1. Mossbauer Spectroscopy
Scott Powers
Molecular Spectroscopy Presentation 1

2. Adolf Mossbauer Born on January 31, 1929 Born in Munich, Germany
Discovered "Mossbauer Effect"
Won Nobel Prize in 1961 in physics
Passed away in 2011 2

3. "Mossbauer Effect"
Based on discovery of recoilless gamma ray emission and absorption
When gamma ray is emitted a nuclei will recoil in conservation of momentum
Not useful for Mossbauer spectroscopy 3

4. Law of Conservation of Momentum
E0 = nuclear transition energy
M = mass of emitting particle
C = speed of light
ER = the energy of the recoil. 4

5. Mossbauer's Breakthrough
Atoms placed in solid matrix have much greater effective mass
Recoil mass of nuclei becomes recoil mass of entire matrix 5

6. Mossbauer's Breakthrough
Phonons emitted from lattice from slight vibrational energy
No recoil energy lost
Only form of energy lost during gamma ray emission
If gamma ray energy is small enough entire systems recoils
Due to not being enough energy to cause vibration in lattice
This is a recoil free event achieving resonance 6

7. Mossbauer's Breakthrough
Random thermal motion of nuclei creates spread of gamma radiation
Mossbauer realized that Doppler effect could be used
Achieve overlap that results in resonance
Create a spread in the energy of emitted gamma ray
Create data that was on a workable scale 7

8. "Mossbauer Effect"
The resonance is not observed if recoil of nuclei occurs
Conservation of momentum induces recoil of nuclei
Tjb - Move this with slide 9
need figure(s) in this slide to demonstrate effect 8

9. "Mossbauer Effect" How does it work
Nuclei in atoms undergo many energy level transitions
Changes occur due to emission and absorption of a gamma ray
Energy levels are determined by the nuclei's surrounding environment
Observed using nuclear resonance fluorescence
Special technique used to gauge distances between chromophores
Only works when separation distance is less than 10nm 9

10. Diagram of Vibrational Energy Levels
En represents ground state energy
En+1 represents the next highest energy
ER represents recoil energy
The first example shows a event resulting in no resonance
The second examples shows an event resulting in resonance
Tjb diagram does not make it clear why resonance occurs, define En+1 and other levels 10

11. "Mossbauer Effect" Gamma ray emission produces signals
Certain states with certain energies
These energies have phonons of specific velocities
These signals can be plotted
Velocity of emitted rays
Time elapsed
Mössbauer Effect tjb- phonon is the occupation of quantized vibrational states, unclear how text explains diagram
The recoil energy associated with absorption or emission of a photon can be described by the conservation of momentum.In it we find that the recoil energy depends inversely on the mass of the system. For a gas the mass of the single nucleus is small compared to a solid.The solid or crystal absorbs the energy as phonons, quantized vibration states of the solid, but there is a probability that no phonons are created and the whole lattice acts as the mass, resulting in a recoilles emission of the gamma ray.  The new radiation is at the proper energy to excite the next ground state nucleus. The probability of recoilles events increases with decreasing transition energy. 11

12. Question
Why is it normal behavior for an atom to recoil in the event of gamma emission?
How was this overcome? 12

13. "Mossbauer Effect"
Two possibilities exist for recoil event based on the energy
Recoil energy<energy of nuclear transitions gives no resonance
Recoil energy>energy of nuclear transitions gives resonance
Resonance achieved by removing loss of the recoil energy 13

14. Circumstances of Resonance
Top figure shows an example of a nucleus that recoils as a result of gamma ray emission
Bottom figure shows an example of a nucleus that does not recoil as a result of gamma ray emission
Resonance results
Tjb – this figure needs to be much earlier, why is nucleus at right above is moving 14

15. Circumstances of Resonance
What does this mean
With the use of the Doppler effect the wavelength of the source gamma rays can be tuned
When this wavelength is the same as the wavelength of emitted gamma ray resonance is achieved 15

16. Question
What type of energy levels are effected by gamma emission? 16

17. Where do the Gamma Rays Come From?
Based from the original discovery that 57Co decomposes readily to 57Fe
57Fe is also unstable and further decomposes
Gives off a gamma ray as well as some other types of energies
Tjb – very important slide, must discuss every part, every transistion and its nature 17

18. Circumstances of Resonance
P is electron density
ED is the variable chosen to describe the spread of the gamma ray energy
For resonance to occur overlap of two ED values for two nuclei must occur
This overlap is generally very small
Tjb – Unclear where spread of gamma energy is explained
Include definitions of ER and P ? 18

