Mössbauer spectroscopy

References:J.P. Adloff, R. Guillaumont: Fundamentals of

Radiochemistry, CRC Press, Boca Raton, 1993

Mössbauer effect:recoil free nuclear resonance absorption of

radiation

0

1,2

-4 1

Energy

RecoilRecoil

Recoil

ABSORPTIONEMISSION

E2

E1

E photon

E photon=(E2-E1)-R E photon=(E2-E1)+R

E

E1

E photon

Line width (W) results from: natural width of E2 level + Doppler widening due to temp.

W ~10-6 eV (natural width) + 10-3 eV (temp. effect) → 10-3 eV (overall effect)

R~100 eV in nuclear processes, R’~10-7 eV in optical processes

Realization of nuclear resonance:

Source and absorber contain the same element (same nuclear energy levels).

Reduction of R by embedding the isotope in a solid crystal matrix,

cooling the sample (reduced oscillation of the atoms, reduced R↔reduced W).

The missing part of „2R” energy can be provided by moving the source due to Doppler effect

57Co271 days

5/2- 137 keV 10-9 secresonance

9% 91% absorption

3/2 14,4 keV 10-7 sec

1/2- ground57Fe 57Fe

emitter absorber

Mössbauer spectrometer:

S A

v

D

S source emitting weak radiation

A absorber moving with velocity

v (mm/s)

D radiation detector

The linear motion represents about 10-8 eV.

The resulting Es energy is derived from the E source energy: Es= E(1±v/c)

Source or absorber is moved. (Emission and absorption spectroscopy, respectively.)

Source or absorber should be in ground state, non-magnetic, symmetric environment precluding hyperfine splitting of nuclear level.

The Mössbauer spectrum Resonance absorption spectrum : radiation intensity vs. velocity

(Energy)

-1,2

0

-10 0 10

velocíty (mm/s); energy (10-7 eV)

cou

nt

rate

Typical Mössbauer emission spectrumas the superposition of 2 single linesaccording to magnetic splitting of the nuclear levels in magnetic field:

5/2-

5/2+

237Np

chemical shift

-1,2

0

1,2

-10 0 10 20 30

velocíty (mm/s); energy (10-7 eV)

cou

nt

rate

Typical Mössbauer emission spectrumas the superposition of 5 single linesaccording to quadrupole splitting of the nuclear levels in electric field:

5/2-

5/2+

237Np

Chemical information in Mössbauer spectra

Spectra reveal splittings of nuclear levels, determined by the electronic environment.

• Isomer shift: position of the centroid of the line,

oxidation state, covalency of the bondings• Quadrupole splitting: multiplets

asymetry in the electronic environment, chemical spin state, intensity of ligand field

• Magnetic splitting: multiplet due to magnetic field

Mössbauer active atoms

• 75 transitions in isotopes of 44 elements• Radionuclide: MBq activity

alpha, beta, EC or ITT1/2: hours-hundreds of years

• Conditions to be fulfilled:- E<100 keV,- emitter should be bound in a lattice- mean life-time of excited level: 1 ns-100 ns- solid, cooled absorber (liquid N2), m>100mg

E.g.: 57Co(EC)57Fe: 14,4 keV241Am(alpha)237Np: 60 keVTc, Th, Pa, U, Np, Pu, Am

Application examples

• Analysis of steels:oxidation state of iron (+2 or +3)chemical form (oxide, sulfate…)magnetic properties

• Analysis of iron oxide layersmagnetite, hematite

• Recoil processes in condensed material• Oxidation states of Np, Am compounds

Other nuclear related methods providing information on chemical environment

• Positron annihilation spectrometry

• Muon spectrometry

• Nuclear magnetic resonance

• Electron spectroscopies:

photoelectron spectroscopy

conversion electron spectroscopy

Auger electron spectroscopy