1 3
Top Curr Chem (Z) (2018) 376:6
single-shot signal detection [77] could accelerate the use of these approaches for
study of condensed phase molecular dynamics.
5 2D THz Magnetic Resonance Spectroscopy
The examples discussed in the above two sections all utilize the THz electric field
for 2D spectroscopy of molecular rotational and vibrational degrees of freedom.
It is also possible to use the THz magnetic field to drive the spin degrees of freedom in molecular and condensed matter systems and to conduct 2D THz magnetic
resonance spectroscopy, which is directly relevant to chemistry and biology. In this
section, we will discuss 2D THz magnetic resonance spectroscopy and show the
first example of its application to collective spin waves (magnons) in a magnetic
material.
5.1 Background and Motivation
Nonlinear manipulation of spins is the basis for all advanced methods in magnetic
resonance including multidimensional nuclear magnetic resonance and electron paramagnetic resonance (EPR) spectroscopies [78, 79], magnetic resonance imaging,
and, in recent years, quantum control over individual spins [80]. The methodology is
facilitated by the ease with which the strong-field regime can be reached for radiofrequency or microwave magnetic fields that drive nuclear or electron spins, respectively, typified by sequences of magnetic pulses that control the magnetic moment
directions [78–80]. The capabilities meet a bottleneck, however, for far-infrared
magnetic resonances, which are characteristic of molecular complexes including
molecular magnets [81] and metalloproteins [82] containing high-spin transitionmetal or rare-earth ions. In these systems, zero-field splittings (ZFSs) due to high
magnetic anisotropy and/or spin-spin interactions result in transition frequencies in
the THz frequency region even in the absence of external magnetic fields. Measurements of the spin resonances originating from ZFSs can provide mechanistic insight
into molecular magnetic properties and protein catalytic function as the ZFSs show
exquisite sensitivity to ligand geometries and transition metal electronic structure.
With strong applied magnetic fields (~ 10 T), resonances of unpaired electron spins
in molecular complexes and metalloproteins can be shifted from the usual microwave regime into the THz range, thereby drastically improving the resolution of
spectral splittings [79, 83, 84].
Despite the critical importance of THz-frequency EPR spectroscopy, current
EPR technology remains limited at THz frequencies because the weak sources
used only permit measurements of linear responses, i.e., 1QC or free-induction
decay (FID) signals. Utilization of the strong THz generation techniques discussed
in Sect. 2.1.1 can circumvent this limitation. Nonlinear and 2D THz spectroscopy
methods can allow the extension of established, commercially available multidimensional EPR spectroscopy from the microwave to the THz frequency range. To date,
the only available example of 2D THz spectroscopy of the spin degree of freedom
309
Reprinted from the journal
Top Curr Chem (Z) (2018) 376:6
single-shot signal detection [77] could accelerate the use of these approaches for
study of condensed phase molecular dynamics.
5 2D THz Magnetic Resonance Spectroscopy
The examples discussed in the above two sections all utilize the THz electric field
for 2D spectroscopy of molecular rotational and vibrational degrees of freedom.
It is also possible to use the THz magnetic field to drive the spin degrees of freedom in molecular and condensed matter systems and to conduct 2D THz magnetic
resonance spectroscopy, which is directly relevant to chemistry and biology. In this
section, we will discuss 2D THz magnetic resonance spectroscopy and show the
first example of its application to collective spin waves (magnons) in a magnetic
material.
5.1 Background and Motivation
Nonlinear manipulation of spins is the basis for all advanced methods in magnetic
resonance including multidimensional nuclear magnetic resonance and electron paramagnetic resonance (EPR) spectroscopies [78, 79], magnetic resonance imaging,
and, in recent years, quantum control over individual spins [80]. The methodology is
facilitated by the ease with which the strong-field regime can be reached for radiofrequency or microwave magnetic fields that drive nuclear or electron spins, respectively, typified by sequences of magnetic pulses that control the magnetic moment
directions [78–80]. The capabilities meet a bottleneck, however, for far-infrared
magnetic resonances, which are characteristic of molecular complexes including
molecular magnets [81] and metalloproteins [82] containing high-spin transitionmetal or rare-earth ions. In these systems, zero-field splittings (ZFSs) due to high
magnetic anisotropy and/or spin-spin interactions result in transition frequencies in
the THz frequency region even in the absence of external magnetic fields. Measurements of the spin resonances originating from ZFSs can provide mechanistic insight
into molecular magnetic properties and protein catalytic function as the ZFSs show
exquisite sensitivity to ligand geometries and transition metal electronic structure.
With strong applied magnetic fields (~ 10 T), resonances of unpaired electron spins
in molecular complexes and metalloproteins can be shifted from the usual microwave regime into the THz range, thereby drastically improving the resolution of
spectral splittings [79, 83, 84].
Despite the critical importance of THz-frequency EPR spectroscopy, current
EPR technology remains limited at THz frequencies because the weak sources
used only permit measurements of linear responses, i.e., 1QC or free-induction
decay (FID) signals. Utilization of the strong THz generation techniques discussed
in Sect. 2.1.1 can circumvent this limitation. Nonlinear and 2D THz spectroscopy
methods can allow the extension of established, commercially available multidimensional EPR spectroscopy from the microwave to the THz frequency range. To date,
the only available example of 2D THz spectroscopy of the spin degree of freedom
309
Reprinted from the journal