Foundations of Modern EPR

The following sections are included:

• PREFACE

• CONTENTS

• CONTRIBUTORS

The golden jubilee of the Dutch Physical Society in April 1971 was concluded with a lecture by Samuel Goudsmit on the history of the discovery of the electron spin. Actually, his could hardly be called a polished lecture; it was a grandiose artistic performance, full of wit and emotional involvement. Goudsmit, then at the end of his scientific career, gave a very personal account of how chance and the guidance by Ehrenfest, their far-sighted supervisor, led him and Uhlenbeck to formulate their remarkable discovery. When, in connection with the present book the question came up how to discuss the early history of electron spin, my thoughts returned to that day, nearly twenty-five years ago, when I had been impressed by Goudsmit's truly humane wisdom. After weighing various alternatives I thought: why not let the master speak for himself?…

The following sections are included:

• Introduction

• Molecular Beams And Predictions Of Resonant Absorption

• Microwave Absorption

• Nonresonant Paramagnetic Relaxation

• Crystal Field Splittings

• Theories Of Paramagnetic Relaxation

• Molecular Beam Resonance Experiment

• NMR, But The Relaxation Time Was Too Long

• Radio Detecting And Ranging

• Discussion

• Acknowledgment

• References

The following sections are included:

• Introduction

• Atoms

• Molecules

• Condensed Matter

• Crystal Field Theory

• Gyromagnetic, Optical, And Thermal Effects

• Spin-Spin Interactions

• Spin-Lattice Interaction

• Conclusion

• References

The following sections are included:

• The Dawn

• Hyperfine Structure

• The Jahn-Teller Effect

• Cryogenic Applications

• Magnetic Cooling And Nuclear Alignment

• The Ordered State

• References

In 1944 Assistant Professor of Kazan State University Evgeny K. Zavoisky detected for the first time the Electron Paramagnetic Resonance absorption in paramagnetic substances. In these experiments, he used manganese and copper sulfates, and anhydrous chromium chloride. E. K. Zavoisky was inspired by Gorter's experiments on paramagnetic relaxation in solid paramagnetic salts. However, he replaced the calorimetric detection method, which was used by C. J. Gorter, by a sensitive electronic method - the method of grid current. For the success of Zavoisky's experiment it was also very important that he decided to apply a low frequency modulation of an external magnetic field.

The following sections are included:

• Introduction

• Acoustic Paramagnetic Resonance (APR)

• Relaxation Processes Of The Resonance Fluorescence Type

• Difficulties In The Theory Of Spin-Lattice Relaxation At Low Temperatures

• Magnetic Resonance In Van Vleck Paramagnets

• Conclusion

• References

The collaboration of three Kazan physicists: E.K. Zavoisky, S.A. Al'tshuler and B.M. Kozyrev, should be mentioned in the context of the history of the electron paramagnetic resonance discovery. Zavoisky investigated the effect of the high frequency electromagnetic field on substances back in the thirties. At that time he invited physical-chemist B.M. Kozyrev and a bit later theorist S.A. Al'tshuler, who were acquainted with him since his studying in Kazan University, to participate in this work. Their collaboration led to a close friendship that lasted for life.

The following sections are included:

• Introduction

• Motivation For Growth

• Paramagnetic Salts

• Line Widths And Shapes

• Dilute Paramagnetic Salts

• Ordered Spin Systems

• Paramagnetic Gases

• Free Radicals

• Radiation Damage

• Alkali Metals In Solution

• Nuclear Spins

• Metals

• Forbidden And Double Transitions

• Triplet States And Biradicals (also See Ch. H.3)

• Dispersion Mode

• Double Resonance and Polarization

• Maser

• Fourier Transform Approach

• The Pioneers

• Acknowledgment

• References

The following sections are included:

• Introduction

• The University of Amsterdam

• Koninklijke/Shell Laboratorium In Amsterdam

• The University Of Nijmegen

• University Of Leiden

• University Of Groningen

• Dutch State Mines (DSM), Geleen

• Technical University Of Delft

• The Technical University Of Eindhoven

• The University Of Utrecht

• Philips Research Laboratories, Eindhoven

• The Agricultural University Of Wageningen

• Epilogue

• Acknowledgement

• References

The following sections are included:

• Prologue

• EPR Studies Of Solids

• Epilogue

• Publications
  • Conduction Electron EPR In Metals
  • Electron–Center EPR
  • Conduction Electron EPR In Semiconductors
  • Impurity Level EPR In Semiconductors

• Conference Reports

• Abstracts And Talks
  • Conduction Electron EPR In Metals And Semiconductors
  • Electron–Center EPR

• Doctoral Dissertations

• Visitors

Harvard University housed a distinguished group of faculty in the Department of Physics and in the Division of Engineering and Applied Physics who had made numerous seminal contributions to magnetic resonance spectroscopy by 1957, the year of my arrival in the Chemistry Department as a freshman Instructor. Among these faculty, Professors G. Benedek, N. Bloembergen, R. V. Pound, E. M. Purcell, N. F. Ramsey, and J. H. van Vleck come quickly to mind. (R. L. Orbach had yet to arrive at Harvard from the Clarendon Laboratories of Oxford University. He was appointed Assistant Professor of Applied Physics at Harvard in 1961.)…

The following sections are included:

• Instrumentation
  • At TIFR
  • At Other Places

• Organic Free Radicals
  • Dynamic Nuclear Polarization(DNP)
  • Chemically Induced Dynamic Electron Polarization (CIDEP) As Studied By Time Domain EPR (TDEPR)

• Relaxation Studies

• Oscillating Reactions

• Radiation-Induced Paramagnetic Centres

• Centres Produced By Doping In The Solid State

• Transition Metal Complexes

• Exchange Interactions

• Phase And Ferroelectric Transitions

• High Pressure Studies

• Glassy And Amorphous Materials

• High T_C Materials And Related Topics

• Biological Studies

• Special Events

• Conclusions And Acknowledgments

• References

The following sections are included:

• Introduction

• Hyperfine Structure In EPR Of Transition Metal Ions Prior To 1953

• Work Done At The University Of California In Berkeley

• After 1957

• Origins Of The Hyperfine Interaction

• References

In the mid-fifties, chemists had already discovered NMR (1) and were beginning to exploit this phenomenon, although it would be many years before it was to become the standard analytical tool that it is today. But there were few taking advantage of ESR despite the fact that it was invented (2) before NMR (3) and was techincally easier to utilize. Of course, Bleaney (4) had already outlined the field of ESR in his work on transition metals, but soon thereafter chemists (5,6) led by the distinguished work of Sam Weissman (5), took advantage of this tool and applied it to organic free radicals as well as to inorganic complexes.

The following sections are included:

• Properties Of Lanthanide Ions

• The Crystal Field
  • The Spin Hamiltonian And EPR

• Orbit-Lattice Interaction And Spin-Lattice Relaxation
  • The Spin-Lattice Hamiltonian And The Effect Of External Stress

• Hyperfine Structure
  • Electron Nuclear Double Resonance (ENDOR)
  • Self ENDOR
  • Ligand ENDOR
  • ENDOR Of Distant Nuclei

• Interactions Between Paramagnetic Ions

• Non-Kramers Ions

• Materials Investigated
  • Structural Phase Transitions
  • Sites Of Low Symmetry

• Conclusion

• References

The following sections are included:

• Introduction

• Copper Acetate

• Isolated Dimers In Frozen Solutions And In Powders

• General Theory Of Coupling

• High Spin Systems

• Dipole-Dipole Interaction

• Conclusions

• References

This chapter deals with some of the early magnetic resonance studies of rates and pathways of exchanges, either via electron transfer or atom transfer, between paramagnetic and diamagnetic molecules. The cited examples are of systems at equilibrium. The average concentrations of the species involved are constant but are accompanied by the fluctuations about the average that are an inevitable consequence of the stochastic nature of dynamic equilibrium.

