PRACTICAL RAMANSPECTROSCOPY –AN INTRODUCTION

PRACTICAL RAMANSPECTROSCOPY –AN INTRODUCTION

Peter VandenabeeleGhent University, Belgium

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Library of Congress Cataloging-in-Publication Data

Vandenabeele, Peter.Practical Raman spectroscopy : an introduction / Peter Vandenabeele, Ghent University, Belgium.

pages cmIncludes bibliographical references and index.ISBN 978-0-470-68319-4 (hardback) – ISBN 978-0-470-68318-7 (paperback) 1. Raman

spectroscopy–Study and teaching. I. Title.QD96.R34V36 2013543′.57–dc23

2013013049

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

Print ISBN: Cloth 9780470683194Paper 9780470683187

Set in 11/13pt Times by Laserwords Private Limited, Chennai, India

1 2013

To my family – for all your support,To my colleagues – that became friends,

To my students – who have never stopped inspiring me.

About the Author

Peter Vandenabeele obtained his masters’ degree in chemistry atGhent University, where he made his masters’ thesis on thermalanalysis of precursors for the synthesis of superconductors. HisPhD research was carried out at the same university, but in thedepartment of analytical chemistry, under the supervision of Prof.Dr. L. Moens. This research was on the optimisation of micro-Raman spectroscopy and total-reflection X-ray fluorescence forart analysis (2000). During his post-doctoral period Peter furtherworked on novel applications of Raman spectroscopy, in art anal-ysis as well as in pharmaceutics, microbiology and astrobiology.In 2007, the author was appointed as research professor in thedepartment of archaeology of Ghent University, where he furthercan apply his analytical skills to study archaeological and artisticobjects.

Peter Vandenabeele has authored almost 100 research papers onRaman spectroscopy and in archaeometry. He has given many pre-sentations on international conferences, of which several invited orplenary oral presentations. On many occasions he has written bookchapters on Raman spectroscopy in archaeometrical research. Asresearch professor, he has limited time to teach, but nevertheless,he enjoys introducing students in archaeometry as well as in Ramanspectroscopy.

Contents

Preface xiii

Acknowledgements xv

Acronyms, Abbreviations and Symbols xvii

Introduction to Raman spectroscopy xix

1 Theoretical Aspects 1

1.1 Classical Approach 11.2 Selection Rule 71.3 Energy Levels and Group Frequencies 131.4 Raman Intensity 191.5 Raman Bandwidth 351.6 The General Appearance of a Raman Spectrum 371.7 Summary 38Intermezzo 1.1 The Harmonic Oscillator and the Position

of the Vibrational Energy Levels 6Intermezzo 1.2 The Anharmonic Oscillator and the

Position of Vibrational Energy Levels 11Intermezzo 1.3 Raman Intensity Annotations 30

x Contents

2 Interferences and Side-effects 39

2.1 Absorption 402.2 Fluorescence 402.3 Thermal Effects, Photodecomposition and Laser

Ablation 432.4 Ambient Light and Background Radiation 442.5 Summary 44

3 Enhancement of the Raman Signal 47

3.1 Resonance Raman (RR) Spectroscopy 483.2 Surface-Enhanced Raman Spectroscopy (SERS) 48

3.2.1 Working Principle of SERS 49(a) Chemical Enhancement 49(b) Field Enhancement 51

3.2.2 Preparation of SERS Substrates 53(a) Colloids 54(b) Sputtering 55(c) Electrochemical Production 55(d) Etching 56

3.2.3 SERS Active Molecules 563.2.4 Advantages and Disadvantages of SERS 57

3.3 Summary 59Intermezzo 3.1 The Analysis of Organic Dyes by SERS 58

4 Raman Instrumentation 61

4.1 Lasers 624.1.1 Classification According to Safety Category 644.1.2 The Operating Principle of the Laser 654.1.3 Lasers for Raman Spectroscopy 71

4.2 Detectors 744.2.1 Single-Channel Detectors 754.2.2 Multi-Channel Detectors 76

Contents xi

4.2.3 Charge-Coupled Device (CCD) Detectors 764.2.4 Semi-conductor Detectors 80

4.3 Filters 814.4 Dispersion Systems 83

4.4.1 Systems Based on Diffraction of Light 834.4.2 Fourier-Transform (FT-) Systems 86

4.5 Components for Transportation of Light 884.6 Sample Chambers and Measurement Probes 914.7 Noise in Raman Spectroscopy 95

4.7.1 Noise Originating from the Sample: σs 964.7.2 Noise Originating from the Raman

Instrument: σi 984.7.3 Noise Originating from the Signal

Processing: σp 994.8 Summary 99Intermezzo 4.1 The Helium-Neon laser 69

5 Raman Spectroscopy in Daily Lab-life 101

5.1 Calibration of a Raman Spectrometer 1025.1.1 X-Axis Calibration 1025.1.2 Y-Axis Calibration 105

5.2 Raman Spectral Post-processing 1085.2.1 Digital Filters 1095.2.2 Scaling 1105.2.3 Deconvolution 1125.2.4 Baseline Corrections 113

5.3 Interpretation of Raman Spectra ofOrganic Molecules 1165.3.1 Raman Spectra of Alkanes and Cycloalkanes 1205.3.2 Raman Spectra of Alkenes and Cycloalkenes 1255.3.3 Raman Spectra of Aromates 129

5.4 Interpretation of Raman Spectra of InorganicMolecules 131

xii Contents

5.5 Quantitative Aspects of Raman Spectroscopy 1345.6 Fingerprinting and Spectral Searching Algorithms 1375.7 Raman Mapping and Imaging 1415.8 Combination with Other Techniques 1455.9 Summary 147Intermezzo 5.1 Micro-Raman Spectroscopy in

Microbiology 107Intermezzo 5.2 Raman Spectroscopy in a Forensics

Laboratory 117Intermezzo 5.3 Pigment Analysis with Raman

Spectroscopy 132Intermezzo 5.4 Detection of Counterfeit Medicines

with Raman Spectroscopy 136Intermezzo 5.5 Exobiology: Raman Spectroscopy Helps

the Search for Life on Mars 139Intermezzo 5.6 Direct Analysis of Precious Art Objects 144

Responses to Questions 149

Bibliography 153

Glossary of Terms 155

Index 157

Preface

Raman spectroscopy is a very versatile molecular spectroscopictechnique, with many different applications in a range of researchfields. Whereas in its early days using this technique was verytime-consuming and complex – and only applied in a few veryspecialised laboratories – today the technique is becoming increas-ingly popular in fundamental research as well as in applied science.Indeed, due to many instrumental evolutions, Raman spectroscopyhas become increasingly more accessible and affordable. As a con-sequence, the technique has moved from the specialised laboratoriestowards more generally oriented laboratories. However, along withthis broadening of applications, there is an increasing chance formisinterpretations and good training in Raman spectroscopy canhelp in avoiding these pitfalls.

This handbook starts with an introduction, where the historyof Raman spectroscopy is sketched. In Chapter 1, the theoreticalbackground of the technique is described. This theory is used tounderstand possible interferences (Chapter 2) and to study possibletechniques to enhance the Raman intensity (Chapter 3). Chapter 4focuses on the technical aspects of Raman spectroscopy: generalaspects of Raman spectrometer construction and the propertiesof the different components. Together with these aspects, someconsiderations about noise in Raman spectra are discussed. Thefinal chapter in this book (Chapter 5) describes aspects from