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The Raman Effect A Unified Treatment of the Theory of Raman Scattering by Molecules Derek A. Long free download
















Contents
Preface xix
Acknowledgements xxiii
Part One Theory 1
1 Survey of Light-scattering Phenomena 3
1.1 Introduction 3
1.2 Some Basic Definitions 4
1.3 Rayleigh and Raman Scattering 5
1.3.1 Description 5
1.3.2 Energy transfer model 7
1.4 Hyper-Rayleigh and Hyper-Raman Scattering 10
1.4.1 Description 10
1.4.2 Energy transfer model 10
1.5 Second Hyper-Rayleigh and Second Hyper-Raman Scattering 11
1.5.1 Description 11
1.5.2 Energy transfer model 11
1.6 Coherent anti-Stokes and Coherent Stokes Raman Scattering 11
1.7 Stimulated Raman Gain and Loss Spectroscopy 13
1.8 Typical Spectra 14
1.9 Bases for the Theoretical Treatment of Rayleigh and
Raman Scattering 16
1.10 Historical Perspective 16
viii Contents
1.11 Caveat 17
References 17
2 Introduction to Theoretical Treatments of Incoherent Light Scattering 19
2.1 General Considerations 19
2.2 Induced Oscillating Electric Dipoles as Sources of
Scattered Radiation 21
2.3 Basis of the Classical Theory of Light Scattering 22
2.4 Basis of the Quantum Mechanical Treatment
of Incoherent Light-Scattering Phenomena: Electric Dipole Case 24
2.5 Extension of Quantum Mechanical Treatment of Incoherent
Light Scattering to Include Magnetic Dipole and Electric
Quadrupole Cases 27
2.6 Comparison of the Classical and Quantum Mechanical
Treatments of Light Scattering 28
2.7 The Way Ahead 29
3 Classical Theory of Rayleigh and Raman Scattering 31
3.1 Introduction 31
3.2 First-order Induced Electric Dipole 31
3.3 Frequency Dependence of the First-order Induced Electric
Dipole 34
3.4 Classical Scattering Tensors aRay and aRam
k 35
3.5 Selection Rules for Fundamental Vibrations 36
3.5.1 General considerations 36
3.5.2 Diatomic molecules 36
3.5.3 Polyatomic molecules 38
3.6 Selection Rules for Overtones and Combinations 43
3.7 Coherence Properties of Rayleigh and Raman Scattering 44
3.8 Limitations of the Classical Theory 45
3.9 Example of Rayleigh and Raman Scattering 45
3.10 Presentation of Raman Spectra 47
References 48
4 Quantum Mechanical Theory of Rayleigh and Raman Scattering 49
4.1 Introduction 49
4.2 Time-dependent Perturbation Theory and a fi 50
4.3 Qualitative Discussion of ˛ fi 54
4.3.1 Frequency denominators 55
4.3.2 Transition electric dipole numerators 56
4.3.3 Selection rules 58
4.4 Tensorial Nature of the Transition Polarizability and its Symmetry 58
4.5 Born–Oppenheimer Approximation and the Transition
Polarizability Tensor 61
Contents ix
4.6 Simplification of ˛ efvf:egvi : General Considerations 64
4.7 Simplification by Radical Approximation: the Placzek
Transition Polarizability 65
4.8 Simplification of ˛ efvf:eivi by Stages 68
4.8.1 Introduction of Herzberg–Teller vibronic coupling 68
4.8.2 Identification of non-resonance and resonance
situations 75
4.9 Normal Electronic (and Vibronic) Raman Scattering 77
4.10 Normal Pure Vibrational Raman Scattering 78
4.11 Electronic (and Vibronic) Resonance Raman Scattering 81
4.12 Vibrational Resonance Raman Scattering 83
4.13 Units and Orders of Magnitude 83
References 84
5 Vibrational Raman Scattering 85
5.1 Introduction 85
5.2 The Placzek Vibrational Transition Polarizability:
Recapitulation 86
5.2.1 Cartesian basis 86
5.2.2 The spherical basis 88
5.3 Definition of Illumination–Observation Geometry 89
5.4 Intensity of Scattered Radiation: Some General Considerations 94
5.4.1 Development of a symbol for scattered intensity 94
5.4.2 Scattering cross-section 95
5.5 Intensity Formulae and Polarization Characteristics for a General
Vibrational Transition in Various Illumination–Observation
Geometries 97
5.5.1 General considerations 97
5.5.2 Linearly polarized incident radiation 98
5.5.3 Natural incident radiation 102
5.5.4 Angular dependence of scattered intensity 103
5.5.5 Circularly polarized incident radiation 106
5.5.6 Symmetry and depolarization ratios, reversal
coefficients and degrees of circularity 109
5.6 Stokes Parameters for Scattered Radiation 113
5.7 Specific Vibrational Transitions 116
5.8 Vibrational Selection Rules 120
5.9 Patterns of Vibrational Spectra 123
5.10 Orders of Magnitude 126
5.11 Epilogue 127
References 131
Reference Tables for Chapter 5 132
Reference Table 5.1: Definitions for I ; ps, pi 132
x Contents
Reference Table 5.2(a) to 5.2(g): Intensities, Polarization
Properties and Stokes Parameters for Vibrational Raman
(and Rayleigh) Scattering 132
Reference Table 5.3: Symmetry classes for x, y, z, the rotations
Rx, Ry and Rz, and the components of the cartesian basis
tensor ca. 145
6 Rotational and Vibration–Rotation Raman Scattering 153
6.1 Introduction 153
6.2 Irreducible Transition Polarizability Components 154
6.3 Symmetric Top 156
6.3.1 Selection rules 156
6.3.2 Placzek invariants G j fi 157
6.3.3 Intensities 167
6.3.4 Subsequent development 169
6.4 Rotational and Vibrational Terms 169
6.5 Statistical Distribution of Molecular Population 171
6.6 Diatomic Molecule 173
6.6.1 Introduction 173
6.6.2 Heteronuclear diatomic molecule: pure rotation 174
6.6.3 Heteronuclear diatomic molecule: vibration–rotation 175
6.6.4 Homonuclear diatomic molecule: nuclear spin degeneracy 179
6.6.5 Intensity distribution 180
6.7 Symmetric Top Molecule 186
6.7.1 Introduction 186
6.7.2 Symmetric top: pure rotation 187
6.7.3 Symmetric top: vibration–rotation 191
6.7.4 Intensities 203
6.8 Linear Molecules 204
6.8.1 Rotation and vibration-rotation Raman spectra 204
6.8.2 Intensities 207
6.9 Contributions from Electronic Orbital and Spin Angular Momenta 208
