Quantitative Millimetre Wavelength Spectrometry (Rsc Analytical Spectroscopy Monographs) - Hardcover

Alder, John F.; Baker, John G.; Royal Society Of Chemistry (Great Britain)

Hardcover

ISBN 10: 0854045759 ISBN 13: 9780854045754

Verlag: Royal Society of Chemistry, 2002

Inhaltsangabe

This unique book demonstrates the current status, and future potential, of millimetre wavelength (MMW) spectrometry as a means of quantitative analysis of gaseous mixtures. After outlining the spectroscopic theory required, the authors then go on to discuss the components of an MMW cavity spectrometer, concentrating on compact, automatic, low-cost instruments. Other topics covered include solid state MMW sources with both cryogenically cooled and room temperature detectors. Post-detector signal processing, smoothing, filtering and spectral profile fitting are also discussed. The book concludes with a look at the future of the technique, in areas such as millimetre wave-over-fibre technology. Quantitative Millimetre Wavelength Spectrometry will be welcomed by practitioners in both industry and academia.

Über diesen Titel

Über die Autorin bzw. den Autor

Professor of Analytical Chemistry. He was a pioneer in introducing Atomic Spectrometry methods in Spain. He is a world-reputed Atomic Spectroscopist who introduces/develops many novel, state-of-the-art atomic techniques. He is a member of the editorial board of ABC and also of the editorial board of the RSC for a series of monographs on Atomic Spectrometry (leaded by Prof. Neil Burnett). He currently develops and tests cutting-edge instruments, even for some commercial firms. He published many papers and books. In 2007 he won the Robert Kellner award (The Robert Keller Lecture will be given at the Euroanalysis 2007).

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Quantitative Millimetre Wavelength Spectrometry

By John F. Alder, John G. Baker

The Royal Society of Chemistry

Copyright © 2002 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85404-575-4

Contents

Glossary of Terms and Symbols, xi, 
Chapter 1 Interaction of Millimetre Wavelength Electromagnetic Radiation with Gases, 1, 
Chapter 2 The Components of a MMW Cavity Spectrometer for Quantitative Measurements, 21, 
Chapter 3 Practical Spectral Sources and Detectors for Analytical Spectrometry, 38, 
Chapter 4 The Quantitative Analysis of Gas Mixtures, 65, 
Chapter 5 Cavity Spectrometer Designs and Applications, 80, 
Chapter 6 A Practical Frequency Modulated Spectrometer and Its Application to Quantitative Analysis, 89, 
Chapter 7 The Future for Quantitative Millimetre Wavelength Spectrometry, 115, 
Subject Index, 119, 

CHAPTER 1

Interaction of Millimetre Wavelength Electromagnetic Radiation with Gases

Millimetre wavelength (MMW) radiation forms the 30–300 GHz band of the electromagnetic spectrum and is used to study the mainly rotational spectra of gaseous molecules. Transitions can also be measured in the inversion spectrum of ammonia, between molecular rotamer configurations and between magnetic fine structure components of molecules possessing a magnetic dipole, e.g. O2 and NO. The absorption spectra normally studied in small molecules (<200 Dalton) become more intense at higher frequencies, as illustrated in Figure 1.1 for sulfur dioxide, so for quantitative work it is usually advantageous to work in the MMW band.

The rotational levels lie at low energies and are all populated at ambient temperatures. This gives rise to abundant spectra of narrow lines, width < 1 MHz at Pascal (Pa) pressures, that are spread over this wide spectral region. Spectral interference between species in real mixtures is unusual and regions can be chosen for measurements where the target species are represented without spectral overlap. This is quite different from other spectrometric methods in the ultraviolet-visible to mid-infrared region where the analyst is usually constrained to work at one of a few frequencies. Separation prior to analysis for gas mixtures is not required apart perhaps from gross scrubbing of suspended solids or liquids, making rotational spectrometry unique amongst quantitative analytical methods.

Rotational spectroscopy is essentially a low-pressure technique if one is to exploit its remarkable selectivity, although quantitative measurements can be made at pressures up to atmospheric. Not all gases are rotationally active; with the exception of dioxygen all the homonuclear diatomic molecules and of course monatomic gases are inactive. Molecules with a high degree of symmetry, notably methane, ethane, ethene, benzene and carbon dioxide, are likewise inactive or have weak spectra, e.g. propane and butane. Oxygen and water, possibly the two most-determined molecular species in the modern world, are rotationally active and can be measured readily in those matrix gases.

