Mössbauer Spectroscopy

Advanced Inorganic Chemistry

Seminar

Presentation in WS 2011/2012

Annkatrin Lennert, Carina Bronnbauer, Eva Kränzlein,

Kerstin Krebs, Kristin Brunner, Sven Herrmann, Dominik Halter

2

Motivation & Utilization

What is Mössbauer Spectroscopy good for?

• Oxidation states

• Spin States (correlated to bond length)

• Magnetic Behaviour

• Ligand Information

Determination of

Characterization of Metal Complexes

Where is Mössbauer Spectrospy applied?

Spectroscopy Principle?

Absorption Spectrum of modulated gamma quanta

Theory of Mössbauer spectroscopy

� Mössbauer effect: recoil-free emission and resonant absorption of ŝ radiation

� Example: 57Fe nuclear resonance

� Heisenberg uncertainty principle: ľ=h

2 Ů Ū

ľ: line widthh: Planck constantŮ: live time

ľ = 6,75e-9 eV

Small changes in frequency inhibt the resonanceeffect between source and absorber

� Recoil energy: ER= = = E

R = 1,95e-3 eV

mv2

2

p2

2m

(hŧ)2

2mc2

p = mv p = (hŧ)/c

Eŝ= ĿE – ER

+ ?

Excited state(live time: Ů = 97 ns)

Eŝ

Ground state

ĿE =14,4keVť = 86 pm

E

Eŝ= ĿE + ER

+ ?

Source Sample

EnergyĿE

-ER

+ER

3

4

Theory of Mössbauer spectroscopy

� Useing the Doppler Effect to affect the frequency:

Moveing the source leads to a change in the frequency of the emitted ŝ quantum

ŧ = ŧ0 + ŧ

0v

a

c

ŧ0: frequency of the emitted photon

ŧa: velocity of the source

ŧ : new frequencyc: light velocity

ŧ0

has to be in the dimension of 1mm s-1 to achieve resonace effects between the sourceand the absorber

Eŝ= ĿE - ER

+ ED

� Construction of the measuring equipment

ŝ quantum

-va

+va Absorber Detector

Source

E

26Co57

26Fe57

ŝ2

(11%)

ŝ1

(85%)

Eŝ

136 keV

14,4 keV

e- capture

5

� Creation of a Mössbauer spectrum

Theory of Mössbauer spectroscopy

va

= 0 Maximum overlap

va

> 0 Partial overlap

va

< 0 Partial overlap

Energy

Energy

Energy

ES

EA

ES

EA

ES

, EA

Resonanceabsorption line

Velocity mm/s

Re

l. T

ran

sm

issio

n [%

]

6

Sample preparation

• Low resonance absorption conditional for Mössbauer spectroscopy

• → solid sample can distribute recoil energy in the crystal lattice

• Energy levels in atom are quantized

→ limited probability f of absorbing a γ-quant

without accompanying phonon transition = recoilless absorption is given by the Lamb-Mössbauerfactor

• → recoilless emission and absorption favoured at deep temperatures

7

Common samples for Mössbauer

spectroscopy

Important factors:- Suitable half-life time of the parent isotop- Low gamma-ray energy to gain sufficent signal-to-noise ratio

Most frequently used isotops are: 57Fe, 119Sn, 121Sb and 129I

8

Isomer Shift δ

9

Reasons for isomer shift:

- electron density at the nucleus is different for ground and excited state

- unlike coloumb interactions

• Electronegativity of ligands

• Oxidation state of the Mössbauer atom.

• Bonding properties in case of coordination compounds (covalency)

• Delocalization of d-electrons due to back-bonding or shielding-effects of s-electrons

Information about:

Measurement of electron density at the nucleus

Isomer Shift δ

10

keep in mind: Due to many possibilities affecting isomer shift there can´t be any

absolute value for δ – but you can tell about trends!

"The higher the oxidation state the lower is the isomer-shift!"

Example: 57Fe-Mössbauer with (δR/R) beeing negative

Isomer Shift δ

11

n

12

y

e.g.

13

Quadrupol Splitting

14

Information

� Molecule symmetry, oxidationstate,

coordination, steroechemistry

� Trans ∆EQ

= 3.0 – 4.0 mm/s

� Cis ∆EQ

= 1.7 – 2.4 mm/s

Sn

X

R

X

R

R

Sn

X

X

R

R

R

15

16

Magnetic Dipol Interactions

17

Magnetic Dipol Interactions

18

Comparison of 57Fe (II) Mössbauerspectra

ɒ large quadrupole splitting (3 mm/s)despite O

hsymmetry

[6]

19

ɒ Oh

symmetry ɒ no quadrupolesplitting

ɒ C4v

symmetry ɒ significantquadrupole splitting

[6]

20

Summary

Observed parameters:

Absorption Spectrum of modulated gamma quanta

(Doppler effect)

Technique:

Isomer Shift Quadrupol Splitting

s-Electr. (nucleus)

• Metal oxidation State

• Spin state ( bondlength)

• Coordination environment

Quadrupol momentum of nucleus

Interacting with Field [ I > ½ ] !

• Metal valence e- [non cubic]

• Ligand charges

Disadvantages: • Radio active source / synchrotron radiation

• Interpretation difficulties due to interaction overlay

21

References

[1] http://www.uni-muenster.de/imperia/md/content/physikalische_chemie/app_moess.pdf

[2] http://uni-leipzig.de/~energy/pdf/freusd9.pdf

[3] http://ruby.chemie.uni-freiburg.de/Vorlesung/methoden_I_5.xhtml

[4] Organometallchemie, Christoph Elschenbroich, Vieweg & Teubner Verlag (2008)

[5] Anorganische Chemie, Erwin Riedel, Gruyter; Auflage: 6. A. (2007)

[6] P. Gütlich, Z. anorg. allg. Chem 2012, 638 (1), 15.