6.10 Spherical Top Molecules 210
6.11 Asymmetric Top Molecules 211
6.12 Epilogue 211
References 213
Reference Tables for Chapter 6 214
Introduction 214
Reference Tables 6.1 to 6.4 216
7 Vibrational Resonance Raman Scattering 221
7.1 Introduction 221
7.2 Vibrational Transition Polarizability Tensor Components in the
Resonance Case, Based on Perturbation Theory 222
Contents xi
7.3 Comparison of the AVI, BVI,CVI and DVI Terms 224
7.3.1 The AVI term 224
7.3.2 The BVI term 227
7.3.3 The CVI term 229
7.3.4 The DVI term 229
7.3.5 Subsequent developments 230
7.4 AVI Term Raman Scattering from Molecules with Totally
Symmetric Modes 231
7.4.1 AVI term Raman scattering from molecules with one
totally symmetric mode 231
7.4.2 AVI term Raman scattering from molecules
with more than one totally symmetric mode: general
considerations 237
7.4.3 AVI term Raman scattering from totally symmetric modes
when k is very small 238
7.5 AVI Term Raman Scattering Involving Non-Totally Symmetric
Modes 239
7.5.1 General considerations 239
7.5.2 AVI term scattering involving a change of molecular
symmetry of the resonant excited state 239
7.5.3 AVI term scattering involving excited state Jahn–Teller
coupling 240
7.5.4 Summary of excited state Jahn–Teller effects in
resonance Raman scattering 240
7.6 BVI Term Scattering Involving Vibronic Coupling of the Resonant
Excited State to a Second Excited State 241
7.6.1 Introduction 241
7.6.2 BVI term scattering from molecules with non-totally
symmetric modes 241
7.6.3 BVI term scattering from molecules with totally
symmetric modes 244
7.7 Symmetry, Raman Activity and Depolarization Ratios 246
7.7.1 General symmetry considerations 246
7.7.2 The AVI term 247
7.7.3 The BVI term 250
7.8 Time-Dependent Formulation of Resonance Raman Scattering 262
7.8.1 Introduction 262
7.8.2 Transformation of the AVI term to a time-dependent
expression 263
7.8.3 The time-dependent interpretation of resonance Raman
scattering 264
7.9 Continuum Resonance Raman Scattering 266
References 270
xii Contents
8 Rotational and Vibration–Rotation Resonance Raman Scattering 271
8.1 Introduction 271
8.2 General Expression for ˛ fi for a Symmetric Top Molecule 272
8.3 General Expression for ˛ j
m fi 274
8.4 Contraction of General Expression for ˛ j
m fi 274
8.5 The Quadratic Term 275
8.6 Selection Rules 276
8.7 Evaluation of j ˛ j
m fij2 277
8.8 Intensities and Depolarization Ratios 279
8.9 An Illustrative Example 283
8.10 Concluding Remarks 287
Reference 287
9 Normal and Resonance Electronic and Vibronic Raman Scattering 289
9.1 Introduction 289
9.2 Normal Electronic and Vibronic Raman Scattering 289
9.2.1 General considerations 289
9.2.2 AIII-term scattering 290
9.2.3 BIII C CIII -term scattering 291
9.2.4 DIII-term scattering 292
9.2.5 Transition tensor symmetry 292
9.3 Resonant Electronic and Vibronic Raman Scattering 292
9.3.1 General considerations 292
9.3.2 AV-term scattering 293
9.3.3 BV-term scattering 296
9.3.4 CV-term scattering 297
9.3.5 DV-term scattering 297
9.4 Selection Rules in Electronic Raman Spectra 297
9.4.1 General symmetry considerations 297
9.5 Intensities and Polarization Properties of Electronic Raman
Scattering 301
9.5.1 Intensities: general considerations 301
9.5.2 Excitation profiles 301
9.5.3 Depolarization ratios 302
10 Rayleigh and Raman Scattering by Chiral Systems 303
10.1 Introduction 303
10.2 Outline of the Theoretical Treatment 305
10.3 Intensities of Optically Active Rayleigh Scattering 310
10.3.1 General considerations 310
10.3.2 Intensity formulae 314
10.3.3 Stokes parameters 317
Contents xiii
10.4 Intensities of Optically Active Raman Scattering 321
10.4.1 General considerations 321
10.4.2 Discussion of intensities and isotropic invariants 323
10.4.3 Placzek polarizability theory and optically
active scattering 324
10.5 Symmetry Considerations 326
10.6 Concluding Remarks 327
Reference 327
Reference Tables for Chapter 10 328
Part Two Appendices 337
Introduction 339
A1 The Right-handed Cartesian Axis System and Related Coordinate
Systems 341
A1.1 Introduction 341
A1.2 The Right-handed Cartesian Axis System 342
A1.3 Cartesian Coordinate System 344
A1.4 Cylindrical Coordinate System 344
A1.5 Polar Coordinate System 345
A1.6 Complex Coordinate Systems 347
A2 The Summation Convention 349
A2.1 General Definitions 349
A3 Direction Cosines 351
A3.1 Introduction 351
A3.2 Definitions and Properties of Direction Cosines 351
A3.3 Definitions of Direction Cosines in Other Coordinate Systems 354
A4 Isotropic Averages of Products of Direction Cosines 355
A4.1 Introduction 355
A4.2 Specific Isotropic Averages of Products of Direction Cosines 356
A4.3 General Formulae 358
References 358
A5 The Euler Angles and the Rotation Operator 359
A5.1 Introduction 359
A5.2 Definitions of the Euler Angles and the Rotation Operator 359
A5.3 The Relationship of the Euler Angles to the Polar Coordinates 362
A5.4 Direction Cosines and Euler Angles 362
References 363
xiv Contents
A6 Complex Numbers and Quantities 365
A6.1 Introduction 365
A6.2 Definitions and Operations 365
A6.3 Graphical Representation of Complex Numbers 367
A6.4 Complex Numbers and Polar Coordinates 369
A6.5 Complex Quantities and Physical Phenomena 370
A6.6 Spherical Coordinates 371
Reference 372
A7 Some Properties of Matrices 373
A7.1 Introduction 373
A7.2 Nomenclature 373
A7.3 Some Special Matrices 374
A7.4 Matrix Representation of Simultaneous Linear Equations 377
A7.5 Eigenvalues and Eigenvectors 377
A7.6 Example of Diagonalization of a Matrix 379
A8 Vectors, I 381
A8.1 Introduction: Scalars, Vectors and Tensors 381
