Mossbauer Spectroscopy

Scott PowersMolecular Spectroscopy Presentation

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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

http://www.nndb.com/people/824/000099527/rudolf-mossbauer-1.jpg2

"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

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Law of Conservation of Momentum

E0 = nuclear transition energyM = mass of emitting particleC = speed of lightER = the energy of the recoil.

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Mossbauer's Breakthrough

• Atoms placed in solid matrix have much greater effective mass

• Recoil mass of nuclei becomes recoil mass of entire matrix

http://www.rsc.org/membership/networking/interestgroups/mossbauerspect/intropart1.asp

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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

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• 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

Mossbauer's Breakthrough

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"Mossbauer Effect"

• The resonance is not observed if recoil of nuclei occurs

• Conservation of momentum induces recoil of nuclei

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"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

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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"

http://www.rsc.org/images/simple_spectrum_tcm18-12013.gif

• 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

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Question

• Why is it normal behavior for an atom to recoil in the event of gamma emission?

• How was this overcome?

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"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

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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

http://www.rsc.org/membership/networking/interestgroups/mossbauerspect/intropart1.asp

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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

http://www.cmp.liv.ac.uk/shrike/mphys/chap3.html

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Question

• What type of energy levels are effected by gamma emission?

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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

http://serc.carleton.edu/research_education/geochemsheets/techniques/mossbauer.html17

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.asp18

Gamma Rays

http://upload.wikimedia.org/wikipedia/commons/thumb/c/c2/Gamma_Decay.svg/2000px-Gamma_Decay.svg.png

• Emission of energy

• Form of light

• Form of energy

• Byproduct of radioactivity

• Not a "particle"

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Gamma Rays

• Interesting challenge faced when dealing with gamma rays

• A gamma ray is extremely high energy

• A gamma ray is small wavelength

http://chandra.harvard.edu/graphics/resources/illustrations/em_spectrum2.jpg

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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.html21

Energy Loss in Nuclei of Atom

https://classconnection.s3.amazonaws.com/799/flashcards/1528799/png/gamma_ray_emission1355022930521.png

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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

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Question

• What is so significant about the wavelength of a gamma ray?

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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 molecules25

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

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Mossbauer Spectroscopy in Biology

• Used in combination with other data to obtain chemical information about proteins

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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

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Mossbauer Spectroscopy in Space

• Used on Mars

• Rovers have miniature mossbauer spectrometers

http://www.medc.dicp.ac.cn/Mesite/images/Klingelhofer2.gif29

Question

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

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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

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What of Usefulness is Observed

• Example of each kind of shift

• Energy diagrams

• Spectral splitting

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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

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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

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Isomer Shift

http://article.sapub.org/image/10.5923.j.ajcmp.20130302.01_004.gif

• General form of an isomer shift

• Single peak

• Slightly shifted from zero

• Can be positive or negative

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Question

• How is the Doppler effect used in Mossbauer spectrscopy?

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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

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Quadrupole Splitting• Shows two samples

• Both show quadrupole splitting

• Show how similar structures give similar signals

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Calculating Energy Difference in Doublets

• Equation to calculate the energy difference between quadrupole shifts

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Quadrupole Splitting

• Equation used to calculate electric field gradient

• Relation of electric field gradient to splitting of energy levels

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Hamiltonian for Quadrupole Splitting

• Interaction between nuclear moment and electric field gradient

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Question

Why is the Doppler effect important to Mössbauer spectroscopy

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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

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Zeeman Effect

• Spectral lines that are normally degenerate become differentiable• Observable splitting of spectral lines• Results from external magnetic field

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/zeeman.html45

Magnetic Splitting

http://ej.iop.org/images/0953-8984/24/15/156001/Full/cm418558f6_online.jpg

• As temperature increases lines increase

• Shows at higher temperature splitting is different

• Higher temperature leads to different shifts

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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

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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

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Simple Explanation of Spectra

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Question

• What type of peak is produced from the three types interactions?

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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

http://serc.carleton.edu/research_education/geochemsheets/techniques/mossbauer.html

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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

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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

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Instrumentation

• Detector

• Two types

• Gas filled detector

• Scintillation detector

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Gas Filled Detector

• Sensitive volume of gas between two electrodes• Not often used for Mossbauer spectroscopy

http://www.lanl.gov/orgs/n/n1/panda/00326398.pdf55

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

http://www.lanl.gov/orgs/n/n1/panda/00326398.pdf

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Scintillation Detectors

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The Chemistry Observed From Spectra

http://pecbip2.univ-lemans.fr/webibame/Photos/Spin3.JPG

• Prominent quadripole• No charge symmetry

• Temperature variation• Spreads peaks• Shows change

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Rearrangement from Temperature Change

http://www.rsc.org/Membership/Networking/InterestGroups/MossbauerSpect/part3.asp

• Simply show how at higher temperatures some interactions change

• Higher temperatures gives loss of quadupole

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Rearrangement from Temperature

http://www.frontiersin.org/files/Articles/19486/fmicb-03-00118-r2/image_m/fmicb-03-00118-g008.jpg

• Simply show how temperature can affect chemical environment

• Quadrupole and magnetic interaction occur at lower temperatures

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Example of Mossbauer Spectra

• Sample Spectra

• Two iron complex

• Isomer shift

• Quadrupole

• Magnetic Splitting

http://www.icame2013opatija.com/system/image/71/Bill.jpg

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Example of Mossbauer Spectra

• Shows patterns for atoms in different locations

• Isomer shift

• Quarupole

• Symmetrical charge distribution

http://www.chem.s.u-tokyo.ac.jp/users/chemreact/image/publication/fig_2011_07.jpg

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Example of Mossbauer Spectra

• Isomer shift

• Quadrupole splitting

• Magnetic splitting

http://www.chem.s.u-tokyo.ac.jp/users/chemreact/image/publication/fig_2007_03.gif63

Example Mossbauer Spectra

• Isomer Shift

• Quadrupole splitting

• Symmetric charge distribution

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Example of Mossbauer Spectra

http://onlinelibrary.wiley.com/store/10.1002/chem.201002060/asset/image_m/mcontent.gif?v=1&s=d2bd6f4dee56f1763a3886db5fb5c8666b2c189d

• Color coded to show spectra of each atom

• Notice all display similar shifts

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Question

• What device is used to focus the gamma radiation between the source and sample?

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Elements That Display Mossbauer Effect

http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Mössbauer_Spectroscopy67

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

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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

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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

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