2 Synchrotron-Radiation-Based Energy-Domain Mössbauer …
71
the measuring time tends to be shorter in NFS because the interference pattern of
all nuclear levels by hyperfine splitting is measured in the time region without any
energy-scanning mechanism. In contrast, the energy dependence of the scattering
intensity is measured with the mechanical motion of the energy reference substance
in SR-based Mössbauer spectroscopy. The efficiency of the measurement system in
NFS is somewhat small in experiments with high-resonant-energy isotopes owing
to the detection efficiency of the APD. Only high-energy γ-rays from the sample
should be detected in NFS, while low-energy fluorescent X-rays and electrons after
the internal conversion processes from the scatterer could be also detected in SRbased Mössbauer spectroscopy. Additionally, it is sometimes difficult to obtain the
interference pattern in the time spectra of NFS when the lifetime of the nuclear excited
level of the probe isotope is short. In those cases, SR-based Mössbauer spectroscopy
is preferable. Thus, SR-based Mössbauer spectroscopy suits Mössbauer spectroscopy
using nuclear levels, whose energy is high and whose lifetime is short.
2.3 Nuclear Resonant Inelastic Scattering
While Mössbauer spectroscopy uses the recoilless nuclear resonant excitation effect,
the energy tunability of SR enables the measurement of the recoil part. Therefore,
nuclear resonant excitation accompanied by phonon excitation can be measured using
SR. Hence, the first experiment was performed in 1994 [5], and phonon measurement
was conducted [41, 42]. The NRIS method has distinct features favorable for studies
concerning the microscopic dynamics (e.g., phonons and molecular vibration) of
materials because it provides the element (isotope)-specific dynamics due to the
resonant excitation of the specific isotope. In solids, partial phonon densities of states
are measured. Furthermore, measurements under extreme conditions, such as high
pressures, small samples, and thin films, are possible because of the high brilliance
of SR. (For the definition of brilliance, see Chap. 1.) Note that the element (isotope)specific nature enables the measurement of very dilute resonant atoms in complicated
materials. Recently, this method has been actively used to study the local structure of
active sites in enzymes along with first-principles density functional theory (DFT).
2.3.1 Instrumentation and Analysis of the Basic Method
of NRIS
The measurement is performed through inelastic processes with photons. The energy
of each photon is equal to the sum of the energy of the nuclear resonant excited state
(based on the ground state) and a phonon (or phonons). Since the typical energy
of phonon is 1–100 meV, spectroscopic resolution required for the measurement is
71
the measuring time tends to be shorter in NFS because the interference pattern of
all nuclear levels by hyperfine splitting is measured in the time region without any
energy-scanning mechanism. In contrast, the energy dependence of the scattering
intensity is measured with the mechanical motion of the energy reference substance
in SR-based Mössbauer spectroscopy. The efficiency of the measurement system in
NFS is somewhat small in experiments with high-resonant-energy isotopes owing
to the detection efficiency of the APD. Only high-energy γ-rays from the sample
should be detected in NFS, while low-energy fluorescent X-rays and electrons after
the internal conversion processes from the scatterer could be also detected in SRbased Mössbauer spectroscopy. Additionally, it is sometimes difficult to obtain the
interference pattern in the time spectra of NFS when the lifetime of the nuclear excited
level of the probe isotope is short. In those cases, SR-based Mössbauer spectroscopy
is preferable. Thus, SR-based Mössbauer spectroscopy suits Mössbauer spectroscopy
using nuclear levels, whose energy is high and whose lifetime is short.
2.3 Nuclear Resonant Inelastic Scattering
While Mössbauer spectroscopy uses the recoilless nuclear resonant excitation effect,
the energy tunability of SR enables the measurement of the recoil part. Therefore,
nuclear resonant excitation accompanied by phonon excitation can be measured using
SR. Hence, the first experiment was performed in 1994 [5], and phonon measurement
was conducted [41, 42]. The NRIS method has distinct features favorable for studies
concerning the microscopic dynamics (e.g., phonons and molecular vibration) of
materials because it provides the element (isotope)-specific dynamics due to the
resonant excitation of the specific isotope. In solids, partial phonon densities of states
are measured. Furthermore, measurements under extreme conditions, such as high
pressures, small samples, and thin films, are possible because of the high brilliance
of SR. (For the definition of brilliance, see Chap. 1.) Note that the element (isotope)specific nature enables the measurement of very dilute resonant atoms in complicated
materials. Recently, this method has been actively used to study the local structure of
active sites in enzymes along with first-principles density functional theory (DFT).
2.3.1 Instrumentation and Analysis of the Basic Method
of NRIS
The measurement is performed through inelastic processes with photons. The energy
of each photon is equal to the sum of the energy of the nuclear resonant excited state
(based on the ground state) and a phonon (or phonons). Since the typical energy
of phonon is 1–100 meV, spectroscopic resolution required for the measurement is