A8.2 Basic Definition of a Vector 382
A8.3 Unit Vectors 383
A8.4 Vector Addition, Subtraction and Multiplication by a Scalar 385
A8.5 Multiplication of Two Vectors 385
A8.6 Triple Products of Vectors 390
A8.6.1 A B Ð C 391
A8.6.2 A Ð B ð C 391
A8.6.3 A ð B ð C 392
A8.6.4 ABC 392
A8.7 Formal Definition of a Vector in Terms of its Transformation
upon Rotation of Axes 393
A8.8 Polar and Axial Vectors: Time Even and Time Odd Vectors 394
A8.9 Vector Differentiation 395
A8.9.1 The operator ∇ 395
A8.9.2 The gradient 396
A8.9.3 The divergence 397
A8.9.4 The curl, ∇ð V 397
A8.9.5 The divergence and the curl illustrated and compared 398
A8.9.6 Composite functions involving ∇ 398
A8.9.7 Successive applications of ∇ 400
A8.9.8 Time derivative of a vector 401
A8.9.9 Caveat 401
A8.10 Change of Basis Vectors and Effect Upon Coordinates
of a Fixed Vector 401
A8.11 The Effect of a Symmetry Operation on Vectors and Basis Vectors 404
Contents xv
A9 Vectors, II 407
A9.1 Introduction 407
A9.2 Cylindrical Coordinates and Basis Vectors 407
A9.3 Polar Coordinates and Polar Basis Vectors 408
A9.4 Spherical Components and Spherical Basis Vectors and Direction
Cosines 409
A9.5 Rotation of Vectors using Spherical Coordinates 413
A9.6 Vectors in n-Dimensional Space 415
References 416
A10 Tensors 417
A10.1 General Definitions 417
A10.2 Representation or Specification of a Tensor 419
A10.3 Transformation of Tensors upon Rotation of Axes 422
A10.4 Some Properties of Tensors 425
A10.4.1 General 425
A10.4.2 Tensors of rank two 425
A10.4.3 Tensors of rank three: the alternating or
Levi–Civit`a tensor 426
A10.4.4 Isotropic tensors of various ranks 427
A10.4.5 Tensor contraction 427
A10.5 Irreducible Tensorial Sets 428
A11 Electrostatics 433
A11.1 Introduction 433
A11.2 Force Between Charges 433
A11.3 Electric Field Strength 435
A11.4 Electrostatic Potential 436
A11.5 Gauss’s Law 437
A11.6 The Equations of Poisson and Laplace 438
A12 Magnetostatics 439
A12.1 Introduction 439
A12.2 Magnetic Forces 441
A12.3 The Magnetic Induction 442
A12.4 The Lorentz Force on a Point Charge Moving in a
Magnetic Field 445
A12.5 The Divergence of the Magnetic Induction B 446
A12.6 The Vector Potential A 446
A13 The Interaction of a System of Electric Charges with Electric
and Magnetic Fields 449
A13.1 Introduction 449
A13.2 Point Charges in Molecular Systems 450
xvi Contents
A13.3 Electric Dipole in a Molecular System 450
A13.4 Basic Treatment of the Energy of a Distribution of Point Charges
at Rest in a Uniform Electric Field 451
A13.5 Basic Treatment of Permanent and Induced Molecular Dipoles
in a Uniform Static Electric Field 454
A13.6 Basic Treatment of Macroscopic Polarization and Electric
Susceptibilities 457
A13.7 Basic Treatment of the Electric Displacement for a Uniform
Static Electric Field 460
A13.8 The Implications of Using Dynamic Electric Fields 461
A13.9 More General Treatment of Energy of Interaction of Point Charges
at Rest in a Static Electric Field 462
A13.10 Interaction of Charges in Motion with a Static Magnetic Field 466
Reference 470
A14 The Polarizability Tensor 471
A14.1 Introduction 471
A14.2 The Polarizability Tensor in the Cartesian Basis 472
A14.2.1 General considerations 472
A14.2.2 Reduction of the tensor 473
A14.2.3 The polarizability ellipsoid 475
A14.2.4 Transformation of ca under rotation of axes 477
A14.3 The Polarizability Tensor in the Spherical Basis 477
A14.3.1 General definitions 477
A14.3.2 Reduction of the tensor sa 479
A14.3.3 Transformation of sa under rotation of axes 481
A14.4 The Relation Between the ˛ and the ˛ 481
A14.5 Irreducible Polarizability Tensors and their Components 482
A14.6 Transformation Properties of the ˛ j
m under Rotations 486
A14.7 Isotropic Averages and Tensor Invariants 486
A14.7.1 General considerations 486
A14.7.2 Isotropic averages and rotational invariants G j in terms
of the ˛ j
m 487
A14.7.3 Isotropic averages and rotational invariants G j in terms
of the ˛ 488
A14.7.4 Isotropic averages and rotational invariants G j in terms
of the ˛ 490
A14.7.5 Isotropic averages and the rotational invariants G j ,
a, υ and
for the cartesian basis 490
A14.7.6 Isotropic averages and rotational invariants, G j , a,
υ and
for the spherical basis 494
References 495
Contents xvii
A15 The Optical Activity Tensors, G, G , A and A 497
A15.1 Introduction 497
A15.2 Isotropic Averages of the Type haG0i 497
A15.3 Isotropic Averages of the Type haAi 499
A15.4 Other Tensor Invariants 500
Reference 501
A16 Maxwell Equations in Vacuum and in Media 503
A16.1 The Fundamental Equations 503
A16.2 Case I: the Maxwell Equations in Vacuum 507
A16.3 Case II: the Maxwell Equations in a Linear Medium
with D 0 and D 0 508
A16.4 Case III: the Maxwell Equations in a Non-Linear Medium
with D 0 and D 0 509
A16.5 Case IV: the Maxwell Equations in a Linear Medium
with 6D 0 and 6D 0 511
A17 Monochromatic Plane Harmonic Waves in Vacuum and in
a Non-absorbing Linear Medium 513
A17.1 General Wave Equation in Vacuum 513
A17.2 Monochromatic Plane Harmonic Electromagnetic Wave in
Vacuum 514
A17.2.1 Solution of the wave equation for the vector E 514
A17.2.2 Solution of the wave equation for the vector B 518
A17.2.3 Energy considerations for a plane harmonic
electromagnetic wave in vacuum 522
A17.3 The Exponential Representation of Harmonic Waves 526
A17.4 Monochromatic Plane Harmonic Wave in a Homogeneous,
Isotropic and Linear Medium 530
A18 The Transition Polarizability Tensor a fi 533
A18.1 Introduction 533
A18.2 The Restrictions r 6D i, f 533
A18.3 The Relative Signs of ir 535
References 536
A19 Clebsch–Gordan Coefficients and Wigner 3-j and 6-j Symbols 537
