Modern RamanSpectroscopy– A PracticalApproach

Ewen SmithStrathclyde University, Glasgow

Geoffrey DentIntertek ASG and UMIST, Manchester

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Modern Raman Spectroscopy– A Practical Approach

Modern RamanSpectroscopy– A PracticalApproach

Ewen SmithStrathclyde University, Glasgow

Geoffrey DentIntertek ASG and UMIST, Manchester

Copyright � 2005 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester,West Sussex PO19 8SQ, England

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Library of Congress Cataloging in Publication Data

Smith, Ewen.Modern Raman spectroscopy : a practical approach / Ewen Smith, Geoff Dent.

p. cm.Includes bibliographical references and index.ISBN 0-471-49668-5 (cloth : alk. paper) — ISBN 0-471-49794-0 (pbk. : alk. paper)1. Raman spectroscopy. I. Dent, Geoffrey. II. Title.QD96.R34S58 2005535.8046—dc22

2004014375

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

ISBN 0-471-49668-5 (Cloth)ISBN 0-471-49794-0 (Paperback)

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Contents

Preface ixAcknowledgements xi

CHAPTER 1 INTRODUCTION, BASIC THEORYAND PRINCIPLES 1

1.1 Introduction 11.2 Basic Theory 21.3 Molecular Vibrations 81.4 Summary 20

CHAPTER 2 THE RAMAN EXPERIMENT – RAMANINSTRUMENTATION, SAMPLE PRESENTATION,DATA HANDLING AND PRACTICAL ASPECTSOF INTERPRETATION 23

2.1 Introduction 232.2 Choice of Instrument 242.3 Visible Excitation 242.4 NIR Excitation 302.5 Raman Sample Preparation and Handling 312.6 Sample Mounting Accessories 412.7 Microscopy 452.8 Calibration 512.9 Data Handling, Manipulation and Quantitation 532.10 Approach to Qualitative Interpretation 612.11 Summary 67

CHAPTER 3 THE THEORY OF RAMAN SPECTROSCOPY 713.1 Introduction 713.2 Absorption and Scattering 723.3 States of a System and Hooke’s Law 743.4 The Nature of Polarizability and the Measurement

of Polarization 76

3.5 The Basic Selection Rule 803.6 Number and Symmetry of Vibrations 803.7 Symmetry Elements and Point Groups 823.8 The Mutual Exclusion Rule 863.9 The Kramer Heisenberg Dirac Expression 863.10 Lattice Modes 903.11 Conclusions 91

CHAPTER 4 RESONANCE RAMAN SCATTERING 934.1 Introduction 934.2 Theoretical Aspects 944.3 Practical Aspects 1014.4 Examples of the Use of Resonance Raman Scattering 1034.5 Conclusions 112

CHAPTER 5 SURFACE-ENHANCED RAMAN SCATTERINGAND SURFACE-ENHANCED RESONANCERAMAN SCATTERING 113

5.1 Introduction 1135.2 Theory 1165.3 Electromagnetic and Charge Transfer Enhancement 1175.4 Selection Rules 1215.5 Applications of SERS 1225.6 Applications of SERRS 1265.7 The Basic Method 127

CHAPTER 6 APPLICATIONS 1356.1 Introduction 1356.2 Inorganics and Minerals 1356.3 Art and Archaeology 1436.4 Polymers and Emulsions 1436.5 Colour 1496.6 Electronics Applications 1586.7 Biological and Pharmaceutical Applications 1606.8 Forensic Applications 1666.9 Plant Control and Reaction Following 1676.10 Summary 172

CHAPTER 7 MORE ADVANCED RAMAN SCATTERINGTECHNIQUES 181

7.1 Flexible Optics 1827.2 Tuneable Lasers, Frequency Doubling and Pulsed Lasers 1877.3 Spatially Resolved Systems 189

vi Contents

7.4 Nonlinear Raman Spectroscopy 1917.5 Time Resolved Scattering 1967.6 Raman Optical Activity 1987.7 UV Excitation 1997.8 Conclusions 201

Index 203

Contents vii

Preface

For many years the practice of Raman spectroscopy was confined to experts indedicated academic or industrial research laboratories. The instruments werelarge, complicated and the experiments could be quite complex. With advancesin modern technology, Raman spectrometers have become small, portable andare regularly used by people who are neither specialist spectroscopists noranalysts. Often instruments are bought for a specific application but eventuallythe user asks, ‘What else can this be used for?’. Whilst much good workcontinues to be carried on by Raman experts in rolling back the frontiers inadvanced techniques, this book is addressed to the more general, modern,application-driven user. Our aim in writing this book is to provide the informa-tion necessary to enable new users to understand and apply the techniquecorrectly. This includes descriptions of the many pitfalls that can be encoun-tered. We wish to aid those with a more sustained interest to gain sufficientknowledge and understanding to make full use of the high information contentthat Raman scattering can afford. With this approach in mind, we haveprovided in the early chapters enough basic theory to make a practical inter-pretation of Raman spectra. The theory is dealt with in a little more depth inlater chapters where the approach is to describe the main equations used toexplain Raman scattering, but concentrating on their meaning and relevancerather than a full mathematical treatment.