Modern teaching in analytical spectrometry deals only rarely with quantitative rotational spectrometry in any depth and this lack of attention gives rise to some misunderstandings about the technique. Much of that derives from the unusual and sometimes seemingly mysterious combination of optical and electronic phenomena that characterise this spectral region. In reality of course, MMW spectrometry is quite simple and has been made much easier in recent years with the advent of solid state electronic instrumentation and devices of very high quality.

The theory too is fundamentally simple and has been very well developed although less emphasis has been placed on the quantitative aspects of the subject than was perhaps desirable, with the notable exception of the book devoted to this topic by Varma and Hrubesh.

The origin and details of molecular rotational spectra are explicitly laid out in the comprehensive texts by Townes and Schawlow, and Gordy and Cook, the latter two being the sources for much of the theory outlined below. The present authors have focused only upon those aspects that are directly relevant to quantitative MMW cavity spectrometry, and the reader is referred to the original texts for a more comprehensive treatment. Readers will soon notice that the original texts, and indeed the majority of all published MMW spectrometry papers and texts to this day, use cgs rather than SI units. Consequently, some of the equations differ from those in other texts and translation of units will be necessary to compare them with other work.

1 Basic Spectroscopic Theory

The purpose of this section is to give readers access to the most important relationships between the spectroscopy and the quantitative measurement of molecular species in mixtures. The theory aims to form a relationship between the composition of a gas mixture and the peak intensity αmax of a spectral transition. The value of αmax is influenced by pressure broadening, temperature effects and power saturation, and takes on different forms with respect to the size and structure of the molecule. These factors all influence the spectral line shape that is directly related to the fractional abundance of that absorbing species in the sample being measured. The theory also permits at least semi-quantitative design of experimental parameters such as optimum working pressure, measurement frequencies for optimum sensitivity and location with concomitant gas absorption lines. It will help understanding of some design requirements and limitations of the technique. Most of all it is intended to remove some of the mystique from the subject. The initial goal is therefore to equate the maximum absorption coefficient αmax to the species' spectroscopic parameters and fractional abundance. It will be approached from first principles, with an eye focused always on the quantitative aspects of the theory.

The main difference between optical and MMW spectrometry lies in the fact that energies involved in the rotational transitions are ~ 100-1000 times less than those involved in the usually more familiar vibrational and electronic transitions of molecular and atomic species.

The Boltzmann equation exponent hv/kT at ambient temperatures is approximately 4.8 X 10-3 σ/cm-1 or 1.6 X 10-4 v/GHz. Thus for rotational energy levels lying typically in the tens-hundreds cm-1 region* above the ground state the exponential term is not much less than unity, indicating that most levels will hold a significant fraction of the overall population of species. This has mostly positive consequences through rich well-distributed spectra and a choice of frequency region for observation of particular gas mixtures.

If a molecule possessing two rotational energy levels E described by the quantum numbers m for the lower level and n for the upper level, is exposed to radiation of frequency v where

hv = En - Em (1.1)

and h is Planck's constant, a transition will take place with probability

Pm [right arrow] n = p(v)Bm [right arrow] n (1.2)

where pm [right arrow] n/s-1 is the rate of change of the probability that the molecule will be found in the upper state; p(v) is the energy density of the radiation per unit frequency interval/J m-3 s and Bm [right arrow] n/J-1 is the Einstein coefficient of absorption for that transition.

In the excited state n the interaction of the radiation field with the molecule induces emission, and...

Details zur Ausgabe

• Verlag: Royal Society of Chemistry
• Erscheinungsdatum: 2002
• Sprache: Englisch
• ISBN 10: 0854045759
• ISBN 13: 9780854045754
• Einband: Gebundene Ausgabe
• Anzahl der Seiten: 122
• Kontakt zum Hersteller: Nicht verfügbar
• Verantwortliche Person: Buchpark GmbH; gpsr@buchpark.de; ; Krügerweg 1; Trebbin; Brandenburg; 14959; Deutschland