19. Gamma Rays Emission of energy Form of light Form of energy
Byproduct of radioactivity
Not a "particle" 19

20. Gamma Rays Interesting challenge faced when dealing with gamma rays
A gamma ray is extremely high energy
A gamma ray is small wavelength 20

21. Challenge of Gamma Rays
Cannot be observed like normal light
Wavelength is on the order of magnitude to penetrate nuclei of atom
Has no mass allowing for specific change in atom
Allows an atom to decay from high energy state to lower, stable energy state
Allows for atomic decay without loss of mass 21

22. Energy Loss in Nuclei of Atom22

23. Energy Diagram of Gamma Decay
Initial energy change and decay from 57Co to 57Fe
From 5/2 to 1/2 gives no gamma emission
From 5/2 to3/2 lead to further transition to 1/2
Transition from 3/2 to 1/2 gives beta emission 23

24. Question What is so significant about the wavelength of a gamma ray?24

25. Mossbauer Spectroscopy in Physics and Chemistry
Used to further pursue the nature of energy states in nuclei
Measure changes in chemical environment of nuclei
Monitor materials during phase changes
Monitor chemical reactions
Determine structures of molecules 25

26. Mossbauer Spectroscopy in Biology
Used In Cancer treatments
Used to analyze red blood cells
Test environmental effects of human body
Can analyze protein structures
Help in function determinations 26

27. Mossbauer Spectroscopy in Biology
Used in combination with other data to obtain chemical information about proteins 27

28. Mossbauer Spectroscopy in Mineralogy and Metallurgy
Can be used to determine metal samples
Determine crystal structures
Molecular arrangements
Chemical compositions
Used to analyze different mineral samples
Determine different crystal structures
Determine compositions
Analyze intergalactic samples for unordinary behaviors 28

29. Mossbauer Spectroscopy in Space
Used on Mars
Rovers have miniature mossbauer spectrometers 29

30. Question
Why is it important for the sample to be in solid or crystalline state? 30

31. What of Usefulness is Observed
Elimination of recoil leaves scientists with hyperfine interactions
Types of major interactions
Isomer Shift
Quadrupole Splitting
Magnetic Splitting
Hyperfine Interactions
These are generally very small
These hyperfine interactions are what are studied to obtain information 31

32. What of Usefulness is Observed
Example of each kind of shift
Energy diagrams
Spectral splitting 32

33. Isomer Shift Occurs when one nuclear isomer replaces another
Provides important information about nuclear structure
Provides information about the l quantum number of a sample
Affected by the charge density of s-electrons
Closest to nuclei
Provide most shielding
Can be slightly affected by s,p,d, and f electrons 33

34. Isomer Shifts
Interactions between volume of nuclei and the charge density of s-electrons
Determine valency states, ligand bonding states, and electron shielding
Leads to monopole interaction changing nuclear energy levels
Differences in the environment between source and detector produce
Shifts in resonance energies
Not directly measurable so measured relative to a known absorption shift
Spectrum shifts either positively or negatively depending on s-electron density centroid
Isomer Shift _ tjb, need to explain why this is called isomer shift, show and examples with explanation, why only s electrons
The isomer shift arises due to the non-zero volume of the nucleus and the electron charge density due to s-electrons within it. This leads to a monopole (Coulomb) interaction, altering the nuclear energy levels. Any difference in the s-electron environment between the source and absorber thus produces a shift in the resonance energy of the transition. This shifts the whole spectrum positively or negatively depending upon the s-electron density, and sets the centroid of the spectrum. Tjb-did you define or show example of centroid alreadY
As the shift cannot be measured directly it is quoted relative to a known absorber. For example 57Fe Mössbauer spectra will often be quoted relative to alpha-iron at room temperature. Tjb where is alpha iron defined.
The isomer shift is useful for determining valency states, ligand bonding states, electron shielding and the electron-drawing power of electronegative groups. For example, the electron configurations for Fe2+ and Fe3+ are (3d)6 and (3d)5 respectively. The ferrous ions have less s-electrons at the nucleus due to the greater screening of the d-electrons. TJB- I don’t believe your wording is accurate. Don’t you mean the s-electron density is effective less because of the screening. Thus ferrous ions have larger positive isomer shifts than ferric ions. TJB – show a simple energy diagram with example(s) 34

35. Isomer Shift
The isomer shift shows a slight elevation in the energy of the ground and excited states
Notice there is no energy level splitting occurring in an isomer shift
Greater s-electron density gives greater shift 35

36. Isomer Shift General form of an isomer shift Single peak
Slightly shifted from zero
Can be positive or negative
Tjb need a couple of bullets?, what is the take home message 36