A19.1 Introduction 537
A19.2 Clebsch–Gordan Coefficients 538
A19.3 Wigner 3-j Symbols 544
A19.4 Wigner 6-j Symbols 546
Reference 550
Reference Table A19.1 551
xviii Contents
A20 Sources of Electromagnetic Radiation 555
A20.1 Introduction 555
A20.2 The Oscillating Electric Dipole as a Source 555
A20.3 The Oscillating Magnetic Dipole as a Source 561
A20.4 The Oscillating Electric Quadrupole as a Source 562
A20.5 Scattering from Chiral Molecules 564
A21 Polarization of Electromagnetic Radiation 565
A21.1 Introduction 565
A21.2 States of Polarization: Monochromatic Radiation 565
A21.2.1 Linear polarization 565
A21.2.2 Elliptical and circular polarization 566
A21.2.3 Stokes parameters 570
A21.2.4 Stokes parameters for scattered radiation 572
A21.3 States of Polarization: Quasi-Monochromatic Radiation 573
A21.4 Change of Polarization: Depolarization Ratios, Reversal
Coefficients and Degrees of Circularity 575
Further Reading 579
Index 585
xix
Preface
Many ingenious practizes in all trades, by a connexion
and transferring of the observations of one Arte, to the
use of another, when the experiences of severall misteries
shall fall under the consideration of one man’s mind.
Francis Bacon
Raman spectroscopy is now finding wide-ranging application in pure and applied science
and the number of original papers devoted to this area of spectroscopy continues to grow.
This is largely the result of significant advances in the equipment available, particularly
laser excitation sources, spectrometers, detectors, signal processors and computers.
It seems timely, therefore, to provide an integrated treatment of the theory underlying
Raman spectroscopy. Of course there are already a number of edited books and reviews
dealing with various aspects of the subject, but this book is the result of the phenomenon of
Raman spectroscopy falling ‘under the consideration of one man’s mind’ as Francis Bacon
put it. My objective has been to present a unified theoretical treatment which is reasonably
complete and adequately rigorous but nonetheless readable. My hope is that this will
provide a sound basis for the effective use of more highly specialized review articles.
As to completeness, I have had to put some restrictions on the coverage, partly because
the subject is so vast and partly because of my own limitations. Therefore the treatments
developed here relate mainly to scattering by a system of freely orienting, non-interacting
molecules or by systems which approximate to this. As to rigour, I have endeavoured to
explain in words, as far as possible, the inwardness of the mathematics and physics which
are necessarily involved. I have particularly tried to avoid taking refuge behind that often
overworked phrase ‘as is well known’.
An effective theoretical treatment demands a variety of carefully honed mathematical
and physical tools. To keep the treatment in the main text uncluttered, these tools are
xx Preface
developed in comprehensive Appendices to which cross-references are made in the main
text. These Appendices should also ensure that the main text is useful to readers with a
wide variety of scientific backgrounds and experience.
As far as possible the symbols used to represent physical quantities are based on the
IUPAC recommendations but to avoid excessive embroidery with subscripts and superscripts
it has been necessary to introduce a few new symbols, all of which are clearly
defined. The SI system of units is used throughout, except in those few instances where
spectroscopists commonly adhere to historical units, as for example the unit cm 1 for
wavenumber and related quantities.
In the main text I have limited citations of the literature. This has enabled me to use the
Harvard system and quote names of authors and the dates of publication directly in the
text. I find this preferable to the anonymity and lack of historical sequence which result
from the use merely of reference numbers in the text. More complete lists of publications
are provided in the section entitled Further Reading, located at the end of the book.
The writing of this book has been a somewhat lengthy process and I have had to learn a
great deal along the way despite more than 50 years of work in this field. Happily I have
been able to find the time and the energy required. The University of Bradford enabled me
to free myself of administrative responsibilities by allowing me to take early retirement
and then reappointed me in an honorary capacity and provided excellent facilities for
research and scholarship. I am very grateful for these arrangements.
I am also much indebted to a number of other universities and institutions which
invited me to spend short periods with them so that I could use their libraries and benefit
from discussions with colleagues. In France I would mention the National Laboratory for
Aerospace Research (ONERA), and the Universities of Bordeaux I, Lille, Paris VI and
Reims. In Italy I would mention the University of Bologna and the European Laboratory
for Nonlinear Spectroscopy (LENS), attached to the University of Florence. In this country
I have made frequent use of the excellent facilities of the Radcliffe Science Library,
Oxford University. My periods in Oxford were made all the more pleasurable by the
kindness of my old Oxford College, Jesus, in making me a supernumerary member of its
Senior Common Room for a period. Nearer home, the J. B. Priestley Library of Bradford
University has also been much consulted and I am particularly indebted to Mr John
Horton, deputy librarian, for his unstinting help.