This chapter is concerned with one radical species only, the thianthrene cation radical, Th⁺. I shall recount the history of the discovery of the structure of Th⁺ not only because it illustrates how EPR was used successfully after initial uncertainties and errors, but also because Th⁺, with its structure known, was able to play an important role later in studies of reactions of aromatic cation radicals with nucleophiles. I shall not go into those studies, however…

The following sections are included:

• Autobiographical Note

• Preface

• How Radical Ions Are Generated From Organic Compounds
  • Radical Anions
  • Radical Cations

• Multiple Resonance Techniques

• Theoretical Models

• Organic Compounds Of Which Radical Ions Have Been Studied
  • Good Electron Acceptors Or Donors
  • Moderate Electron Acceptors And Donors
  • Poor Electron Acceptors And Donors

• Outlook

• Acknowledgments

• References

I did my graduate research at the California Institute of Technology during 1947-1950. This was followed by postdoctoral research in the Physics Department of the University of Chicago, in the research group of Robert Mulliken, during 1950-52. Although I was keenly interested in the problem of the electronic structures of molecules, I was highly skeptical of the utility of both Pauling's valence bond theory and Mulliken's molecular orbital theory by the time I left Chicago. There was not one single experimental observable for “large” molecules (> diatomic) that could be used to test the various theoretical approximations. Theoretical calculations of bond dissociation energies were of little value, since they are differences between large numbers, each of which had uncertainties of the order of the dissociation energy itself. Excited state energies were particularly uncertain because of strong configuration interaction. Electric dipole moment calculations were uncertain, since they are sensitive to the long tails of wave functions, which were not known accurately. There were of course many hand-waving theories of chemical phenomena, but the overall picture was quite discouraging to me. In the early 1950's the central question was--were these theories of the electronic structures of molecules on the right track? Were they even approximately correct? Paramagnetic resonance spectroscopy did answer these questions in the affirmative and provided much impetus to further development and fruitful application of molecular electronic structure theory…

The following sections are included:

• Introduction
  • Early Beginnings
  • PBN
  • Spin Trapping
  • Other Radical Trapping Research Groups
  • Early Advantages And Disadvantages
  • DMPO
  • ND
  • POBN
  • Applications

• Milestones
  • DMPO and Hydroxyl/Hydroperoxyl-Superoxide Radicals
  • Kinetics and Rates of Spin Trapping
  • Spin Trapping of Gas Phase Radicals
  • EPR Tricks of the Trade
    • Use of Isotopes
    • Computer Simulation Program
    • Solvent Effects
    • ENDOR of Spin Adducts
  • Biological Applications
    • Biochemical Molecules
    • In vitro Systems
    • In vivo Live Animal
  • Instrumentation
  • Other Biological Effects

• Conclusions

• Acknowledgements

• References

The following sections are included:

• Oxygen, O²

• Nitric Oxide, NO

• Nitrogen Peroxide, NO²

• Atoms

• Apparatus

• References

The following sections are included:

• Initial Sparks For EPR In A Gas

• The Early Yale EPR Spectrometer

• Stable Paramagnetic Gases

• EPR Linewidths And Absorption Intensity

• Nearly Stable Nitrogen Dioxide

• Short-Lived Gases

• Applications Of EPR In Gases

• References

The following sections are included:

• In Situ Electrolysis

• Extra Muros Generation

• Controlled Potential And Controlled Current Electrolysis

• Flowing Solutions

• Spin Traps

• Inorganic Materials

• Adsorption on Electrode Surfaces

• Applications To Current Problems

• Conclusions

• The Future Of Electrochemistry – Electron Paramagnetic Resonance

• References

The following sections are included:

• Introduction

• Why Was I Interested?

• A Chemist's View Of The Action Of Ionizing Radiation

• Isoelectronic Families

• Hyperfine Coupling To α- And β-Protons

• R₃X Radicals

• Radicals Of Type R-CO, R₂C=O⁺, RCN⁻, And R₂CN

• Specific Electron Addition

• Examples Of Specific Electron-Addition

• Specific Electron-Loss

• σ*-Radicals

• Pairwise-Trapping

• Proteins
  • Radiation Processes In Proteins
  • DNA

• EPR Techniques

• The Future?