37. Question
How is the Doppler effect used in Mossbauer spectrscopy? 37

38. Quadrupole Splitting
Induced by electric quadrupole moment of the nuclei and change in the electric field due to an electron interactions
Gives information about charge symmetry around nuclei
Nuclear energy level splitting due to symmetrical electric field
Electrons with l>.5 have non-spherical charge distribution and produce a nuclear quadripole moment
Tjb -show a diagram of a simple quadripole. Nuclei in states with an angular momentum quantum number I>1/2 have a non-spherical charge distribution. This produces a nuclear quadrupole moment. In the presence of an asymmetrical electric field (produced by an asymmetric electronic charge distribution or ligand arrangement) this splits the nuclear energy levels. The charge distribution is characterised by a single quantity called the Electric Field Gradient (EFG).
In the case of an isotope with a I=3/2 excited state, such as 57Fe or 119Sn, the excited state is split into two substates mI=±1/2 and mI=±3/2. This is shown in Fig2, giving a two line spectrum or 'doublet'.
Tjb-Missing figure 2 Fig2: Quadrupole splitting for a 3/2 to 1/2 transition. The magnitude of quadrupole splitting, Delta, is shown tjb shown where?
Fig2: Quadrupole splitting for a 3/2 to 1/2 transition. The magnitude of quadrupole splitting, Delta, is shown 38

39. Quadrupole Splitting Shows two samples Both show quadrupole splitting
Show how similar structures give similar signals
Tjb needs bullets, take home message? 39

40. Calculating Energy Difference in Doublets
Equation to calculate the energy difference between quadrupole shifts
Tjb-there is no Hamiltonian in this slide.
The Hamiltonian for quadrupole interaction using 57 Fe
nuclear excited state is given by
Where the nuclear excited states are split into two degenerate doublets in the absence of magnetic interactions.  For the asymmetry parameter
Η=0
Doublets are labeled with magnetic quantum numbers
Doublet has the higher energy.  The energy difference between the doublets is thus 40

41. Quadrupole Splitting
Equation used to calculate electric field gradient
Relation of electric field gradient to splitting of energy levels 41

42. Hamiltonian for Quadrupole Splitting
Interaction between nuclear moment and electric field gradient
Tjb -Need some bullets, take home message 42

43. Why is the Doppler effect important to Mössbauer spectroscopy
Question
Why is the Doppler effect important to Mössbauer spectroscopy 43

44. Magnetic Splitting In presence of a magnetic field
This magnetic field is often called the hyperfine field
Nuclear spin moment feels a dipole interaction through Zeeman splitting
Zeeman splitting
Atomic energy levels are split into a larger number of energy levels
Magnetic field applied to split energy levels
Spectral lines are split along with atomic energy levels
Tjb-Bullets unclear, Zeeman splitting is the differentiation of otherwise degenerate states by a magnetic field, thus they are not degenerate in the magnetic field
In the presence of a magnetic field the nuclear spin moment experiences a dipolar interaction with the magnetic field ie Zeeman splitting. There are many sources of magnetic fields that can be experienced by the nucleus. The total effective magnetic field at the nucleus, Beff is given by:
Tjb Equation needs to me in slide!!! Beff = (Bcontact + Borbital + Bdipolar) + Bapplied
the first three terms being due to the atom's own partially filled electron shells. Bcontact is due to the spin on those electrons polarising the spin density at the nucleus, Borbital is due to the orbital moment on those electrons, and Bdipolar is the dipolar field due to the spin of those electrons.
This magnetic field splits nuclear levels with a spin of I into (2I+1) substates. This is shown in Fig3 for 57Fe. Tjb- next slide? Figure three is silde 23. Transitions between the excited state and ground state can only occur where mI changes by 0 or 1. This gives six possible transitions for a 3/2 to 1/2 transition, giving a sextet as illustrated in Fig3, with the line spacing being proportional to Beff.
Fig3: Magnetic splitting of the nuclear energy levels
The line positions are related to the splitting of the energy levels, but the line intensities are related to the angle between the Mössbauer gamma-ray and the nuclear spin moment. The outer, middle and inner line intensities are related by:
3 : (4sin2theta)/(1+cos2theta) : 1
meaning the outer and inner lines are always in the same proportion but the middle lines can vary in relative intensity between 0 and 4 depending upon the angle the nuclear spin moments make to the gamma-ray. In polycrystalline samples with no applied field this value averages to 2 (as in Fig3) but in single crystals or under applied fields the relative line intensities can give information about moment orientation and magnetic ordering.
These interactions, Isomer Shift, Quadrupole Splitting and Magnetic Splitting, alone or in combination are the primary characteristics of many Mössbauer spectra. The next section will show some recorded spectra which illustrate how measuring these hyperfine interactions can provide valuable information about a system. 44

