FOURIER TRANSFORM INFRARED A Constantly Evolving Technology SEAN JOHNSTON M.SCB.SC. Instrument Manager Laser Monitoring Systems Ltd, Humberside, UK ERRATA Johnston: FOURIER TRANSFORM INFRARED 1. Back cover, 2nd line from bottom: chartered pharmacist should read chartered physicist 2. Back cover, 16th line from bottom: Flegett should read Fellgett 3. P.340 (index), second-to-last entry: ZDP should read ZPD ELLIS HORWOOD NEW YORK LONDON TORONTO SYDNEY TOKYO SINGAPORE
Table of Contents PREFACE 17 1 INTRODUCTION 21 PARTI: CLASSICAL TIMES 2 ORIGINS 27 2.1 The Status in 1800 27 2.1.1 Light waves vs. particles 28 2.2 The discovery of interference 28 2.3 Growth of the undulatory theory 29 2.3.1 The interference of light 29 2.3.2 Polarization 30 2.3.3 Research into interference phenomena 31 2.4 Harmonie analysis 31 2.5 The beginnings of optical spectroscopy 32 2.5.1 Invention of the spectroscope 34 2.5.2 Emission spectra 37 2.5.3 Absorption spectra 37 2.5.4 Molecular spectra 38 References 38 3 THE BIRTH OF INTERFERENTIAL SPECTROSCOPY 40 3.1 The interferometer 40 3.2 The interferogram 43 3.3 The Fourier transform 46 3.4 The harmonic analyser 46 3.5 Experimental limitations 49 3.6 Infrared and far infrared 51
6 Table of contents 3.7 The measurement of infrared radiation 51 3.8 Interferential spectroscopy in the infrared 52 References 55 PART II: DARKAGES 4 THE DECLINE OF INTERFERENTIAL SPECTROSCOPY 59 4.1 Spreading the word 59 4.2 Ambiguities 60 4.3 Restriction to emission spectra 60 4.4 The missing link 61 4.5 Final decline 63 References 64 5 COMPETING TECHNOLOGIES 66 5.1 Spectral units 66 5.2 Spectral ränge 67 5.3 Spectral resolution 67 5.4 Multiple-beam spectroscopic instruments 67 5.4.1 The echelon spectroscope 67 5.4.2 The Fabry-Perot etalon 69 5.4.3 The Lummer-Gehrcke plate 70 5.5 Dispersive spectroscopic instruments 72 5.5.1 Prism spectrometers 72 5.5.2 The diffraction grating 73 5.6 A typical spectroscopic implementation in the late 1920s....74 References 76 6 COMMERCIAL DISPERSIVE SPECTROMETERS 77 6.1 Improvements to classical detection methods 77 6.1.1 The Pfund resonance radiometer 77 6.1.2 The Firestone amplifier 77 6.1.3 Electronic amplifiers 78 6.2 Recording spectrophotometers 79 6.3 Servomechanisms 81 6.4 Fourier transformation before Computers 82 6.5 The growth of commercial spectroscopy 83 6.6 Double beam vs. single beam instruments 83 6.6.1 Optical null instruments 84 6.6.2 Energy control 85 6.6.3 Electronic null instruments 85 6.6.4 Recording Systems 86
Table of contents 7 6.6.5 The dispersive infrared laboratory 86 References 87 PART III: RENAISSANCE 7 POST-WAR DEVELOPMENTS 91 7.1 Computer development 91 7.1.1 Optical processing 91 7.2 Information theory 92 7.3 Instrument science 92 7.4 The multiplex advantage 93 7.5 The Jacquinot advantage 95 7.6 Other contributions 96 References 97 8 EXPERIMENTAL INTERFEROMETRIC SPECTROSCOPY....98 8.1 Fellgett's demonstration 98 8.2 The Johns Hopkins group 99 8.3 Lawrence Mertz at Baird Associates Ine 101 8.4 The road to transformation 101 8.5 Jacquinot's group 103 8.6 The 1957 Bellevue Conference 103 8.7 Connes' thesis 104 8.8 Work at NPL 106 8.8.1 Michelson vs. lamellar interferometers 106 8.8.2 Early NPL instruments 107 8.8.3 A near infrared FTIR 107 8.8.4 The compensator plate 107 8.8.5 An evacuable Michelson interferometer 110 8.9 Moire position measurement 110 8.10 Advances in detectors 112 8.10.1 The Golay cell 112 8.10.2 The photoelectric infrared detectors 113 8.10.3 Photomultiplier tubes 113 8.11 The state-of-the-art: the Connes' planetary spectra 114 References 115 9 WORKING OUT THE DETAILS 117 9.1 Apodization 118 9.2 Digital sampling 120 9.3 Surface flatness vs. wavelength 120 9.4 The effect of beam divergence in an interferometer 121
