UNIT V
BASIC PRINCIPLES OF ESR
Electron Spin resonance spectroscopy is based on the absorption of microwave radiation by an
unpaired electron when it is exposed to a strong magnetic field. Species that contain unpaired
electrons (namely free radicals, odd-electron molecules, transition metal complexes, rare earth
ions, etc.) can therefore be detected by ESR.
When an atomic or molecular system with unpaired electrons is subjected to a magnetic field, the
electronic energy levels of the atom or molecule will split into different levels. The magnitude of
the splitting is dependent on the strength of the applied magnetic field. The atom or molecule can
be excited from one split level to another in the presence of an external radiation of frequency
corresponding to the frequency obtained from the difference in energy between the split levels.
Such an excitation is called magnetic resonance absorption. The atom or molecule under
investigation may be in different environments in an actual sample. The magnetic resonance
frequency will hence be influenced by the local environment of the atom or molecule. The
electron spin resonance technique is therefore, a probe for a detailed identification of the various
atomic and molecular systems and their environments and all associated parameters.
INSTRUMENTATION
EPR spectroscopy can be carried out by either 1) varying the magnetic field and holding the
frequency constant or 2) varying the frequency and holding the magnetic field constant (as is the
case for NMR spectroscopy). Commercial EPR spectrometers typically vary the magnetic field
and holding the frequency constant, opposite of NMR spectrometers. The majority of EPR
spectrometers are in the range of 8-10 GHz (X-band), though there are spectrometers which
work at lower and higher fields: 1-2 GHz (L-band) and 2-4 GHz (S-band), 35 GHz (Q-band) and
95 GHz (W-band).
Figure 1: Block diagram of a typical EPR spectrometer.
1
EPR spectrometers work by generating microwaves from a source (typically a klystron), sending
them through an attenuator, and passing them on to the sample, which is located in a microwave
cavity (Figure 1). Microwaves reflected back from the cavity are routed to the detector diode,
and the signal comes out as a decrease in current at the detector analogous to absorption of
microwaves by the sample.
Samples for EPR can be gases, single crystals, solutions, powders, and frozen solutions. For
solutions, solvents with high dielectric constants are not advisable, as they will absorb
microwaves. For frozen solutions, solvents that will form a glass when frozen are preferable.
Good glasses are formed from solvents with low symmetry and solvents that do not hydrogen
bond. Drago provides an extensive list of solvents that form good glasses.
EPR spectra are generally presented as the first derivative of the absorption spectra for ease of
interpretation. An example is given in Figure 2.
Figure 2: Example of first and second derivative EPR spectrum.
Magnetic field strength is generally reported in units of Gauss or mTesla. Often EPR spectra are
very complicated, and analysis of spectra through the use of computer programs is usual. There
are computer programs that will predict the EPR spectra of compounds with the input of a few
parameters.
ESR SPECTRUM HYPERFINE STRUCTURE
When the molecules of a solid exhibit paramagnetism as a result of unpaired electron spins,
transitions can be induced between spin states by applying a magnetic field and then supplying
electromagnetic energy, usually in the microwave range of frequencies. The resulting absorption
spectra are described as electron spin resonance (ESR) or electron paramagnetic resonance
(EPR). Electron spin resonance has been used as an investigative tool for the study of radicals
formed in solid materials, since the radicals typically produce an unpaired spin on the molecule
from which an electron is removed. Particularly fruitful has been the study of the ESR spectra of
2
radicals produced as radiation damage from ionizing radiation. Study of the radicals produced by
such radiation gives information about the locations and mechanisms of radiation damage.
The interaction of an external magnetic field with an electron spin depends upon the magnetic
moment associated with the spin, and the nature of an isolated electron spin is such that two and
only two orientations are possible. The application of the magnetic field then provides
a magnetic potential energy which splits the spin states by an amount proportional to the
magnetic field (Zeeman effect), and then radio frequency radiation of the appropriate frequency
can cause a transition from one spin state to the other. The energy associated with the transition
is expressed in terms of the applied magnetic field B, the electron spin g-factor g, and the
constant mB which is called the Bohr magneton.