45. Zeeman Effect
Spectral lines that are normally degenerate become differentiable
Observable splitting of spectral lines
Results from external magnetic field 45

46. Magnetic Splitting As temperature increases lines increase
Shows at higher temperature splitting is different
Higher temperature leads to different shifts
Tjb -Need some bullets, take home message
Need to explain temperature dependence, and why there are more lines at 300 K
Magnetic Splitting
Magnetic splitting of seen in Mössbauer spectroscopy can be seen because the nuclear spin moment undergoes dipolar interactions with the magnetic field
E(mI)=−gnβnBeffmI
Where Gn Is the nuclear g-factor and βn is the nuclear magneton. In the absence of quadrupole interactions the Hamiltonian splits into equally spaced energy levels of The allowed gamma stimulated transitions of nuclear excitation follows the magnetic dipole transition selection rule:
ΔI=1andΔmI=0, +−1mI
Is the magnetic quantum number and the direction of  β
Defines the nuclear quantization axis. If we assumeg
and A are isotropic (direction independent) where
gx=gy=gz
And B is actually a combination of the applied and internal magnetic fields: 46

47. Magnetic Splitting Quantitatively
Magnetic fields split one quadrupole shift into 2l+1 magnetic shifts
Different aspects of magnetism in species can be analyzed
Beff = (Bcontact + Borbital + Bdipolar) + Bapplied
Important magnetic information can be obtained
Tjb -Need some bullets, take home message, Table with definitions
where gn
is the nuclear g-factor and βn
is the nuclear magneton. In the absence of quadrupole interactions the Hamiltonian splits into equally spaced energy levels of 47

48. Putting These Shifts Together
Figure to the right shows spectral examples of
Blue shows just an isomer shift
Red is Isomer shift with quadripole splitting
Green shows the hyperfine interactions
Tjb should show this diagram before each of the before the actual data of slide 20, 23, and 7. 48

49. Simple Explanation of Spectra
Tjb –these also should be shown much earlier and more than once. Put a chemical example under each 49

50. Question
What type of peak is produced from the three types interactions? 50

51. Instrumentation of a Mossbauer Spectrometer
Possible arrangements of instrumentation
Mossbauer Drive
Used to move the source relative to sample
57Co Source
Source of gamma ray emission
Collimator
Used to narrow gamma rays 51

52. Instrumentation Sample Contains the material being analyzed
Must be in solid phase
Must be in crystalline structure
Usually requires a large amount of sample
Applied as a thin layer on sample holder and irridatiated
Tjb – be more specific what is in the sample, give examples
What is a resonance detector, how does it work, materials and construction 52

53. Instrumentation Detector
Choice of detector depends of gamma ray energies
Cannot be seen using traditional examination methods of electromagnetic radiation
Observe affect of gamma rays on a material that absorbs them
Resonance detectors
Distance and angle of detector is crucial to Mossbauer spectroscopy 53

54. Instrumentation Detector Two types Gas filled detector
Scintillation detector 54

55. Gas Filled Detector Sensitive volume of gas between two electrodes
Not often used for Mossbauer spectroscopy 55

56. Scintillation Detectors
Sensitive material is luminescent material
Gamma rays interact with the luminescent material
Gamma rays are detected by an optical detector
Used in Mossbauer spectroscopy 56

57. Scintillation Detectors57

58. The Chemistry Observed From Spectra
Prominent quadripole
No charge symmetry
Temperature variation
Spreads peaks
Shows change
Tjb NEED bullets, explain why there is quadripole splitting?
FeII(phen)3]X2 (phen = 1.10-phenanthroline) is a typical low spin compound with the characteristic Mössbauer spectra as shown on the left; the isomer shift is ca. 0.2 and the quadrupole splitting ca. 0.5 mm s-1, nearly independent of temperature.
If one of the relatively strong phen ligands, which occupies two coordination positions of the octahedron, is replaced by two monofunctional NCS- groups, the average ligand field strength becomes weaker than the mean spin pairing energy and the compound [FeII(phen)2(NCS)2] adopts high spin character at room temperature. TJB show electron population in energy diagram. The Mössbauer spectrum at 300 K shows the typical features of an iron(II) HS compound with isomer shift of ca. 1 mm s-1 and large quadrupole splitting of ca. 3 mm s-1. However, the compound [FeII(phen)2(NCS)2] fulfils the condition for thermal spin crossover to occur, viz. ΔEHL » kBT. As the temperature is lowered, the compound changes spin state from high spin to low spin near 180 K as is well documented by the Mössbauer spectra as a function of temperature, which was first reported by I. Dezsi et al. in Since then more than 200 spin crossover compounds of iron(II) and iron(III) have been studied by Mössbauer spectroscopy (see e.g. P. Gütlich, H.A. Goodwin (eds.), Spin Crossover in Transition Metal Compounds, Springer Series “Topics in Current Chemistry, Vol. 233, 234, 235, Berlin Heidelberg 2004). more 58