With this background the much more detailed world is revealed in whichaspects of Raman spectroscopy can provide unique information for a limitednumber of analytical problems. A full mathematical approach to the theory ofRaman spectroscopy is outside the scope of this book. For those who readthrough to the end, the book will provide a firm grounding, with appropriatereferences given, from which to approach more in-depth studies of specificaspects of Raman spectroscopy. In writing this book some difficult choiceshave had to be made particularly around the presentation of the theory. Manycurrent users of Raman spectroscopy have little idea of the underlying moderntheory and as a result are at risk of misinterpreting their results. However,whilst a full explanation of theory has to have some mathematics, in theauthors’ experience many users do not have the time or the background to

understand a fully rigorous mathematical exposition. The non-rigorous math-ematical approach is almost essential. We have used as few equations aspossible to show how the theory is developed and those are deliberately notin the first chapter. The equations are explained rather than derived so thatthose with little knowledge of mathematics can understand the physical mean-ing described. This level of understanding is sufficient for most purposes.Where a more in-depth approach is sought, the explanation would serve as agood starting point. Two theories are often used in Raman spectroscopy –classical theory and quantum theory. A consequence of our approach to thetheory is the omission of classical Raman theory altogether. Classical theorydoes not use quantum mechanics. In the authors’ opinion the lack of quantumtheory to describe vibrations means that it does not deliver the informationrequired by the average Raman spectroscopist.

One of the practical difficulties faced is in compliance with the IUPACconvention in the description of spectrum scales. Whilst the direction of thewavenumber shift should always be consistent, this is not the practice in mostscientific journals or by software writers for instrument companies. Unfortu-nately the modern practitioner has to view original and reference spectra indiffering formats. To illustrate applications we have used the format in whichthe user is most likely to see a reference spectrum. Equally, where we have used,with permission, literature examples in this book, it would not be possible tochange these round to fit the convention. Raman scattering is a shift from anexciting frequency and should be labelled D cm�1. However it is commonpractice to use cm�1 with the delta implied. Changing labels on previouslypublished examples would not be permitted so for simplicity and consistencywe have used the common format. We apologise to the purists who wouldprefer complete compliance with the IUPAC convention, but we have foundthat it is not practicable.

It is the authors’ hope that those who are just developing or reviving aninterest in Raman spectroscopy will very quickly gain a practical understandingfrom the first two chapters. Furthermore they will be inspired by the eleganceand information content of the technique to delve further into the rest of thebook, and explore the vast potential of the more sophisticated applications ofRaman spectroscopy.

x Preface

Acknowledgements

We thank members of Professor Smith’s group at the University of Strathclydewho read chapters and supplied diagrams – Rachael Littleford, GraemeMcNay, Prokopis Andrikopoulos, Dale Cunningham and Maarten Scholtes;Gillian Neeson and Liz Keys who did much of the typing; Intertek ASG forpermisson to publish; and our respective wives, Frances and Thelma, forputting up with us.

Chapter 1

Introduction, Basic Theory andPrinciples

1.1 INTRODUCTION

The main spectroscopies employed to detect vibrations in molecules are basedon the processes of infrared absorption and Raman scattering. They are widelyused to provide information on chemical structures and physical forms, toidentify substances from the characteristic spectral patterns (‘fingerprinting’),and to determine quantitatively or semi-quantitatively the amount of a sub-stance in a sample. Samples can be examined in a whole range of physicalstates; for example, as solids, liquids or vapours, in hot or cold states, in bulk,as microscopic particles, or as surface layers. The techniques are very wideranging and provide solutions to a host of interesting and challenging analyticalproblems. Raman scattering is less widely used than infrared absorption,largely due to problems with sample degradation and fluorescence. However,recent advances in instrument technology have simplified the equipment andreduced the problems substantially. These advances, together with the ability ofRaman spectroscopy to examine aqueous solutions, samples inside glass con-tainers and samples without any preparation, have led to a rapid growth in theapplication of the technique.

In practice, modern Raman spectroscopy is simple. Variable instrumentparameters are few, spectral manipulation is minimal and a simple interpret-ation of the data may be sufficient. This chapter and Chapter 2 aim to set outthe basic principles and experimental methods to give the reader a firm under-standing of the basic theory and practical considerations so that the technique

Modern Raman Spectroscopy – A Practical Approach W.E. Smith and G. Dent

� 2005 John Wiley & Sons, Ltd ISBNs: 0-471-49668-5 (HB); 0-471-49794-0 (PB)