Mössbauer Nomenclature
Recommendations on Nomenclature and Conventions for Reporting Mössbauer Data
IBAME (the International Board on the Applications of the Mössbauer Effect) has submitted this document to IUPAC for their review and approval. Comments are welcome and may be submitted to any of the members of the IBAME Sub-Committee on Nomenclature:

Frank Berry (f.j.berry@open.uk)
Hendrick de Waard (h.de.waard@phys.rug.nl)
John Stevens (stevens@unca.edu)

Introduction
A long time has elapsed since the previous recommendations were presented in the Information Bulletin published in August 1973 by The International Union of Pure and Applied Chemistry (Nr33 of Appendices on Tentative Nomenclature Symbols, Units and Standards). That document was drawn up by Commission I.1 (on Physico-Chemical Symbols, Terminology and Units) and Commission I.5 (on Molecular Structure and Spectroscopy) of the Physical Chemistry Division of IUPAC.

The extent to which Mössbauer Spectroscopy is used in physics and chemistry has now stabilized, and the customs for presenting Mössbauer data have become more standardized. The materials used for standard line sources and for calibration absorbers are now listed in the industrial specifications. For isomer shift standards, reference is best made to the tables published by the Mössbauer Effect Data Center(1) and the list of materials presented in the previous compilation has now been omitted. However, conventions for reporting results remain very useful and have been amended here. We recommend their use in any publication involving results of Mössbauer spectroscopy.

A. Nomenclature and conventions for reporting Mössbauer spectroscopic data

I. Text
The text should contain at least the following information:
 
(a) approximate source strength and composition

(b) the method of absorber mounting: thickness and confinement; precise composition if not standard

(c) the physical form of the absorber: single crystal, polycrystalline powder, inert filler (if used), evaporated film, rolled foil, isotopic enrichment, texture (see Note A1)

(d) the detector and data acquisition equipment, associated electronics, and method of detection, if unusual

(e) the type of experiment (emission, absorption, transmission, scattering, in-beam, angular dependent)

(f) use of critical absorbers or other filters

(g) method of data reduction (visual, by computer, curve fitting procedure) (see Note A2)

(h) the isomer shift convention used and the isomer shift relative to a standard reference absorber (or source) (see also Note B1)


II. Numerical and tabulated data
These should include:

(a) source and absorber temperatures

(b) values of the parameters used to characterize the relevant features of the Mössbauer spectra (in mm/s or other appropriate units) with estimated errors, in particular:

-isomer shift reference point

-line widths, defined as the full-width at half maximum peak-height (may be abbreviated to 'half-width')

-line intensities defined as fractions of background intensity (see Note A3)

-estimates of systematic and statistical errors of the parameters derived from the spectra

 
III. Figures illustrating spectra
Communications in which Mössbauer effect measurements constitute a primary source of information should include at least one illustrative spectrum (i.e. a plot of transmission or of number of counts per channel vs velocity). Such figures should include the following information:

(a) velocity units along the horizontal scale

(b) percentage or fractional transmission along the vertical scale (see Note A4)

(c) statistical error of the number of counts for at least one channel

(d) individual data points and (whenever possible) a computer fit, given as a continuous line clearly distinguishable from the experimental points, plus the subspectra if these cannot be readily discerned from the overall fit


B. Terminology, symbols and units for Mössbauer spectroscopy

(if non SI units are used they should be explicitly defined in the text)

 
Name
Symbol
SI Unit
Suggested Unit
Definition & Comment
isomer shift
δ
m/s
mm/s
Note B1
nuclear quadrupole moment (spectroscopic)
Q
m2
b(arn)
(10-28m2)
Note B2
electric field gradient (EFG) tensor
q
V/m2
V/cm2
Note B2
principal component of EFG
Vzz
V/m2
V/cm2
Note B2
quadrupole coupling constant
e2qQ/h
Hz
MHz
Note B2
quadrupole splitting
Δ
m/s
mm/s
Note B2, Note B3
quadrupole shift
ε
m/s
mm/s
e2qQ/4*(3cos2Θ-1)/2
asymmetry
η
(Vxx-Vyy)/Vzz
line width
Γexp
m/s
mm/s
full width at half the maximum of the observed resonance line
natural line width
Γnat
m/s
mm/s
usually derived from lifetime
resonance effect
l

%
difference in transmitted/scattered intensity at resonance and off-resonance divided by intensity off resonance
recoilless fraction
ƒ


fraction of gamma-rays emitted (ƒs) or (ƒa) without energy change
effective thickness



Note B4
resonance cross-section
σO
m2
b(arn)
(10-28m2

magnetic hyperfine field
B
T(WB/m2)

Note B5


Notes

A1 The concept of texture implies a non-random orientation of crystal axes, spins, etc. in a polycrystalline sample

A2 In the analysis of complex spectra, constraints should be specified (e.g. fixed line widths, intensity ratios)

A3 Line intensities may also be expressed as percentages (percentage - 100 x fraction). Since these numbers depend on detector quality etc., it is often advisable to give relative intensities. Line areas and relative line areas are sometimes preferred (but they should never be equaled to relative fractions of atoms in different sites, since the effect of differences of recoilless fractions is then disregarded)

A4 It is customary to display data obtained in transmission geometry with the resonance maximum down and scattering data with the maximum up. In both cases data should be shown far enough from the resonance peaks to firmly establish the non-resonant base line.

B1 In an absorber experiment, where absorber quantities are determined, the isomer shift is the energy difference between the absorber and the source transition energies (Ea-Es), usually given in terms of the Doppler velocity shift S = c(Ea - Es)/Eγ, where Eγ is the Mössbauer gamma energy and c is the speed of light in vacuum. Here, the isomer shift of the absorber relative to the source is positive if Ea > Es; i.e. if the source must be moving towards the absorber (or the absorber towards the source) to achieve resonance. In this case, the velocity shift and the isomer shift are equal.

In an emission experiment, where the isomer shift of the source relative to the absorbers considered, the velocity shift of the source relative to the absorber needed to achieve resonance is the opposite of the source isomer shift. Since absorber experiments are much more common than source experiments, a statement to this effect should be made in the text. To avoid confusion, it is advisable to always label the horizontal scale in a Mössbauer spectrum 'velocity scale'.

The isomer shift defined as above includes a term corresponding to a relativistic effect known as the 'second order Doppler shift'. The term that only depends on the change of the electron density at the nucleus is sometimes specified as the 'chemical isomer shift'. When this designation is used other effects should have been corrected for or stated to be negligible.

B2 The nuclear quadrupole moment describes the effective deviation of the shape of the nuclear charge distribution from spherical; e is the positive elementary charge; a prolate nucleus has Q > 0, an oblate one Q < 0.

In its interaction with a field gradient, the quadrupole moment of a general charge distribution has tensor character. By using a transformation of the coordinate axes, this interaction can be reduced to one with only two components: the z-component of the field gradient, Vzz and the asymmetry parameter, η.

B3 To facilitate a direct comparison with NMR and NQR data, quadrupole splittings are frequently reported in MHz. If this unit is used, the conversion factor to Doppler shift values should be stated.

B4 The reduced thickness is usually calculated for a thin absorber from the relation t=nσoƒ   in which n is the number of Mössbauer active nuclei per unit area in the path of the gamma rays, σo is the cross section for recoilless scattering and ƒ is the recoilless fraction.

B5 1 T = 104 Gauss



Ref. [1] W L Gettys and J G Stevens, Isomer Shift Reference Scales (Mössbauer Effect Data Center, University of North Carolina at Asheville, Asheville, N.C. 28804) 1979.

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