59. Rearrangement from Temperature Change
Simply show how at higher temperatures some interactions change
Higher temperatures gives loss of quadupole
TJB – structure should be on slide!!! Off-center impurities are those which can be displaced from their regular positions in a crystal lattice. They can be considered as existing in an asymmetric double potential well. Such atoms can change their position as the temperature changes. Unfortunately there are often many other phenomena in such systems that can mask the off-centering effect.
Mössbauer spectroscopy provides a good tool for observing this effect. Firstly the movement of the off-center atom within the lattice will change the symmetry of the electric field it is in: hence changing the quadrupole splitting. Mössbauer spectroscopy is also isotope and site specific, meaning we can observe the off-center single component without any masking from other elements or effects.
A compound which was thought to exhibit off-centering is Pb0.8Sn0.2Te0.8Se0.2, with tin as an off-center atom. Spectra are shown in Fig2 from this sample at 200K and 20K. There are two components: one from an off-center site and one from a normal single-potential site. It can be seen in the highlighted region that the small green component develops from a single line to a (broad) doublet. Explain the temperature effect. Why are there two peaks at low temp and why do they collapse?The quadrupole splitting is increasing, indicating the electric field environment around these particular atoms is become more asymmetrical. This is consistent with an atom moving within an asymmetric potential well. 59

60. Rearrangement from Temperature
Simply show how temperature can affect chemical environment
Quadrupole and magnetic interaction occur at lower temperatures
Tjb- Need bullets, take home message, show the structure. 60

61. Example of Mossbauer Spectra
Sample Spectra
Two iron complex
Isomer shift
Quadrupole
Magnetic Splitting 61

62. Example of Mossbauer Spectra
Shows patterns for atoms in different locations
Isomer shift
Quarupole
Symmetrical charge distribution 62

63. Example of Mossbauer Spectra
Isomer shift
Quadrupole splitting
Magnetic splitting 63

64. Example Mossbauer Spectra
Isomer Shift
Quadrupole splitting
Symmetric charge distribution 64

65. Example of Mossbauer Spectra
Color coded to show spectra of each atom
Notice all display similar shifts 65

66. Question
What device is used to focus the gamma radiation between the source and sample? 66

67. Elements That Display Mossbauer Effect
Tjb –slide needs to be much later, slide 9 should be here or after slide 4!!
Mössbauer Isotopes
By far the most common isotopes studied using Mössbauer spectroscopy is 57Fe, but many other isotopes have also displayed a Mössbauer spectrum.  Two criteria for functionality are Tjb Need bullets for the following
The excited state is of very low energy, resulting in a small change in energy between ground and excited state.  This is because gamma rays at higher energy are not absorbed in a recoil free manner, meaning resonance only occurs for gamma rays of low energy.
The resolution of Mössbauer spectroscopy depends upon the lifetime of the excited state. The longer the excited state lasts the better the image.  
Both conditions are met by 57Fe and it is thus used extensively in Mössbauer spectroscopy.  In the figure to the right the red colored boxes of the periodic table of elements indicate all elements that have isotopes visible using the Mössbauer technique.  67

68. Elements That Display Mossbauer Effect
Requirements
Excited state be of relatively low energy
Small change of energy between ground and excited state
Too large a change of energy results in no resonant emission
Relatively long lasting life of excited state 68

69. Drawbacks of Mossbauer Spectroscopy
Must be in solid crystalline structure
Minute hyperfine interactions
Overcome with the use of Doppler Effect
Major limitation is that it is a “bulk” technique
Often times large amounts of sample are needed for analysis
Recent improvements in electronics and detectors are helping to overcome
Slide should be near end when audience understand why there are drawbacks? 69

70. Conclusions Wide application across multiple scientific disciplines
Relatively cheap method
Relatively fast method
Give valuable information on chemical environment within molecule
Isomer Shifts
Quadrupole splitting
Magnetic splitting 70