The following friends in the community of Raman spectroscopists have kindly read
and commented fruitfully upon sections of this book: A. Albrecht, D. L. Andrews,
L. D. Barron, J. Bendtsen, H. Berger, A. D. Buckingham, R. J. H. Clark, T. J. Dines,
H. Hamaguchi, S. Hassing, L. Hecht, M. Hollas, W. J. Jones, W. Kiefer, I. M. Mills,
O. Mortensen, H. W. Schr¨otter, G. Turrell, A. Weber, R. Zare and L. D. Ziegler. I
acknowledge with gratitude their efforts which have eliminated many errors and
ambiguities. I am also grateful to Claude Coupry and Marie-Th´er`ese Gousset for providing
Plate 5.1 and to H. G. M. Edwards for providing Plate 5.2.
I would also like to record my appreciation of the patience and continued support
of John Wiley and Sons. I would mention particularly Dr Helen McPherson, Chemistry
Publisher, who has been most considerate throughout; and Mr Martin Tribe who has been
an efficient and imperturbable Production Coordinator.
Preface xxi
Very special thanks are due to my wife Moira. She has very ably undertaken most
of the word-processing of the text, helped considerably with the style and presentation,
and provided loving encouragement. Rupert, another member of the family, has also
earned honourable mention. His insistence upon regular walks has helped to offset the
sedentary effects of authorship and his relaxed presence in my study has been calming
and companionable.
It would be unrealistic to expect that this wide-ranging book will be entirely free of
errors. In the words of Evan Lloyd, an eighteenth century Welshman, also a graduate of
Jesus College, Oxford, I can only plead that
‘Earnest is each Research, and deep;
And where it is its Fate to err,
Honest its Error, and Sincere.’
December 2001 Derek A. Long
Acknowledgements
Comme quelqu’un pourrait dire de moi
que j’ai seulement fait ici un amas de fleurs ´etrang`eres,
n’ayant fourni du mien que le filet `a les lier.
M. E. Montaigne
I am grateful for permission to reproduce the material listed below.
Figure 5.11, Plate 5.2 – Edwards, H. G. M., Russell, N. C. and Wynn-Williams, D. D.
(1997) Fourier-Transform Raman Spectroscopic and Scanning Electron Microscopic Study
of Cryptoendolithic Lichens from Antarctica. J. Raman Spectrosc., 28, 685, John Wiley
& Sons, Ltd, Chichester. Reproduced with permission from John Wiley & Sons, Ltd,
Chichester.
Figure 6.13 – from Bendtsen, J. (1974). J. Raman Spectrosc., 2, 133. Reproduced with
permission from John Wiley & Sons, Ltd, Chichester.
Figure 6.14 – from Bendtsen, J. and Rasmussen, F. (2000). J. Raman Spectrosc., 31, 433.
Reproduced with permission from John Wiley & Sons, Ltd, Chichester.
Table 7.1 – from Mortensen, O. S. and Hassing, S. (1980). Advances in Infra-red and
Raman Spectroscopy, volume 6, 1, eds. R. J. Clark and R. E. Hester, Wiley-Heyden,
London. Reproduced with permission from John Wiley & Sons, Ltd, Chichester.
Figures 7.6–7.12 inclusive – from Clark, R. J. H. and Dines, T. J. (1986). Resonance
Raman Spectroscopy and its Application to Inorganic Chemistry. Angew. Chem. Int. Ed.
Engl., 25, 131. Reproduced with permission from Wiley-VCH Verlag GmbH, Weinheim.
xxiv The Raman Effect
Figures 7.14–7.18 inclusive – from Kiefer, W. (1995). Resonance Raman Spectroscopy
in Infrared and Raman Spectroscopy, ed. B. Schrader. VCH Verlag GmbH, Weinheim.
Reproduced with permission from Wiley-VCH Verlag GmbH, Weinheim.
Figures 8.2, 8.3, 8.5, 8.6 – from Ziegler, L. D. (1986). J. Chem. Phys., 84, 6013. Reproduced
with permission of the American Institute of Physics.
Table 8.2 – from Ziegler, L. D. (1986). J. Chem. Phys., 84, 6013. Reproduced with permission
of the American Institute of Physics.
Figure 10.1 – from Nafie, L. A. and Che, D. (1994). Theory and Measurement of Raman
Optical Activity, in Modern Non-linear Optics, part 3, eds. M. Evans and S. Kielich.
Reproduced with permission from John Wiley & Sons, Inc., New York.
Figures A8.6, A8.7, A8.8 – from Barron, L. D. (1983). Molecular Light Scattering and
Optical Activity. Cambridge University Press, Cambridge. Reproduced with permission
from Cambridge University Press.
Figure A8.10a, b, c – from Atkins, P. W. (1983). Molecular Quantum Mechanics, Oxford
University Press, Oxford. Reproduced by permission of Oxford University Press.
Reference Table A19.1 – from Zare, R. N. (1988). Angular Momentum, John Wiley &
Sons, Inc.: New York. Reproduced with permission from John Wiley & Sons, Inc., New
York.
I also acknowledge my indebtedness to the literature of Raman spectroscopy in general
and in particular to the following authors and their publications upon which I have
drawn for the topics stated. For scattering by chiral systems, reviews and books by
L. D. Barron, L. Hecht and L. A. Nafie; for rotational and vibration-rotation Raman scattering,
reviews by S. Brodersen, by W. J. Jones and by A. Weber; for vibrational and
electronic resonance Raman scattering, reviews by R. J. H. Clark and T. J. Dines, by
H. Hamaguchi and by W. Kiefer; for rotational and vibration-rotation resonance Raman
scattering, reviews by L. Ziegler; and for irreducible transition polarizability tensors, a
review by O. S. Mortensen and S. Hassing. Full references will be found in the Section
entitled Further Reading.
The treatments of the classical theory of Rayleigh and Raman scattering, vibrational
Raman scattering and the properties of electromagnetic radiation and many of the Reference
Tables are based upon my earlier book, the copyright of which has been assigned
to me by the original publishers.
