FOURIER TRANSFORM INFRARED

Transcription

1 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

2 Table of Contents PREFACE 17 1 INTRODUCTION 21 PARTI: CLASSICAL TIMES 2 ORIGINS The Status in Light waves vs. particles The discovery of interference Growth of the undulatory theory The interference of light Polarization Research into interference phenomena Harmonie analysis The beginnings of optical spectroscopy Invention of the spectroscope Emission spectra Absorption spectra Molecular spectra 38 References 38 3 THE BIRTH OF INTERFERENTIAL SPECTROSCOPY The interferometer The interferogram The Fourier transform The harmonic analyser Experimental limitations Infrared and far infrared 51

3 6 Table of contents 3.7 The measurement of infrared radiation Interferential spectroscopy in the infrared 52 References 55 PART II: DARKAGES 4 THE DECLINE OF INTERFERENTIAL SPECTROSCOPY Spreading the word Ambiguities Restriction to emission spectra The missing link Final decline 63 References 64 5 COMPETING TECHNOLOGIES Spectral units Spectral ränge Spectral resolution Multiple-beam spectroscopic instruments The echelon spectroscope The Fabry-Perot etalon The Lummer-Gehrcke plate Dispersive spectroscopic instruments Prism spectrometers The diffraction grating A typical spectroscopic implementation in the late 1920s References 76 6 COMMERCIAL DISPERSIVE SPECTROMETERS Improvements to classical detection methods The Pfund resonance radiometer The Firestone amplifier Electronic amplifiers Recording spectrophotometers Servomechanisms Fourier transformation before Computers The growth of commercial spectroscopy Double beam vs. single beam instruments Optical null instruments Energy control Electronic null instruments Recording Systems 86

4 Table of contents The dispersive infrared laboratory 86 References 87 PART III: RENAISSANCE 7 POST-WAR DEVELOPMENTS Computer development Optical processing Information theory Instrument science The multiplex advantage The Jacquinot advantage Other contributions 96 References 97 8 EXPERIMENTAL INTERFEROMETRIC SPECTROSCOPY Fellgett's demonstration The Johns Hopkins group Lawrence Mertz at Baird Associates Ine The road to transformation Jacquinot's group The 1957 Bellevue Conference Connes' thesis Work at NPL Michelson vs. lamellar interferometers Early NPL instruments A near infrared FTIR The compensator plate An evacuable Michelson interferometer Moire position measurement Advances in detectors The Golay cell The photoelectric infrared detectors Photomultiplier tubes The state-of-the-art: the Connes' planetary spectra 114 References WORKING OUT THE DETAILS Apodization Digital sampling Surface flatness vs. wavelength The effect of beam divergence in an interferometer 121

5 10 Table of contents LVDTs Fringe monitoring Associated spectrometer optics Jacquinot stop Source and source optics Sample optics Detectors Detector optics 214 References THE OPTIMIZATION OF FTIR Interferometer efficiency Dynamic ränge Amplifier linearity Filtering Beamsplitter design Wavefront distortion Spectral band Polarization effects 228 References FTIR SINCE Analect Beckman Block Engineering Bomem Bran+Luebbe Bruker Chelsea Instruments Digilab Hewlett-Packard Hitachi IBM Instruments Idealab Janos Jasco Jeol Kayser-Threde Lloyd Instruments Mattson 238

6 Table of contents Midac Nicolet Perkin-Elmer Philips Analytical Specac Soviet instruments Epilogue: companies no longer manufacturing FTIRs References FTIR IN SPACE Ground-based astronomy Airborne Fourier spectrometers JPL University of Arizona Other airborne instruments Space-borne interferometers Block Engineering Nimbus satellites EXCEDE experiment Mariner Mars probe Voyager space probe Cosmic Background Explorer Tropospheric emission spectrometer (TES) Soviet space-borne interferometers Manned Space flight 258 References BEYOND FTIR Double-beam FTIR Double-beam configurations and applications Technical advantages of double-beam configurations Comparison with dispersive double-beam instruments Factors affecting Performance Summary Asymmetrie FTIR Field-widened interferometers Principle of field-widening Practical instruments Imaging interferometers Polarizing interferometers 284

7 12 Table of Contents Wire-grid polarizers Mertz's polarizing interferometer Martin and Puplett design Polarizing interferometer for dichroism measurements FT spectrometry in the ultraviolet Optical quality Scanning accuracy Alignment Practical instruments Types of noise and their effect on the multiplex advantage The Jacquinot advantage in FT-UV Gas chromatography and IR Thermogravimetric analysis and IR Raman spectroscopy Sampling techniques Infrared microsampling Diffuse reflectance Attenuated total reflectance Photo-acoustic spectroscopy Long-path gas cells Fibre optics interfaces 308 References THE SHAPE OF THINGS TO COME Changing markets Advances in technology Data analysis Neural nets Expert Systems Communication Interferometer design FTIR with no moving parts FTIR within an optical fibre Advances in sampling techniques The eventual demise of FTIR Scanning laser spectroscopy Infrared diode-array spectrometers The weaknesses of FTIR The strengths of FTIR 321

8 Table of contents FTS or FTIR? 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