• Acknowledgments

• References

The following sections are included:

• Introduction

• Steady-State Methods

• Kinetic Measurements

• CIDEP

• Current Applications

• Acknowledgments

• References

Almost ten years elapsed following Zavoiski's discovery of the ESR phenomenon before the methodology was applied to the study of irradiated biomaterials. In 1954 Combrisson and Uebersfeld published a report of an ESR signal obtained from irradated glycine among other irradiated amino acids and carbohydrates (1). Shortly thereafter Professor Gordy took up the study of irradated biomaterials at Duke University (2). Gordy introduced a long succession of graduate students to this field of investigation. I became acquainted with several of his students and postdoctorals at American Physical Society meetings, particularly Howard Shields, Ichiro Miyagawa and Chester Alexander Jr. The springtime APS meetings held in Washington are especially memorable. It was a pleasant time of year in the capital city and Gordy's group always helped to make the meeting more interesting…

The following sections are included:

• Introduction

• Free Radicals Formed by Pyrolysis

• Irradiation Damage

• Enzymes

• Molecules Containing Transition Group Atoms

• Conclusion

• References

The following sections are included:

• Low Spin Ferric Heme

• Hemichromes

• Other Low Spin Forms

• Chlorin Proteins

• Tetrahydroporphyrin Proteins

• Conclusions

• References

The following sections are included:

• Scope Of The Chapter

• Imaging And Spectroscopy

• Why Perform In Vivo EPR Studies?

• Potential Constraints On In Vivo EPR
  • Concentration Of Paramagnetic Species
  • Bioreduction
  • Excretion
  • Physiological Motions
  • Accurate Localization Of Paramagnetic Materials
  • Penetration Of Microwaves
  • Sensitivity In In Vivo EPR

• Results Of In Vivo EPR Spectroscopy Of Functional Biological Systems

• Results Of In Vivo EPR Imaging Of Functional Biological Systems
  • Imaging Studies Of Pharmacokinetics Of Nitroxides
  • Imaging Studies Of The Concentration Of Oxygen
  • Imaging Of Diffusion In Skin And Related Systems
  • Imaging Studies At The Microscopic Level
  • Use Of Gated And Pulsed Methods In Obtaining Images
  • Results Of EPR Spectral-Spatial Imaging Of Functional Biological Systems

• Summary And Conclusions

• References

The following sections are included:

• Some Recollections

• A Brief Survey
  • Fe(II)
  • Mononuclear Fe PII)
  • Low-spin Fe(III), (S=1/2)
  • High-spin Fe(III), (S=5/2)
  • Multinuclear Fe Clusters In Proteins
  • Fe-oxo Proteins
  • Fe-S Proteins

• Recent Developments

• Iron-Storage Proteins

• The Future

• References

The following sections are included:

• Introduction

• Applications To Membranes

• Conclusion

• Acknowledgments

• References

The following sections are included:

• Introduction
  • Nitroxide Spin Labeling
  • Philosophy Of Structure Determination By Site-Directed Spin Labeling

• Analysis Of EPR Environmental Variables In Terms Of Protein Structure
  • Side Chain Dynamics
  • Side Chain Accessibilities
  • Distance Determination
  • Time Dependent Changes During The Photocycle Of Bacteriorhodopsin
  • The Future Of Site-Directed Spin Labeling As A Structural Technique For Proteins

• References

The following sections are included:

• Symposium On Free Radicals In Biological Systems (1960)

• Flavoproteins

• Substrate Free Radicals

• Ascorbic Acid Free Radical

• Semiquinone Formation Constant

• Protein Radicals

• Free Radicals In Prosthetic Groups

• Miscellaneous

• Acknowledgement

• References

The following sections are included:

• Current Status Of EPR Studies Of Cells And Tissues

• References

The following sections are included:

• Inhomogeneous Broadening

• Spectral Diffusion

• All That
  • Homogeneous And Inhomogeneous Broadening
  • Spin Diffusion, Conduction, And Localized States

• References

The following sections are included:

• Introduction

• Historical Remarks Of One Of The Authors (K.H.H.)

• Relaxation and Linewidth

• Examples

• Experimental
  • Oxygen
  • Technical Aspects

• References

The following sections are included:

• Early Observations

• The Search For The Identity Of The Intermediate State

• Research Following That Of Lewis et al.