D. A. Long.

The Mathematical Beauty of PHysics A Memorial Volume By J M Doruffe and J B Zuber free download

The Fundamental Constants A Mystery of Physics free download





PREFACE
It was already an important question to the philosophers of antiquity: of
what does matter fundamentally consist? When repeatedly dividing some
piece of matter such as wood or metal or a diamond, does one reach a
limit? If so, how does this limit manifest itself? Are there indivisible
objects; is there a smallest possible object? Or does the limit exist only
in the sense that any further division would seem to make no sense or be
experimentally impossible?
Every careful observer of natural phenomena is bound to be impressed
by the colorful and stunning diversity of the material world. One soon
notes, however, that there does not exist total chaos in these phenomena.
Things repeat themselves. A diamond here and another in some other
place are as alike as peas in a pod. The leaves of an oak in Boston are
indistinguishable from those of an oak in Denver.
So, within the uncountable diversity of phenomena, there are
also constants, things that repeat themselves. It was this duality of
vi THE FUNDAMENTAL CONSTANTS
multiplicity and constancy that inspired the Greek philosophers, most
notably Leucippus of Milet and his student Democritus of Abdera in the
5th century BC, to formulate the hypothesis that the universe is made
up of many small, indivisible building blocks called atoms (derived
from the Greek word atomos, which means essentially the same thing as
indivisible). A small number of different atoms and their unending new
combinations should suffice, so they claimed, to make up the diversity
of things. “Nothing exists,” spoke Democritus, “except atoms and empty
space.”
A slightly different concept was brought into play by Anaxagoras
around 500 BC. He spoke of an infinity of basic elements which, by
mixing, would produce the diversity of objects in the world. These basic
elements were furthermore said to be indestructible, and the changes
observable in physical objects were considered the result of motion
causing new combinations of the elements. Empedocles, who was ten
years younger than Anaxagoras, postulated that the basic materials of
the world were the four elements earth, water, air, and fire.
It is interesting to note that it is in the conceptualization of the atom
that empty space plays a role for the first time. Up to this point, space
had been seen as filled with matter, and the idea of empty space was
therefore unthinkable. In the context of the theory of the atom, empty
space took on an important function. It became the bearer of geometry,
the structure in which the atoms moved. So now matter and geometry
are two different things.
Atoms move in space and they have geometrical qualities.
Democritus said: “Just as tragedy and comedy can be written down with
the same letters, different phenomena can be realized in our world by
the same atoms, provided they assume different positions and different
movements. Some given matter may have the appearance of a given
color; may appear to us as tasting sweet or bitter — but in reality there
are only atoms and empty space.”
PREFACE vii >>>
Later the Greek philosophy adopted the elements of the theory of
the atom and developed the idea further. Plato, in his dialogue Timaeus,
discusses possible connections between atoms and the Pythagorean
theory of the harmony of numbers. For instance, he identified the
atoms of the elements earth, water, air, and fire with the regular solids,
the cube, octahedron, icosahedron, and tetrahedron. In speaking of the
motion of the atoms, special reference is made to natural causality.
Atoms are not moved by forces like love and hate, rather their motion is
the consequence of true natural laws.
What started two and a half millennia ago on what is nowadays the
west coast of Turkey was nothing less than the beginning of a revolution
that continues today. For thousands of years before then, mankind had
seen what occurred in the world as coming from some primarily mystic
source. Magic and superstition ruled the world.
That changed 2500 years ago on the Ionian coast. The time and place
were no accident. In the city states of the Ionian coast democratic values
had taken hold. New ideas were accepted easily and could spread quickly.
This was due in part to the switch around that time from hieroglyphic
symbols to an alphabet. Religion played no or only a subordinate role.
Thus the idea that our world is somehow in the end knowable, and that
natural processes can be analyzed with a rational mind, gained ground.
“Atomism” was the very beginning of this development. The thread that
winds its way through history from the Ionian coast 2500 years ago to
this highly scientific and technological present day essentially concerns
our knowledge of the building blocks of matter.
Many details about atomic theory as taught in antiquity were preserved
for us by a chance event that occurred in Italy in 1417: a manuscript by
Lucretius, a Roman poet and philosopher, was discovered. In this script,
De rerum natura, which is composed in a measured hexameter,
Lucretius not only describes the ideas of Leucippus and Democritus but
also further develops them. In the work of Lucretius, the atomism of
antiquity reaches its highest form. This book was one of the first to be
viii THE FUNDAMENTAL CONSTANTS
made after the invention of printing. Copies spread all over Europe and
have influenced scientists ever since.
In the work of Lucretius, one finds the best and most detailed
description of the atomic theory of antiquity, but, in the end, the theory
could not prevail against Plato’ and Aristotle’s system of ideal forms.
His work combined scientific questioning and the demystification of
nature with a deep respect for nature and its immutable rules.
Had Lucretius’s teachings prevailed two millennia ago, the course of
world history would have been different. It would have been less marked
by religious excesses and the religious wars in Europe and Asia. Alas,
the reality was different.
After the collapse of the Roman Empire, the western world sank
into intellectual oblivion for more than a millennium, dominated
by religious fanaticism and superstition. It was not until the Italian
Renaissance that the brilliant intellectual clarity of Greek thought
came to wide parts of Europe again, after being lost for more than a
thousand years. The scientific epoch commenced then, led by such
heroes of the mind as Copernicus, Leonardo da Vinci, Johannes Kepler,
and Galileo Galilee.
In the 17th century, the atomism of the philosophers of antiquity was
combined with scientific ideals for the first time. At this time, scientists
came to the conclusion that chemical elements such as hydrogen, oxygen,
and copper were composed of similar atoms. Isaac Newton, the inventor
of mechanics and thereby also the inventor of theoretical physics, was
even of the opinion that the consistency of materials, the hardness of a
metal for example, was somehow linked to the forces between atoms.
In the second half of the 19th century, atomism was applied with
success to the field of chemistry. Chemists found that chemical reactions
were best understood if one assumed the substances involved to be made
up of small, indivisible building blocks, or atoms. A chemical element,
hydrogen say, was thought to consist of a single type of atom. And
chemical methods made it possible to determine the approximate size of
PREFACE ix >>>
such an atom: 10−8 cm. One billion hydrogen atoms stacked on top of
one another would reach a height of around 10 cm.
