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 Line width (W) results from: natural width of E 2 level + Doppler widening due to temp. W ~10 -6 eV (natural width) eV (temp. effect) → 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

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 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)

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 IT T1/2: 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 N 2 ), m>100mg E.g.: 57 Co(EC) 57 Fe: 14,4 keV 241 Am(alpha) 237 Np: 60 keV Tc, 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 layers magnetite, 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