Modern Raman Spectroscopy – A Practical Approach Ewen Smith Strathclyde University, Glasgow Geoffrey Dent Intertek ASG and UMIST, Manchester Modern Raman Spectroscopy – A Practical Approach Modern Raman Spectroscopy – A Practical Approach Ewen Smith Strathclyde University, Glasgow Geoffrey Dent Intertek ASG and UMIST, Manchester Copyright Ó 2005 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England Telephone (þ44) 1243 779777 Email (for orders and customer service enquiries): [email protected] Visit our Home Page on www.wileyeurope.com or www.wiley.com All Rights Reserved. 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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 2005 535.8046—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) Typeset in 10/12 pt Times by Integra Software Services Pvt. Ltd, Pondicherry, India Printed and bound in Great Britain by Antony Rowe, Chippenham, Wiltshire This book is printed on acid-free paper responsibly manufactured from sustainable forestry in which at least two trees are planted for each one used for paper production. Contents Preface ix Acknowledgements xi CHAPTER 1 INTRODUCTION, BASIC THEORY AND PRINCIPLES 1 1.1 Introduction 1 1.2 Basic Theory 2 1.3 Molecular Vibrations 8 1.4 Summary 20 CHAPTER 2 THE RAMAN EXPERIMENT – RAMAN INSTRUMENTATION, SAMPLE PRESENTATION, DATA HANDLING AND PRACTICAL ASPECTS OF INTERPRETATION 23 2.1 Introduction 23 2.2 Choice of Instrument 24 2.3 Visible Excitation 24 2.4 NIR Excitation 30 2.5 Raman Sample Preparation and Handling 31 2.6 Sample Mounting Accessories 41 2.7 Microscopy 45 2.8 Calibration 51 2.9 Data Handling, Manipulation and Quantitation 53 2.10 Approach to Qualitative Interpretation 61 2.11 Summary 67 CHAPTER 3 THE THEORY OF RAMAN SPECTROSCOPY 71 3.1 Introduction 71 3.2 Absorption and Scattering 72 3.3 States of a System and Hooke’s Law 74 3.4 The Nature of Polarizability and the Measurement of Polarization 76 vi Contents 3.5 The Basic Selection Rule 80 3.6 Number and Symmetry of Vibrations 80 3.7 Symmetry Elements and Point Groups 82 3.8 The Mutual Exclusion Rule 86 3.9 The Kramer Heisenberg Dirac Expression 86 3.10 Lattice Modes 90 3.11 Conclusions 91 CHAPTER 4 RESONANCE RAMAN SCATTERING 93 4.1 Introduction 93 4.2 Theoretical Aspects 94 4.3 Practical Aspects 101 4.4 Examples of the Use of Resonance Raman Scattering 103 4.5 Conclusions 112 CHAPTER 5 SURFACE-ENHANCED RAMAN SCATTERING AND SURFACE-ENHANCED RESONANCE RAMAN SCATTERING 113 5.1 Introduction 113 5.2 Theory 116 5.3 Electromagnetic and Charge Transfer Enhancement 117 5.4 Selection Rules 121 5.5 Applications of SERS 122 5.6 Applications of SERRS 126 5.7 The Basic Method 127 CHAPTER 6 APPLICATIONS 135 6.1 Introduction 135 6.2 Inorganics and Minerals 135 6.3 Art and Archaeology 143 6.4 Polymers and Emulsions 143 6.5 Colour 149 6.6 Electronics Applications 158 6.7 Biological and Pharmaceutical Applications 160 6.8 Forensic Applications 166 6.9 Plant Control and Reaction Following 167 6.10 Summary 172 CHAPTER 7 MORE ADVANCED RAMAN SCATTERING TECHNIQUES 181 7.1 Flexible Optics 182 7.2 Tuneable Lasers, Frequency Doubling and Pulsed Lasers 187 7.3 Spatially Resolved Systems 189 Contents vii 7.4 Nonlinear Raman Spectroscopy 191 7.5 Time Resolved Scattering 196 7.6 Raman Optical Activity 198 7.7 UV Excitation 199 7.8 Conclusions 201 Index 203 Preface For many years the practice of Raman spectroscopy was confined to experts in dedicated academic or industrial research laboratories. The instruments were large, complicated and the experiments could be quite complex. With advances in modern technology, Raman spectrometers have become small, portable and are regularly used by people who are neither specialist spectroscopists nor analysts. Often instruments are bought for a specific application but eventually the user asks, ‘What else can this be used for?’. Whilst much good work continues to be carried on by Raman experts in rolling back the frontiers in advanced 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 technique correctly. This includes descriptions of the many pitfalls that can be encoun- tered. We wish to aid those with a more sustained interest to gain sufficient knowledge and understanding to make full use of the high information content that Raman scattering can afford. With this approach in mind, we have provided 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 in later chapters where the approach is to describe the main equations used to explain Raman scattering, but concentrating on their meaning and relevance rather than a full mathematical treatment. With this background the much more detailed world is revealed in which aspects of Raman spectroscopy can provide unique information for a limited number of analytical problems. A full mathematical approach to the theory of Raman spectroscopy is outside the scope of this book. For those who read through to the end, the book will provide a firm grounding, with appropriate references given, from which to approach more in-depth studies of specific aspects of Raman spectroscopy. In writing this book some difficult choices have had to be made particularly around the presentation of the theory. Many current users of Raman spectroscopy have little idea of the underlying modern theory and as a result are at risk of misinterpreting their results. However, whilst a full explanation of theory has to have some mathematics, in the authors’ experience many users do not have the time or the background to x Preface understand a fully rigorous mathematical exposition. The non-rigorous math- ematical approach is almost essential. We have used as few equations as possible to show how the theory is developed and those are deliberately not in the first chapter. The equations are explained rather than derived so that those 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 a good starting point. Two theories are often used in Raman spectroscopy – classical theory and quantum theory. A consequence of our approach to the theory is the omission of classical Raman theory altogether. Classical theory does not use quantum mechanics. In the authors’ opinion the lack of quantum theory to describe vibrations means that it does not deliver the information required by the average Raman spectroscopist. One of the practical difficulties faced is in compliance with the IUPAC convention in the description of spectrum scales. Whilst the direction of the wavenumber shift should always be consistent, this is not the practice in most scientific journals or by software writers for instrument companies. Unfortu- nately the modern practitioner has to view original and reference spectra in differing formats. To illustrate applications we have used the format in which the 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 to change these round to fit the convention. Raman scattering is a shift from an exciting frequency and should be labelled D cmÀ1.