Nowadays, we know of 110 different elements, that is to say, 110
different kinds of atom. This fact would have posed a serious problem
for the ancient Greek philosophers: they would certainly never have
considered it possible that more than 100 different atoms exist. For
the first time, there were doubts about whether the atom was truly an
indivisible entity. In the end, it was physicists, not chemists, who
realized, at the start of the 20th century, that atoms are not indivisible in
the sense understood by the ancient Greeks. They discovered that atoms
are made up of smaller components: electrons, the particles that make up
the atomic shells, and the atomic nucleus, which constitutes the greatest
part of the mass of an atom. Atoms became complicated systems.
In the 1920s and 1930s, atomic physics had its great breakthrough.
With the help of the newly developed field of quantum mechanics, it was
possible for the first time to understand atoms, and hence the structure
of atomic matter, both quantitatively and qualitatively on the basis of
a small number of principles. Most of the problems that physicists and
chemists had grappled with in the previous century could now be solved
elegantly.
Physicists then applied the same principles to the atomic nucleus,
in the hope that they would quickly reach a similar, more profound
understanding of the nucleus. But this hope was not to be fulfilled.
They discovered that the atomic nucleus, far from being an indivisible
object, is made up of nuclear particles called protons and neutrons.
This knowledge, however, hardly helped to reveal very much about the
properties of the atomic nucleus itself.
Soon it was observed that when particles are collided at very high
energy, new particles are created. Nobody had anticipated this. Einstein’s
now famous equation E = mc2, which states that matter and energy are
interchangeable, was seen in full effect. A whole zoo of new particles
x THE FUNDAMENTAL CONSTANTS
was discovered. Some physicists despaired at this confusing diversity
and compared subnuclear physics to botany.
Finally, between 1960 and 1980, a breakthrough was achieved that
brought order to the chaotic world of subnuclear physics. The 20th
century will enter into history as the epoch in which the substructure
of matter was largely elucidated. Today we know that normal matter is
made up of quarks, which are the building blocks of the atomic nucleus,
and electrons. In the 1970s, a clear picture of the microstructure of
matter finally evolved, often prosaically named the Standard Model of
particle physics. This model also describes the fundamental interactions
qualitatively and quantitatively in a simple form. The interactions are the
chromodynamic force, which acts between quarks, and the electroweak
force, which acts between quarks and leptons, such as electrons.
The Standard Model is, however, far more than a theoretical model of
the elementary particles and their interactions. Its claim to fame is that of
a complete unified theory for all the observed phenomena associated with
elementary particles. For specialists, the whole theory can be reduced
to a couple of lines. This makes it something like the Weltformel that
theoretical physicists such as Albert Einstein and Werner Heisenberg
had unsuccessfully looked for in the past.
Could this theory prove to be a last and thereby final truth? Are
electrons and quarks indeed nature’s elementary objects, meaning that
physicists have finally found the atoms of Democritus and Lucretius?
The answer to this question remains undecided. The Standard Model
has a number of unsatisfactory characteristics, so many physicists today
assume that it is merely an approximation, albeit a very well-functioning
approximation, to a more comprehensive theory. If so, physicists should
soon find evidence in their experiments for phenomena not explained by
the Standard Model, perhaps even evidence for a new substructure of
matter.
Electrons and quarks are not simple building blocks that can be
combined at will. They are subject to forces such as electromagnetic
PREFACE xi >>>
forces which are, in turn, transmitted by small particles. That is why
in particle physics we should avoid speaking of fundamental forces
acting on particles, rather we should speak of interactions between
particles.
It turns out that interactions in the Standard Model are governed by
very specific laws that are based on symmetry. Symmetry in nature and
elementary particles are closely connected. Plato had already referred to
such a connection in antiquity. Werner Heisenberg, one of the founders
of quantum mechanics and one of the most important physicists of the
20th century, had the following to say: “For Plato is the elementary
particle not an unchangeable and indivisible object. The elementary
particle is reduced to mathematics. The roots of the observed phenomena
are not matter, but the mathematical law and the underlying symmetry.”
Are ideas finally more important than matter? Or does the distinction
between matter and ideas disappear as we attempt to describe the limits
of particle physics? Until today, the answer to this question is undecided;
it is even unclear whether this is a valid question at all.
Our lives are full of change. Everything is in flux, nothing stays as it
was. But this is not the whole truth. There is a continuity in the world
that allows us to predict objects and occurrences. We scientists discover
that some things stay the same, the laws of nature, for instance. These
laws, however, depend on strange numbers that we call the constants
of nature. Experiments allow us to determine these numbers with ever
increasing precision. But the more precise our answer, the stranger these
numbers seem.
The constants of nature reflect a profound knowledge of the universe.
They characterize our universe. The fact that they exist tells us that, in
other regions of our universe, laws similar to our own apply. But other
universes could exist and could have different natural constants. Some
physicists don’t even speak of “the” universe anymore, because it may
not exist in such a form. They speak of a whole collection of universes,
called the multiverse.
xii THE FUNDAMENTAL CONSTANTS
At the same time, these natural constants stand for our knowledge
of the universe and they stand for our ignorance. We do not know how
their values come to be. Introducing values that cannot be derived but
which must be fixed by experimentation is not satisfactory. Scientists
cannot be reconciled to this situation.
Are the values of the natural constants a consequence of some hitherto
unknown natural laws, or are they merely a random product of the Big
Bang? We do not know. To date, no one has been able to explain the
value of any of the natural constants. This is one of the mysteries of
science, perhaps the biggest mystery in our world.
Life in the universe is only possible for certain very special values of
these natural constants. Why do the natural constants take precisely these
values here in our universe? We do not know. What is clear, however, is
that if they had other values, life could not exist and there would be no
one around to ask the question.
The problem of the natural constants arose due to the increased
precision in determining their values that particle physicists have been
capable of since the 1970s. That is why the discussions in this book
center around particle physics. Quantum mechanics is also discussed
along with questions concerning the Big Bang and astrophysics.