If the radio frequency excitation was supplied by a klystron at 20 GHz, the magnetic field
required for resonance would be 0.71 Tesla, a sizable magnetic field typically supplied by a large
laboratory magnet.
If you were always dealing with systems with a single spin like this example, then ESR would
always consist of just one line, and would have little value as an investigative tool, but several
factors influence the effective value of g in different settings. Much of the information obtainable
from ESR comes from the splitting caused by interactions with nuclear spins in the vicinity of
the unpaired spin, splitting called nuclear hyperfine structure.
APPLICATIONS OF ESR
The technique of ESR has been used to study many types of systems and we will comment on a
few of them.
1. Biological Systems
ESR has been applied quite extensively to biological systems. One can follow the variations that
occur under changing environmental conditions by monitoring the intensity of a free radical
signal. For example, the presence of free radicals has been studied in healthy and diseased tissue.
If a transition metal ion is present, as in case of the Fe ions of hemoglobin, then changes in its
valence state may be studied by ESR. Early concrete evidence for the role of free radicals in
3
photosynthesis, was provided by the observation of a sharp ESR resonance line. When incident
light was turned off the resonance soon weakened or disappeared.
An inconvenience associated with the analysis of biological samples is the presence of water
which produces high dielectric losses, making it necessary to use a flat quartz sample cell. Some
typical systems which have been studied by ESR are hemoglobin, nucleic acids, enzymes,
irradiated chloroplasts, riboflavin (before and after UV irradiation), and carcinogens.
2. Chemical Systems
A number of chemical substances such as synthetic polymers and rubber contain free radicals.
The nature of the free radical depends upon the method of synthesis and on the history of the
substance. Recent refinements in instrumentation have permitted the detection of free radical
intermediates in chemical reactions.
Typical chemical systems that have been studied by ESR are polymers, catalysts, rubber, along
lived free radical intermediates, charred carbon, and chemical complexes with transition metals.
3. Conduction Electrons
Conduction electrons have been detected by both conventional electron spin resonance methods,
and by the related cyclotron resonance technique. The latter employs and ESR spectrometer and
the sample is located in a region of strong microwave electric field strength. In the usual ESR
arrangement, on the other hand, the sample is placed at a position of strong microwave magnetic
field strength. Conduction electrons have been detected in solutions of alkali metals in liquid
ammonia, alkaline earth metals (fine powders), alloys (e. g., small amounts of paramagnetic
metal alloyed with another metal), nonresonant absorption of microwaves by superconductors,
and graphite.
4. Free Radicals
A free radical is a compound which contains and unpaired spin, such as the methyl radical CH3
produced through the breakup of methane
CH4 CH3 + H (5)
where both the hydrogen atom and the methyl radical are electrically neutral. A charged free
radical or radical ion is a neutral molecule which has gained or lost an electron
C6H6 + e C6H6 -(cation)
(6)
C6H6
e + C6H6+ (anion)
Free radicals have been observed in gaseous, liquid, and solid systems. They are sometimes
stable, but can also be shod lived intermediates in chemical reactions.
Free radicals and radical ions ordinarily have g factors close to the free electron value of 2.0023
(e. g., for the stable free radical , ' diphenyl -picrylhydrazyl, referred to as DPPH, g =
2.0036). In low viscosity solutions, radicals exhibit hyperfine patterns with a typical overall
spread of about 2.5 millitesla. The scrupulous removal of oxygen often reveals hitherto
unresolved structure. In a high concentration solid a single exchange narrowed resonance
appears ( B~0.3 millitesla for DPPH prepared from benzene solution), whereas DPPH in a
dilute oxygen free solution of the solvent tetrahydrofurane produces a spectrum with more than
100 hyperfine components. In irradiated single crystals the free radicals may have strongly
directionally dependent or anisotropic hyperfine interactions and slightly anisotropic g factors.
