Mossbauer Spectroscopy
Scott Powers
Molecular Spectroscopy Presentation
1
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
http://www.nndb.com/people/824/000099527/rudolf-mossbauer-1.jpg
"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
http://www.rsc.org/membership/networking/interestgroups/mossbauerspect/intropart1.asp
3
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
Mossbauer's Breakthrough
• Atoms placed in solid matrix have much greater effective mass
• Recoil mass of nuclei becomes recoil mass of entire matrix
5
http://www.rsc.org/membership/networking/interestgroups/mossbauerspect/intropart1.asp
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
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
"Mossbauer Effect"
• The resonance is not observed if recoil of nuclei occurs
• Conservation of momentum induces recoil of nuclei
8
"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
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
10
"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
11
http://www.rsc.org/images/simple_spectrum_tcm18-12013.gif
Question
• Why is it normal behavior for an atom to recoil in the event of gamma
emission?
• How was this overcome?
12
"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
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
14
http://www.rsc.org/membership/networking/interestgroups/mossbauerspect/intropart1.asp
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
http://www.cmp.liv.ac.uk/shrike/mphys/chap3.html
Question
• What type of energy levels are effected by gamma emission?
16
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
17
http://serc.carleton.edu/research_education/geochemsheets/techniques/mossbauer.html
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
http://www.rsc.org/Membership/Networking/InterestGroups/MossbauerSpect/Intropart1.asp
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Gamma Rays
• Emission of energy
• Form of light
• Form of energy
• Byproduct of radioactivity
• Not a "particle"
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http://upload.wikimedia.org/wikipedia/commons/thumb/c/c2/Gamma_Decay.svg/2000px-Gamma_Decay.svg.png
Gamma Rays
• Interesting challenge faced when dealing with gamma rays
• A gamma ray is extremely high energy
• A gamma ray is small wavelength
20
http://chandra.harvard.edu/graphics/resources/illustrations/em_spectrum2.jpg
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
http://www.pbs.org/wgbh/nova/gamma/spec_gamma.html
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Energy Loss in Nuclei of Atom
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https://classconnection.s3.amazonaws.com/799/flashcards/1528799/png/gamma_ray_emission1355022930521.png
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
Question
• What is so significant about the wavelength of a gamma ray?
24
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
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
Mossbauer Spectroscopy in Biology
• Used in combination with other
data to obtain chemical
information about proteins
27
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
Mossbauer Spectroscopy in Space
• Used on Mars
• Rovers have miniature
mossbauer spectrometers
29
http://www.medc.dicp.ac.cn/Mesite/images/Klingelhofer2.gif
Question
Why is it important for the sample to be in solid or crystalline state?
30
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
What of Usefulness is Observed
• Example of each kind of shift
• Energy diagrams
• Spectral splitting
32
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
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
34
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
Isomer Shift
• General form of an isomer shift
• Single peak
• Slightly shifted from zero
• Can be positive or negative
36
http://article.sapub.org/image/10.5923.j.ajcmp.20130302.01_004.gif
Question
• How is the Doppler effect used in Mossbauer spectrscopy?
37
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
38
Quadrupole Splitting
• Shows two samples
• Both show quadrupole
splitting
• Show how similar
structures give similar
signals
39
Calculating Energy Difference in Doublets
• Equation to calculate the energy difference between quadrupole shifts
40
Quadrupole Splitting
• Equation used to calculate electric field gradient
• Relation of electric field gradient to splitting of energy levels
41
Hamiltonian for Quadrupole Splitting
• Interaction between nuclear moment and electric field gradient
42
Question
Why is the Doppler effect important to Mössbauer spectroscopy
43
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
44
Zeeman Effect
• Spectral lines that are normally degenerate become differentiable
• Observable splitting of spectral lines
• Results from external magnetic field
45
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/zeeman.html
Magnetic Splitting
• As temperature increases lines
increase
• Shows at higher temperature
splitting is different
• Higher temperature leads to
different shifts
46
http://ej.iop.org/images/0953-8984/24/15/156001/Full/cm418558f6_online.jpg
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
47
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
48
Simple Explanation of Spectra
49
Question
• What type of peak is produced from the three types interactions?
50
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
http://serc.carleton.edu/research_education/geochemsheets/techniques/mossbauer.html
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
52
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
Instrumentation
• Detector
• Two types
• Gas filled detector
• Scintillation detector
54
Gas Filled Detector
• Sensitive volume of gas between two electrodes
• Not often used for Mossbauer spectroscopy
55
http://www.lanl.gov/orgs/n/n1/panda/00326398.pdf
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
http://www.lanl.gov/orgs/n/n1/panda/00326398.pdf
Scintillation Detectors
57
The Chemistry Observed From Spectra
• Prominent quadripole
• No charge symmetry
• Temperature variation
• Spreads peaks
• Shows change
58
http://pecbip2.univ-lemans.fr/webibame/Photos/Spin3.JPG
Rearrangement from Temperature Change
• Simply show how at higher
temperatures some interactions
change
• Higher temperatures gives loss of
quadupole
59
http://www.rsc.org/Membership/Networking/InterestGroups/MossbauerSpect/part3.asp
Rearrangement from Temperature
• Simply show how temperature can affect
chemical environment
• Quadrupole and magnetic interaction occur at
lower temperatures
60
http://www.frontiersin.org/files/Articles/19486/fmicb-03-00118-r2/image_m/fmicb-03-00118-g008.jpg
Example of Mossbauer Spectra
• Sample Spectra
• Two iron complex
• Isomer shift
• Quadrupole
• Magnetic Splitting
61
http://www.icame2013opatija.com/system/image/71/Bill.jpg
Example of Mossbauer Spectra
• Shows patterns for atoms in
different locations
• Isomer shift
• Quarupole
• Symmetrical charge distribution
62
http://www.chem.s.u-tokyo.ac.jp/users/chemreact/image/publication/fig_2011_07.jpg
Example of Mossbauer Spectra
• Isomer shift
• Quadrupole splitting
• Magnetic splitting
63
http://www.chem.s.u-tokyo.ac.jp/users/chemreact/image/publication/fig_2007_03.gif
Example Mossbauer Spectra
• Isomer Shift
• Quadrupole splitting
• Symmetric charge distribution
64
Example of Mossbauer Spectra
• Color coded to show spectra of
each atom
• Notice all display similar shifts
http://onlinelibrary.wiley.com/store/10.1002/chem.201002060/asset/imag
e_m/mcontent.gif?v=1&s=d2bd6f4dee56f1763a3886db5fb5c8666b2c189d
65
Question
• What device is used to focus the gamma radiation between the
source and sample?
66
Elements That Display Mossbauer Effect
67
http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Mössbauer_Spectroscopy
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
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
69
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