It was the Scottish physicist James Clerk Maxwell who first
suggested basing certain standards on microscopic objects that are to be
found everywhere, such as molecules. Up to that point, standards had
been set using objects specially designed for the purpose. Maxwell was
impressed by the fact that hydrogen molecules, for example, are the same
everywhere, unlike large bodies which have their own particularities. As
president of the British Association for the Advancement of Science,
a society formed in the 19th century and modeled on the Gesellschaft
Deutscher Naturforscher und Ärzte (a society of German scientists and
medical doctors), Maxwell wrote: “If, then, we wish to obtain standards
of length, time, and mass which shall be absolutely permanent, we must
seek them not in the dimensions, or the motion, or the mass of our planet,
PREFACE xiii >>>
but in the wavelength, the period of vibration, and the absolute mass of
these imperishable and unalterable and perfectly similar molecules.”
And indeed these ideas of Maxwell are now followed. Our measure
of length, for instance, is set by the wavelength of light emitted by atoms
of krypton-86. Time is measured by caesium clocks using an atomic
transition of caesium.
A large portion of this book deals with questions related to the natural
constants that have been introduced in today’s Standard Model, or rather,
that have had to be introduced in this model. These constants define, to
a large degree, the structure of our world. This gives me the opportunity
to report on some questions on which I worked with Murray Gell-Mann
at the California Institute of Technology in the ’70s.
Towards the end of the book, a different question is addressed: are the
natural constants really constant? Or are they, however minimally, time
dependent? By examining the light emitted by distant quasars, which
takes billions of years to reach us, it is possible to study the natural
constants in the past. People have found a small yet measurable time
dependency of the fine structure constant. Should these measurements
truly be correct, the consequences are not yet foreseen. The natural
constants could also be minimally dependent on the location in space.
That means that the natural constants could have different values in
different parts of the universe.
The natural constants confront us with one of the most profound
riddles of our universe. Where do they come from? Are they really
absolutely constant? Are they the same everywhere? Are they dependent
on one another? At this point in time, we can give no definitive answers
to these questions. Albert Einstein believed the natural constants to be
fixed by the interactions, thus excluding any freedom. To date, however,
we have not seen any way of verifying this. Presumably there is freedom
in the choice of the constants. It may take hundreds of years to be sure.
At the beginning of the new millennium, particle physics is
faced with new challenges, and in all probability physics stands at
xiv THE FUNDAMENTAL CONSTANTS
Murray Gell-Mann (right) and the author Harald Fritzsch (left) in Berlin in 1995.
the threshold of new and important discoveries. In the 20th century,
physicists delved ever deeper into the inner workings of matter. New,
previously unknown worlds were accessed, new horizons opened up.
The structure of the microphysical world became visible. This complex
world can be described by a surprisingly simple theory, a theory that can
be formulated mathematically.
The questions to be answered become ever more fundamental.
Where does matter come from? What happens to it in the distant future?
Where do the natural constants come from? Particle physics remains
a great adventure. When a new experiment begins, usually after years
of preparation, the physicists involved start on a journey into terra
incognita.
The abyss between the world of particle physics and the everyday
world has become huge. This book aims to reduce this abyss. Why
should one set oneself the goal of exploring particles and phenomena
that have practically nothing to do with our everyday life? The reasons
PREFACE xv >>>
are the same as those that drive scientists to explore outer space, to push
ever further into the depths of our oceans, or to overcome other frontiers.
As with all fundamental research, particle physics is part of our culture,
part of our effort to gain a rational understanding of the cosmic order.
Fundamental research in physics is, to a large extent, particle physics.
The considerable investments made in this field have played a big role
in the development of the open and enlightened societies that we find in
most parts of the world today. The fascinating insights into the world of
the microcosmos that particle physics has allowed us can be counted as
one of the lasting achievements of the last century.
One of the essential goals of this book is to inform the general public,
not primarily physicists, about the problem of the natural constants.
To this end, I use a format that has been used with success before: an
imagined dialogue between real and fictitious persons. In this book the
persons are Isaac Newton, Albert Einstein, and a modern-day physicist
named Adrian Haller, who comes from the University of Bern and is
serving as a guest professor at the California Institute of Technology
(Caltech) in Pasadena.
The dialogue form has been known since Plato’s dialogues in ancient
Greece. The famous dialogue Timaeus, for example, features the three
figures of Critias, Socrates, and Timaeus. Another example is Galileo
Galilee’s dialogues published in the famous Discorsi from the Middle
Ages. The format is useful because the reader is often confronted with
questions that he may have wanted to ask, and these questions are then
answered. The discussions in this book take place in California, in places
where I was active years ago.
I wish the reader pleasure, new insights and success in understanding
the problem of the fundamental constants.

SUPERSYMMETRY AND COSMOLOGY By Jonathan L. Feng free download











Contents
1 Introduction 3
2 Supersymmetry Essentials 4
2.1 A New Spacetime Symmetry . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Supersymmetry and the Weak Scale . . . . . . . . . . . . . . . . . . . 5
2.3 The Neutral Supersymmetric Spectrum . . . . . . . . . . . . . . . . . . 7
2.4 R-Parity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5 Supersymmetry Breaking and Dark Energy . . . . . . . . . . . . . . . 9
2.6 Minimal Supergravity . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3 Neutralino Cosmology 15
3.1 Freeze Out and WIMPs . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2 Thermal Relic Density . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.2.1 Bulk Region . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2.2 Focus Point Region . . . . . . . . . . . . . . . . . . . . . . . . 20
3.2.3 A Funnel Region . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.2.4 Co-annihilation Region . . . . . . . . . . . . . . . . . . . . . . 22
3.3 Direct Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.4 Indirect Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.4.1 Positrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.4.2 Photons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.4.3 Neutrinos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4 Gravitino Cosmology 34
4.1 Gravitino Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.2 Thermal Relic Density . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.3 Production during Reheating . . . . . . . . . . . . . . . . . . . . . . . 38
4.4 Production from Late Decays . . . . . . . . . . . . . . . . . . . . . . . 39
4.5 Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.5.1 Energy Release . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.5.2 Big Bang Nucleosynthesis . . . . . . . . . . . . . . . . . . . . 43
4.5.3 The Cosmic Microwave Background . . . . . . . . . . . . . . . 47
2
4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5 Prospects 49
5.1 The Particle Physics/Cosmology Interface . . . . . . . . . . . . . . . . 49
5.2 The Role of Colliders . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.3 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
6 Acknowledgments 55
References 55