4
Radical ions of many organic compounds can be produced in an electrolytic cell which is usually
a flat quartz cell with a mercury pool cathode and a platinum anode. This electrolytic cell may be
mounted in a flat measuring cell located in the resonance cavity. When the applied voltage in the
electrolytic system is increased, the current will first increase, but soon levels off to a plateau.
Radical ions are formed in this plateau region. Radical formation can sometimes be observed
visually because of color changes in the solution. Since oxygen is also paramagnetic, dissolved
air must be scrupulously removed prior to the experiment. The best method is to use the freeze
pump thaw technique where the sample is first frozen and then connected to a high vacuum
source. After closing off the vacuum pump, the sample is melted and refrozen. Several cycles are
sufficient to remove the oxygen.
5. Irradiated Substances
A considerable amount of work has been done on radiation induced free radicals in organic
compounds and paramagnetic color centers in ionic solids. Most irradiations are carried out with
X-rays, gamma rays, or electrons whose energies far exceed chemical bond energies, although
paramagnetic spins can also be produced by less energetic ultraviolet light or neutrons. Most of
these spectra are obtained after the sample is irradiated, and many paramagnetic centers are
found to be sufficiently long lived to warrant such a
procedure. More sophisticated experimental techniques entail simultaneous irradiation and ESR
detection. Low temperature irradiation and detection can reveal the presence of new centers
which can be studied at gradually increasing temperatures to elucidate the kinetics of their
recombination.
6. Naturally Occurring Substances
Most of the systems studied by ESR are synthetic or manmade. Nevertheless, from the beginning
of the field, various naturally occurring substances have been studied, such as: (1) minerals with
transition elements [e. g., ruby (Cr/Al2O3), dolomite Mn/(Ca, Mg(Co3)]; (2) minerals with
defects (e. g., quartz); (3) hemoglobin (Fe); (4) petroleum; (5) coal; (6) rubber; and (7) various
biological systems.
7. Spin labels
There are a number of free radicals called spin labels which can attach themselves to particular
sites in biological systems and produce spectra which provide information on changes in the
chemical and physical characteristics in the neighborhood of the site. An example of a spin label
has its unpaired electron on the NO group of a nitroxyl compound such as 2, 2, 6, 6tetramethyl4piperadone1oxyl
The unpaired electron shown on the lower oxygen, spends time on
and strongly interacts with the I = 1 nuclear spin of the nitrogen atom
to produce a 3line hyperfine spectrum. When the solvent has a low
viscosity (flows very freely) the radical tumbles rapidly to average
out the anisotropies, and a spectrum of three narrow lines is obtained.
At lower temperatures the viscosity increases and the solvent
materials flows very slowly so the nitrogen radical can no longer
tumble rapidly, and a smeared out spectrum is produced.
When the spin label attaches itself to a particular site in a biological
molecule its spectrum reveals the extent to which free motion or very restricted motion occurs at
the site. Different spin labels have been synthesized which attach themselves to very specific
sites. For example the spin label ndoxylstearic has the nitroxide doxyl group attached to different
5
positions n between 5 and 16 on the long stearic acid molecule. This spin label can be inserted
into a cell membrane so each value of n corresponds to a different depth in the membrane, and a
sequence of labels provides information on the activities of the membrane at various depths
below the surface.
MOSSBAUER SPECTROSCOPY
THE MOSSBAUER EFFECT
Nuclei in atoms undergo a variety of energy level transitions, often associated with the emission
or absorption of a gamma ray. These energy levels are influenced by their surrounding
environment, both electronic and magnetic, which can change or split these energy levels. These
changes in the energy levels can provide information about the atom's local environment within a
system and ought to be observed using resonance-fluorescence. There are, however, two major
obstacles in obtaining this information: the 'hyperfine' interactions between the nucleus and its
environment are extremely small, and the recoil of the nucleus as the gamma-ray is emitted or
absorbed prevents resonance.