10 Table of contents 13.5.2 LVDTs 204 13.5.3 Fringe monitoring 204 13.6 Associated spectrometer optics 208 13.6.1 Jacquinot stop 208 13.6.2 Source and source optics 209 13.6.3 Sample optics 212 13.6.4 Detectors 213 13.6.5 Detector optics 214 References 217 14 THE OPTIMIZATION OF FTIR 219 14.1 Interferometer efficiency 219 14.2 Dynamic ränge 223 14.3 Amplifier linearity 224 14.4 Filtering 225 14.5 Beamsplitter design 226 14.5.1 Wavefront distortion 226 14.5.2 Spectral band 227 14.5.3 Polarization effects 228 References 229 15 FTIR SINCE 1980 230 15.1 Analect 231 15.2 Beckman 232 15.3 Block Engineering 232 15.4 Bomem 232 15.5 Bran+Luebbe 234 15.6 Bruker 234 15.7 Chelsea Instruments 235 15.8 Digilab 235 15.9 Hewlett-Packard 235 15.10 Hitachi 236 15.11 IBM Instruments 236 15.12 Idealab 236 15.13 Janos 236 15.14 Jasco 236 15.15 Jeol 236 15.16 Kayser-Threde 237 15.17 Lloyd Instruments 237 15.18 Mattson 238
Table of contents 11 15.19 Midac 239 15.20 Nicolet 239 15.21 Perkin-Elmer 242 15.22 Philips Analytical 244 15.23 Specac 245 15.24 Soviet instruments 246 15.25 Epilogue: companies no longer manufacturing FTIRs....246 References 246 16 FTIR IN SPACE 248 16.1 Ground-based astronomy 248 16.2 Airborne Fourier spectrometers 249 16.2.1 JPL 249 16.2.2 University of Arizona 250 16.2.3 Other airborne instruments 252 16.3 Space-borne interferometers 252 16.3.1 Block Engineering 252 16.3.2 Nimbus satellites 252 16.3.3 EXCEDE experiment 255 16.3.4 Mariner Mars probe 255 16.3.5 Voyager space probe 256 16.3.6 Cosmic Background Explorer 257 16.3.7 Tropospheric emission spectrometer (TES) 257 16.3.8 Soviet space-borne interferometers 258 16.3.9 Manned Space flight 258 References 258 17 BEYOND FTIR 260 17.1 Double-beam FTIR 260 17.1.1 Double-beam configurations and applications....261 17.1.2 Technical advantages of double-beam configurations 268 17.1.3 Comparison with dispersive double-beam instruments 270 17.1.4 Factors affecting Performance 271 17.1.5 Summary 274 17.2 Asymmetrie FTIR 275 17.3 Field-widened interferometers 278 17.3.1 Principle of field-widening 278 17.3.2 Practical instruments 280 17.3.3 Imaging interferometers 283 17.4 Polarizing interferometers 284
12 Table of Contents 17.4.1 Wire-grid polarizers 284 17.4.2 Mertz's polarizing interferometer 285 17.4.3 Martin and Puplett design 286 17.4.4 Polarizing interferometer for dichroism measurements 288 17.5 FT spectrometry in the ultraviolet 289 17.5.1 Optical quality 289 17.5.2 Scanning accuracy 289 17.5.3 Alignment 289 17.5.4 Practical instruments 290 17.5.5 Types of noise and their effect on the multiplex advantage 291 17.5.6 The Jacquinot advantage in FT-UV 293 17.6 Gas chromatography and IR 294 17.7 Thermogravimetric analysis and IR 299 17.8 Raman spectroscopy 299 17.9 Sampling techniques 301 17.9.1 Infrared microsampling 301 17.9.2 Diffuse reflectance 302 17.9.3 Attenuated total reflectance 303 17.9.4 Photo-acoustic spectroscopy 304 17.9.5 Long-path gas cells 306 17.9.6 Fibre optics interfaces 308 References 310 18 THE SHAPE OF THINGS TO COME 312 18.1 Changing markets 312 18.2 Advances in technology 313 18.3 Data analysis 313 18.3.1 Neural nets 313 18.3.2 Expert Systems 314 18.3.3 Communication 314 18.4 Interferometer design 315 18.4.1 FTIR with no moving parts 315 18.4.2 FTIR within an optical fibre 317 18.5 Advances in sampling techniques 317 18.6 The eventual demise of FTIR 318 18.6.1 Scanning laser spectroscopy 318 18.6.2 Infrared diode-array spectrometers 320 18.7 The weaknesses of FTIR 320 18.8 The strengths of FTIR 321
Table of contents 13 18.9 FTS or FTIR? 321 18.10 In the crystal ball 322 References 324 Appendix PRACTICAL EVALUATION OF AN FTIR SPECTROMETER 325 ALI The single-beam spectrum: alignment and purging 325 AI.2 The 100% line: alignment, stability, speed Variation, and noise 325 AI.3 The polystyrene test: amplifier linearity and frequency scale 328 AI.4 Absorbance ränge and linearity 329 AI.5 Substitution of a frequency generator for detector and preamplifier 330 AI.6 Interferogram examination: modulation efficiency and long-term stability 332 Reference 333 INDEX 334