70
M. Seto et al.
including experiments by NFS and conventional Mössbauer spectroscopy will help
the analysis. A substantial description on various isotopes by Greenwood and Gibb
[2], Shenoy and Wagner [38], and Gütlich et al. [39] can be very useful.
2.2.4 Comparison with Other Methods Using SR
Here, the properties of the SR-based Mössbauer spectroscopy are compared with
those of the SMS and NFS, the two major methods to observe the hyperfine structure
of samples using SR. These methods are in a complementary relation. The differences
in these methods described here will help in selecting the best method for a study.
As for the SMS, energy-domain spectra are obtained similarly to SR-based Mössbauer spectroscopy. One large difference between the two is the nuclide availability.
In the SMS, a unique component produces Mössbauer radiation, that is, SR whose
energy width is as narrow as the natural linewidth of a Mössbauer isotope. This
component is a nuclear monochromator at which the nuclear diffraction is allowed,
although the usual diffraction by electrons is prohibited [40]. Nuclear monochromator
is successfully developed only for
57 Fe, and thus, the SMS is limited to
57 Fe Mössbauer spectroscopy, which is different from SR-based Mössbauer spectroscopy, in
which many isotopes can be used. Furthermore, the nuclear monochromator produces
Mössbauer radiation without a time window, and thus, the SMS method has no limit
on the time structure on SR in principle. SR-based Mössbauer spectroscopy requires
at least some kind of pulsed time structure of SR with the time period for the time
window by which the delayed nuclear resonant scattering is selected. Although this
difference is only a technical point, it is important to the actual experiment because
various bunch modes exist in SR facilities. Neither width-narrowing effect nor the
wavy background is seen in the SMS, and thus, the analysis of the SMS spectra is
usually more intuitive.
As for the NFS, the hyperfine structure of various isotopes can be observed by
both NFS and SR-based Mössbauer spectroscopy. A large difference is noticeable
in the appearance of the spectra. The time spectra showing beat patterns due to
the interference of nuclear hyperfine splitting at the sample are obtained in NFS.
Therefore, NFS is suitable in tracing a reaction depending on the time in the scale of
nanoseconds, such as pump-probe spectroscopy. For stable samples, this difference
in appearance affects the analyzing model at an initial guess: the energy spectra
of SR-based Mössbauer spectroscopy are intuitive. When there are two or more
components in the sample, this intuition becomes important. In the experimental
viewpoint, NFS requires an appropriate time structure of SR to obtain the time
spectra. This limitation is stricter in NFS than in SR-based Mössbauer spectroscopy.
The required period between SR pulses in NFS depends on the magnitude of hyperfine
splitting to be evaluated. However, the magnitude is not usually obvious. In actual
experiments, the period with more than the lifetime of the nuclear excited state is
usually enough (the longer the period, the better). This limitation sometimes restricts
isotope availability. When we have the suitable timing bunch mode of SR facility,
M. Seto et al.
including experiments by NFS and conventional Mössbauer spectroscopy will help
the analysis. A substantial description on various isotopes by Greenwood and Gibb
[2], Shenoy and Wagner [38], and Gütlich et al. [39] can be very useful.
2.2.4 Comparison with Other Methods Using SR
Here, the properties of the SR-based Mössbauer spectroscopy are compared with
those of the SMS and NFS, the two major methods to observe the hyperfine structure
of samples using SR. These methods are in a complementary relation. The differences
in these methods described here will help in selecting the best method for a study.
As for the SMS, energy-domain spectra are obtained similarly to SR-based Mössbauer spectroscopy. One large difference between the two is the nuclide availability.
In the SMS, a unique component produces Mössbauer radiation, that is, SR whose
energy width is as narrow as the natural linewidth of a Mössbauer isotope. This
component is a nuclear monochromator at which the nuclear diffraction is allowed,
although the usual diffraction by electrons is prohibited [40]. Nuclear monochromator
is successfully developed only for
57 Fe, and thus, the SMS is limited to
57 Fe Mössbauer spectroscopy, which is different from SR-based Mössbauer spectroscopy, in
which many isotopes can be used. Furthermore, the nuclear monochromator produces
Mössbauer radiation without a time window, and thus, the SMS method has no limit
on the time structure on SR in principle. SR-based Mössbauer spectroscopy requires
at least some kind of pulsed time structure of SR with the time period for the time
window by which the delayed nuclear resonant scattering is selected. Although this
difference is only a technical point, it is important to the actual experiment because
various bunch modes exist in SR facilities. Neither width-narrowing effect nor the
wavy background is seen in the SMS, and thus, the analysis of the SMS spectra is
usually more intuitive.
As for the NFS, the hyperfine structure of various isotopes can be observed by
both NFS and SR-based Mössbauer spectroscopy. A large difference is noticeable
in the appearance of the spectra. The time spectra showing beat patterns due to
the interference of nuclear hyperfine splitting at the sample are obtained in NFS.
Therefore, NFS is suitable in tracing a reaction depending on the time in the scale of
nanoseconds, such as pump-probe spectroscopy. For stable samples, this difference
in appearance affects the analyzing model at an initial guess: the energy spectra
of SR-based Mössbauer spectroscopy are intuitive. When there are two or more
components in the sample, this intuition becomes important. In the experimental
viewpoint, NFS requires an appropriate time structure of SR to obtain the time
spectra. This limitation is stricter in NFS than in SR-based Mössbauer spectroscopy.
The required period between SR pulses in NFS depends on the magnitude of hyperfine
splitting to be evaluated. However, the magnitude is not usually obvious. In actual
experiments, the period with more than the lifetime of the nuclear excited state is
usually enough (the longer the period, the better). This limitation sometimes restricts
isotope availability. When we have the suitable timing bunch mode of SR facility,