In a free nucleus during emission or absorption of a gamma ray it recoils due to conservation of
momentum, just like a gun recoils when firing a bullet, with a recoil energy ER. This recoil is
shown in Fig1. The emitted gamma ray has ERless energy than the nuclear transition but to be
resonantly absorbed it must be ER greater than the transition energy due to the recoil of the
absorbing nucleus. To achieve resonance the loss of the recoil energy must be overcome in some
way.
Fig1: Recoil of free nuclei in emission or absorption of a
gamma-ray
As the atoms will be moving due to random thermal motion the gamma-ray energy has a spread
of values ED caused by the Doppler effect. This produces a gamma-ray energy profile as shown
in Fig2. To produce a resonant signal the two energies need to overlap and this is shown in the
red-shaded area. This area is shown exaggerated as in reality it is extremely small, a millionth or
less of the gamma-rays are in this region, and impractical as a technique.
6
Fig2: Resonant overlap in free atoms. The overlap shown
shaded is greatly exaggerated
What Mössbauer discovered is that when the atoms are within a solid matrix the effective mass
of the nucleus is very much greater. The recoiling mass is now effectively the mass of the whole
system, making ER and ED very small. If the gamma-ray energy is small enough the recoil of the
nucleus is too low to be transmitted as a phonon (vibration in the crystal lattice) and so the whole
system recoils, making the recoil energy practically zero: a recoil-free event. In this situation, as
shown in Fig3, if the emitting and absorbing nuclei are in a solid matrix the emitted and absorbed
gamma-ray is the same energy: resonance!
Fig3: Recoil-free emission or absorption of
a gamma-ray when the nuclei are in a solid
matrix such as a crystal lattice
If emitting and absorbing nuclei are in identical, cubic environments then the transition energies
are identical and this produces a spectrum as shown in Fig4: a single absorption line.
7
Fig4: Simple Mössbauer spectrum from identical source and
absorber
Now that we can achieve resonant emission and absorption can we use it to probe the tiny
hyperfine interactions between an atom's nucleus and its environment? The limiting resolution
now that recoil and Doppler broadening have been eliminated is the natural linewidth of the
excited nuclear state. This is related to the average lifetime of the excited state before it decays
by emitting the gamma-ray. For the most common Mössbauer isotope, 57Fe, this linewidth is
5x10-9ev. Compared to the Mössbauer gamma-ray energy of 14.4keV this gives a resolution of 1
in 1012, or the equivalent of a small speck of dust on the back of an elephant or one sheet of
paper in the distance between the Sun and the Earth. This exceptional resolution is of the order
necessary to detect the hyperfine interactions in the nucleus.
As resonance only occurs when the transition energy of the emitting and absorbing nucleus
match exactly the effect is isotope specific. The relative number of recoil-free events (and hence
the strength of the signal) is strongly dependent upon the gamma-ray energy and so the
Mössbauer effect is only detected in isotopes with very low lying excited states. Similarly the
resolution is dependent upon the lifetime of the excited state. These two factors limit the number
of isotopes that can be used successfully for Mössbauer spectroscopy. The most used is 57Fe,
which has both a very low energy gamma-ray and long-lived excited state, matching both
requirements well.
EXPERIMENTAL METHODS
In 1957 Rudolf Mossbauer achieved the first experimental observation of the resonant absorption
and recoil-free emission of nuclear γ-rays in solids during his graduate work at the Institute for
Physics of the Max Planck Institute for Medical Research in Heidelberg Germany. Mossbauer
received the 1961 Nobel Prize in Physics for his research in resonant absorption of γ-radiation
and the discovery of recoil-free emission a phenomenon that is named after him. The Mossbauer
effect is the basis of Mossbauer spectroscopy.
