 Monitoring Exhaled CO 228
8.4.3 Analysis of Blood NO 229
8.5 Conclusion and Perspectives 232
Acknowledgements 233
References 233
Contents ix
9 Studies into the Growth Mechanism of a-Si:H Using in situ
Cavity Ring-Down Techniques 237
M.C.M. van de Sanden, I.M.P. Aarts, J.P.M. Hoefnagels, B. Hoex,
R. Engeln and W.M.M. Kessels
9.1 Introduction 237
9.2 Gas Phase CRDS on SiH Radicals 240
x
9.2.1 Production and Loss Processes of Radicals Under
Plasma Conditions 240
9.2.2 Experimental Set-Up for Plasma Deposition of a-Si:H
and CRDS Measurements 242
9.2.3 Cavity Ring-Down Measurements During ETP
Deposition of a-Si:H 246
9.3 Thin Film CRDS on Dangling Bonds in a-Si:H Films (ex situ) 252
9.3.1 General Considerations 254
9.3.2 Measuring ex-situ Dangling Bonds in a-Si:H Films 260
9.4 Evanescent Wave CRDS on Dangling Bonds During a-Si:H
Film Growth 263
9.4.1 The Evanescent Wave CRDS Set-Up 264
9.4.2 Measuring Dangling Bonds During a-Si:H Film Growth 265
Acknowledgements 268
References 269
10 Cavity Ring-Down Spectroscopy for Combustion Studies 273
Xavier Mercier and Pascale Desgroux
10.1 Introduction 273
10.2 General Description of Cavity Ring-Down
Spectroscopy in Flames 277
10.3 Experimental Set-Up 280
10.3.1 Burners and Flames 280
10.3.2 Laser Sources 282
10.3.3 Ring-Down Cavity for Low Pressure Flames 283
10.3.4 Typical Designs of Cavities for Flame Experiments 283
10.3.5 Mode Matching under Flame Conditions 284
10.3.6 Cavity Alignment 286
10.3.7 Detection Scheme 288
10.4 Quantitative Concentration Measurements by CRD
Spectroscopy in Flames 289
10.4.1 Concentration Determination for Diatomic Species 290
10.4.2 Precautions 290
10.4.3 Absolute Concentration Measurements
for an Ideal Case 291
10.4.4 A Numerical Example: CH Absorption 294
10.5 Concentration Profile Determination 295
10.5.1 General Method 295
10.5.2 Case of a Nonhomogeneous Concentration Field 296
x Contents
10.6 Specific Difficulties in Combustion Studies 300
10.6.1 CRD Measurements Under Strong Absorption Conditions 300
10.6.2 Temperature and Thermal Gradient Effects 302
10.6.3 Stable Species 302
10.7 Case of Particles: Soot Volume Fraction Determination 303
10.8 Conclusion and Prospects 307
Acknowledgements 307
References 307
Index 313
Preface
Directabsorptionspectroscopyisprobablythemostwidespreadanalyticaltechniqueusedto
studyatomsandmoleculesinthegasandcondensedphases.Itprovidesanabsolutevaluefor
the frequency dependent absorption coefficient, which is the product of the frequency-
dependent absorption cross-section and the number density. Ina direct absorption experi-
ment,theintensityofthelighttransmittedthroughasampleismeasured.Theattenuationof
thelightfollowstheBeer–Lambertlawsothattheintensitydecaysexponentiallyasafunction
of absorption coefficient and path length through the sample. The sensitivity of direct
absorptiontechniquesisoftenlimitedbyintensityfluctuationsofthelightsource.Atbest,a
relativeintensitychangeoftheorderof0.001canbemeasured.Manyschemeshavebeen
developedtoincreasethesensitivity,suchasmulti-passgeometriestoincreasetheabsorption
pathlengthsandmodulationtechniquestominimisetheeffectsofnoiseinthemeasurement
system.Alternatively,indirectabsorptiontechniquesthatarebaseduponthemeasurementof
aneffectinducedbytheabsorptionoflight,ratherthantheabsorptionoflightitself,canbe
used.Althoughtheseindirecttechniquesaremanyordersofmagnitudemoresensitivethan
directabsorption,thedrawbackisthatthesetechniquesarenotself-calibrating;thatis,theydo
notprovidetheabsorptioncoefficientonanabsolutescale.
Cavityring-downspectroscopy(CRDS)isadirectabsorptiontechniquewithasignifi-
cantly higher sensitivity than conventional absorption spectroscopy. It is based on the
measurement of the rate of absorption, rather than the magnitude of absorption, of light
circulatinginanopticalcavity.LetusconsiderasimpleCRDSexperiment.Ashortlaser
pulseiscoupledintoanopticalcavityconsistingoftwohighlyreflectivemirrors.Thelaser
pulseisreflectedbackandforthinsidethecavity,theso-called‘ring-downcavity’.Every
timethepulseisreflectedbyoneofthemirrors,asmallfractionofthelightistransmitted
through that mirror. A fast detector measures the intensity of the transmitted light as a
functionoftime,andtheresultwillbeanexponentiallydecayingintensity.Thedecaytime
is called the cavity ring-down time, and is inversely proportional to all losses inside the
cavity.Thus,bymeasuringthedecaytime–insteadofthetotalintensity–afterthecavity,
therateofabsorptionisdetermined,directlyprovidingthelossesonanabsolutescale.Inan
empty cavity, the losses are only determined by the reflectivity of the cavity mirrors.
Insertinganabsorbingsampleinsidethecavityleadstoalargercavityloss,andthereforeto
a shorter ring-down time.
The sensitivity enhancement of the CRDS technique arises from two effects. The
effectiveabsorption path length, which dependsonthe reflectivityofthe cavitymirrors,
can bevery long (up to several kilometres) while the sample volume can be kept rather
small. Additionally, since the absorption is determined from the time behaviour of the