• EPR Spectroscopy Of The Phosphorescing Molecules

• EPR Spectroscopy Of The Phosphorescing Molecules
  • Single Crystals Of Naphthalene In Durene
  • Studies Of Randomly Oriented Molecules

• Early EPR Studies Of Other Phosphorescent Molecules In Single Crystals

• References

The following sections are included:

• Introduction

• Zero-Field Basis And The “Half-Field” Signal

• In Quest Of Optical Detection

• EPR In Zero Field With Optical Detection

• Acknowledgment

• References

The following sections are included:

• Introduction

• The First Experiments

• The Early Experiments

• The Idea Of Triplet Excitons

• The Exchange Anomaly

• Resolution Of The Exchange Anomaly

• Conclusion

• References

Most chemistry textbooks treat free radicals as transient reactive species, so it was surprising to GRE (then an undergraduate student) that William N. Lipscomb suggested that Alex Kaczmarczyk might have produced a stable radical of a boron hydride in 1961. This hypothesis, which turned out to be wrong, led to a particularly memorable visit to Gus Maki's lab to search for an EPR signal. Later, while serving in the Navy GRE was perplexed by Chemical Abstracts entries about “spin sounds.” Since the original articles were not accessible then, it took some time to realize that this was a mis-translation of the French sonde! The next exposure to EPR came in an assignment as a first-year graduate student to develop, at the initiative of Richard Holm, new undergraduate laboratory exercises that combined synthesis of substituted quinones, electrochemical generation of the semiquinone radicals, and EPR. This was done on the first Varian E3. One day, while browsing in the MIT library, all of these pieces came together when an article about nitroxyl radicals was found serendipitously. This happened about the time that GRE met SSE, also a graduate student at MIT, and we got manied. We developed the idea of making complexes that included nitroxyl radicals in ligands to paramagnetic transition metals (1). If our proposal to work on this as part of our Ph.D. thesis work had been accepted, the work probably would have stopped when we graduated. However, the proposal was rejected as not worth working on, so we could only think and plan for the future. We came to the University of Denver in part because the school was willing to invest in an EPR spectrometer for someone who was not an EPR spectroscopist. Our research program owes a lot to Dwight M. Smith, the Chairman of the Department, who believed in our quest, and gave support at critical junctures…

The following sections are included:

• Introduction

• Iron-Sulfur Clusters

• Heme Proteins

• Iron-oxo Clusters

• Bioinorganic Model Complexes

• Outlook

• References

E. Zavoisky started the field of Electron Paramagnetic Resonance (EPR) in 1945 (1). About a year later Bloch et al. (2) and Purcell et al. (3) independently developed Nuclear Magnetic Resonance (NMR)¹. A decade later Electron Nuclear Double Resonance (ENDOR) was developed (6,7). ENDOR essentially combines both NMR and EPR and thus can be viewed as an extension of both techniques, in which the advantages of each are being utilized. As we are celebrating the 50th anniversary of Zavoisky's seminal work and he unfortunately is no longer among us to tell his story it might be of interest to tell the origin of ENDOR, which may be considered an extension of his work.

The following sections are included:

• Introduction

• Selected Key Experiments In The History Of ENDOR In Liquids
  • Metal-Ammonia Complexes
  • Organic Radicals
  • Liquid Phase ENDOR Intensities And Lineshapes
  • ENDOR In Liquid Crystal
  • TRIPLE Resonance As An Extension of ENDOR In Solution
  • Time-Resolved CIDEP-Enhanced ENDOR

• Selected Examples Of State-Of-The-Art Experiments
  • Multispin Systems
  • Novel Porphyrinoid π-Systems
  • Chlorophyll Ions
  • Primary Donor In Bacterial Photosynthesis

• Outlook

• Acknowledgment

• References

The following sections are included:

• Introduction

• Towards An ELDOR Spectroscopy

• Notes On ELDOR History

• ¹⁴N-¹⁵N ELDOR
  • Theory
  • Summary Of The Literature To 1989
  • ELDOR of ¹⁴N-¹⁵N Spin-Label Pairs Since 1989

• Future Methodological Developments
  • Multiquantum ELDOR
  • Multifrequency ELDOR
  • Bimodal Loop-Gap Resonator
  • Pulse ELDOR
  • ELDOR In The Presence Of A Relaxing Agent

• Acknowledgment

• References

The following sections are included:

• Introduction

• History

• Polarization Origins
  • The Net Contribution
  • The Multiplet Contribution
  • Both TM And RPM Contributions
  • Polarization In The Spin-Correlated Radical Pair
  • Hyperfine-Dependent Single-Phase Polarization
    • ST-₁ Mixing
    • The Radical-Triplet Pair Mechanism (RTPM)
  • Polarization By Cross Relaxation
  • Polarization In Secondary Radicals

• The Future

• Acknowledgments

• References

An acronym RYDMR (reaction-yield-detected magnetic resonance) proposed by E. Frankevich (1) is now used to denote methods for recording the EPR spectra of short-lived spin-correlated pairs of paramagnetic particles based on the influence of resonant microwave (mw) radiation on the yield of the reaction products of these pairs. Spin-correlated pairs arise from a precursor of a given spin multiplicity (e.g. from a singlet or triplet excited molecule) and acquire this multiplicity upon formation. In the RYDMR method a concept “reaction yield” is used in a general sense to denote any change in the system due to pair reaction. This can be the yield of final or intermediate reaction products, the intensity of excited product luminescence, the electrical conductivity of a sample, the nuclear polarization of reaction product, etc…

The following sections are included:

• Introduction

• Theory

• Instrumentation
  • Microwave Bridge
  • Resonators
  • Receivers

• Saturation-Recovery Spectroscopy

• Future Prospects
  • Instrumental
  • Applications

• Acknowledgement

• References

The following sections are included:

• Early Motivation For Relaxation Data

• Were All EPR Lines Inhomogenous?

• Optimal Concentration Of Paramagnetic Centers

• How To Identify The Relaxation Mechanisms Active In A Given Sample?

• Development Of A Pulsed Field Sweep Technique

• Relaxation As Seen By A Fast Field Sweep Technique

• Burning Holes In EPR Lines During Pulsed Field Sweep

• Relaxation In A Maser Material

• Anecdotes On Specific Defect-Host Combinations
  • Isolated F Centers in KCl crystals
  • Interacting F Centers

• Atomic Hydrogen In CaF₂

• Oxygen Vacancies (E' Centers) In Crystalline SiO₂

• Hydrogen Atoms In SiO₂ And CaF₂

• Transition Metal Ion Centers

• References

The following sections are included:

• Electric Field Effects In EPR

• What Has Been Done And What Remains To Do?

• References And Footnotes

In 1961 a unique laboratory was brought by V.V. Voevodsky from the Semenov Institute in Moscow to Siberia to the Institute of Chemical Kinetics and Combustion. In this laboratory physical chemists (researchers) were mixed with engineers, specialists in electronics, and theoretical physicists. The laboratory was prepared to solve various problems chemical physics, particularly in the highly professional environment of colleagues from the other Institutes of young Novosibirsk Akademgorodok…

The following sections are included:

• Beginnings

• Analysis Of Nuclear Modulation

• Early Applications To Single Crystals

• Early Applications To Disordered Systems

• The Role Of ESEM In Solvated Electron Structure - A Personal Account (45)

• Acknowledgment

• References

My first encounter with what was going to grow into Fourier transform EPR occurred, in retrospect, very early in my graduate studies with Professor Larry Kevan at Wayne State University. I arrived in his lab at the beginning of August 1971. Larry suggested, that in the month before classes started, I see what I could do with the saturation recovery bridge that someone else had started to put together before switching to a more productive project. The aim was to see if the T₁ of trapped electrons in alkaline ice glasses or hydrogen atoms in acid glasses could tell something about the structure of the traps. There was much interest in the trapped or solvated electron then not only from its role in radiolysis, but also from its interaction with the solvent as the simplest of anions. I started with a V153C klystron as a power source and a Philco-Ford waveguide diode switch as a modulator and a Varian V-line cavity as resonator and by the end of the month recorded my first saturation recovery curve. I then proceeded to make a few trivial improvements in my bridge and it did not function for another two years…

The following sections are included:

• Continuous Wave ESR Theory And Methods

• Time Domain ESR Theory And Methods

• Acknowledgments

• References

The following sections are included:

• The Early Days Of EPR Imaging

• Techniques
  • Gradient Coils
  • Fixed Gradients
  • Modulated Gradients
  • Spectral-Spatial Imaging
  • Dimensions
  • Microscopic EPR Imaging