8
The Mossbauer effect can be described very simply by looking at the energy involved in the
absorption or emission of a γ-ray from a nucleus. When a free nucleus absorbs or emits a γ-ray to
conserve momentum the nucleus must recoil, so in terms of energy:
Eγ-ray = Enuclear transition - Erecoil
When in a solid matrix the recoil energy goes to zero because the effective mass of the nucleus is
very large and momentum can be conserved with negligible movement of the nucleus. So, for
nuclei in a solid matrix:
Eγ-ray = Enuclear transition
This is the Mossbauer effect which results in the resonant absorption/emission of γ-rays and
gives us a means to probe the hyperfine interactions of an atoms nucleus and its surroundings.
A Mossbauer spectrometer system consists of a γ-ray source that is oscillated toward and away
from the sample by a “Mossbauer drive”, a collimator to filter the γ-rays, the sample, and a
detector.
Figure 1: Schematic of Mossbauer Spectrometers. A = transmission; B = backscatter set up.
The above figure shows the two basic set ups for a Mossbauer spectrometer. The Mossbauer
drive oscillates the source so that the incident γ-rays hitting the absorber have a range of energies
due to the Doppler effect. The energy scale for Mossbauer spectra (x-axis) is generally in terms
of the velocity of the source in mm/s. The source shown (57Co) is used to probe 57Fe in iron
containing samples because 57Co decays to 57Fe emitting a γ-ray of the right energy to be
absorbed by 57Fe. To analyze other Mossbauer isotopes other suitable sources are used. Fe is the
9
most common element examined with Mossbauer spectroscopy because its 57Fe isotope is
abundant enough (2.2), has a low energy γ-ray, and a long lived excited nuclear state which are
the requirements for observable Mossbauer spectrum. Other elements that have isotopes with the
required parameters for Mossbauer probing are seen in Table 1.
Most commonly examined
Fe, Ru, W, Ir, Au, Sn, Sb, Te, I, W, Ir, Au, Eu, Gd, Dy, Er, Yb, Np
elements
Elements that
Mossbauer effect
exhibit K, Ni, Zn, Ge, Kr, Tc, Ag, Xe, Cs, Ba, La, Hf, Ta, Re, Os, Pt, Hg, Ce,
Pr, Nd, Sm, Tb, Ho, Tm, Lu, Th, Pa, U, Pu, Am
TABLE 1: Elements with known Mossbauer isotopes and most commonly examined with
Mossbauer spectroscopy.
THE HYPERFINE INTERACTIONS
The nuclear isomer shift – electric monopole interaction:
The isomer shift originates from the Coulomb interaction of the nuclear charge distribution over
the radius of the nucleus in its ground and excited state, and, the electron charge density AT the
nucleus. It results in a shift of the overall spectrum to higher and lower energies. The isomer
shift depends most strongly on the ionization state of the atom, as shielding effects due to
valence electrons will influence the s-electron density at the nucleus.
The nuclear quadrupole splitting – electric quadrupole interaction.
The quadrupole splitting results from the interaction between the Electron Field Gradient (EFG)
at the nucleus and the electric quadrupole moment eQ of the nucleus itself. More specifically, the
EFG at the nucleus will split the Fe57 nuclear excited I = 3/2 state into a pair of doublets: Iz =
+/- 1/2 and +/- 3/2.
The nuclear Zeeman effect – magnetic dipole interaction.
The nuclear magnetic dipole moment interacts with an applied magnetic field B to produce this
splitting of the energy levels at the nucleus.
Besides these three hyperfine interactions there are other measurable interactions called
the relativistic effects. These are caused by a temperature or pressure changes, and acceleration
and gravitational fields.
10
Nuclear Hyperfine Interactions Observable with Mossbauer Spectroscopy
Observed
Effect
Illustration
Observed Spectrum
Isomer Shift
Interaction of
the nuclear
charge
distribution
with the
electron
cloud
surrounding
the nuclei in
both the
absorber
and source.
Zeeman
Effect
(Dipole
Interaction)
Interaction of
the nuclear
magnetic
dipole
moment with
the external
applied
magnetic
field on the
nucleus.
Quadrupole
Splitting
Interaction of
the nuclear
electric
quadrupole
moment with
the EFG and
the nucleus
11