• Applications
  • Radiation-Induced Defects
  • Conducting Materials
  • Magnetic Susceptibility Effects
  • Dielectric Effects On Microwave Fields
  • In Vivo EPR Imaging And Localized Spectroscopy

• The State-Of-The-Art

• Prognosis

• References

The following sections are included:

• Instrumental Development – 1954-1960

• Marketing

• Technical Information Bulletin

• NMR/EPR Workshops

• Trade Shows

• Postdoctoral Fellowships

• “EPR At Work” And The EPR Applications Laboratory

• EPR At Varian In The Sixties And Early Seventies
  • Small Accessories Including Cavities
  • Major Accessories

• E-Line and Century Series

• Spin Immunoassay – The FRAT System

• Varian In The Soviet Union

• Epilogue

• Acknowledgment

• References

The following sections are included:

• My Way To EPR

• The Early Days Of BRUKER

• The EPR Lab

• EPR At BRUKER Spectrospin Wissemboug, France

• The ER 200tt

• The ENDOR Accessory EN 200 S/E/T

• The ER 200D SERIES

• The EPR World Market In The Seventies And Eighties

• The Soviet Union

• The United States

• The China Adventure

• The Cooperation with IBM

• The Fourier Transform EPR Decade

• The Personal Computer Fever

• Multi-Frequency And Microwave Technology

• Epilogue

The following sections are included:

• Historical Preview

• Personal Reminiscences

• Progress In VHF EPR

• Modern State-of-Art
  • Magnetic Field
  • Microwave Equipment And Probe Heads
  • Pulsed EPR Experiments
  • ENDOR Experiments
  • EPR Imaging
  • Commercial Instrumentation

• Concluding Remarks

• References

The following sections are included:

• Introduction

• Development Of MQ-EPR
  • Detection Of Modulation Of M_Z (The “Naive” Phase)
  • Parallels Between MQ-EPR And Early NMR (The “Historical” Phase)
  • Intermodulation Sidebands (The “Modern” Phase)
  • MQ-EPR Instrumentation

• Multiquantum Double Resonance

• Discussion

• Acknowledgements

• References

The following sections are included:

• Introduction

• What Happened Before

• The First Experiments

• Experimental Applications And Developments

• The New Working Out Of The Method

• Conclusion

• References

The following sections are included:

• Introduction
  • The Spin-Lattice Relaxation Time T₁
  • What Is The Range Of T₁ To Be Measured In EPR And In NMR?
  • Why Develop In 1959 A Method To Measure Short Values Of T₁?

• Elementary Theory For The Modulation Method
  • The Origin Of MM
  • The EPR Signal S
  • Measurement Of Very Short Values Of T₁(3, 31)

• The Spectrometers
  • The First Modulation Spectrometer (10)
  • The Amplitude Modulation of the Microwave Field Used Only for EPR Detection
  • The Variant Developed by H. Place (34)
  • The Second Modulation Spectrometer (3, 28, 35)
  • The Third Modulation Spectrometer

• Information Obtained From The Modulation Method

• Some Applications Of The Modulation Method
  • Relaxation In Gadolinium Pure Metal Above The Curie Temperature (3, 4)
  • Magnetic Field Dependence Of Spin-Lattice Relaxation In Three Iron Group Salts (28)
  • Relaxation Mechanisms In ID Systems (41, 42, 43)

• Comparison With Two Important Techniques: ESE And MQ EPR

• Conclusion

• References

It is widely accepted that the upper sensitivity limit in conventional (CW X-band) ESR is 10¹⁰ electron spins. There are two basic reasons for this relatively small sensitivity: The first one is the small thermal population difference. The second reason is the fact that low energy photons are detected which leads to small detector efficiency and to relatively high thermal noise level. While optical photons can be detected with an efficiency near unity, many microwave photons are required to exceed the noise level. In the first part of this chapter, a very brief and partial description of the different solutions to the sensitivity problem will be given…

Many scientific fields become senescent and of mere historical interest 50 years after the initial discovery. EPR at 50 years, however, is only approaching adolescence. To look forward a bit to what EPR will develop into, let us first look for patterns from the past that continue today. In many of the contributions to this book three themes recur: personal communication, advances in instrumentation, and collaborations between spectroscopists and people with difficult problems…

The following sections are included:

• Index