Top Curr Chem (Z) (2018) 376:6
1 3
elaboration of the different signal contributions to 2D rotational spectra at third and
fifth order. The results suggest possible experimental advances and a wide range of
new information that can be uncovered. For example, by including a third THz pulse
at a controlled delay with respect to the second pulse [33] such that the rotational
population time can be varied, 2D rotational spectroscopy with three THz pulses
can enable measurements of the time-dependent evolution of off-diagonal spectral
peaks and spectral diffusion as in other 2D spectroscopies [69, 70]. Such measurements may reveal specific rotational energy transfer and relaxation pathways (due to
dipole-dipole interactions among the molecules under study, collisions with other
species, etc.) by comparing the strengths and line shapes of the 2D spectral peaks
for different population times. Independent control of the THz pulse polarizations
could reveal the dynamics of relaxation among the M sublevels of the rotational J
levels. The use of stronger THz pulses will allow measurement of still higher-order
signal contributions, which will reveal additional correlations among rotational transitions and will allow the method to be used on molecules with moderate or small
dipole moments. In addition to THz excitation pulses, optical excitation can be used
to achieve enhanced control over rotational dynamics in linear and nonlinear molecules [71, 72]. Thus, a wide range of experimental refinements is possible, offering prospects for new insights into molecular rotational dynamics and the molecular
interactions that mediate them.
4 2D THz and Hybrid 2D THz‑Raman Vibrational Spectroscopies
Multidimensional IR vibrational spectroscopy has proved to be a powerful tool to
study complex liquid-state vibrational dynamics, for example, the hydrogen bonds
in water [5, 73] and intramolecular vibrations in proteins and DNA [6, 7]. Multidimensional THz vibrational spectroscopy of liquids could extend the range to
low-frequency molecular vibrations and intermolecular motions that may provide
additional insights, for example, into the structural dynamics of water, proteins, and
DNA and to their chemical properties. However, 2D THz vibrational spectroscopy
has so far been realized only on lattice vibrations in semiconductors. On the other
hand, hybrid 2D spectroscopy combining THz and optical excitation or detection
methods has enabled 2D THz-Raman variations that are complementary to 2D THz
and 2D Raman spectroscopies. As dipole and polarizability interactions are both
involved, the hybrid methods can provide enhanced sensitivity to study some vibrational modes and their interactions. In this section, we discuss some of the 2D THz
and 2D THz-Raman vibrational spectroscopies and their applications to the study
of lattice vibrations in semiconductors, molecular vibrations in halogenated liquids,
and intermolecular dynamics of the hydrogen-bond networks in water and ionic
aqueous solutions.
298
Reprinted from the journal
1 3
elaboration of the different signal contributions to 2D rotational spectra at third and
fifth order. The results suggest possible experimental advances and a wide range of
new information that can be uncovered. For example, by including a third THz pulse
at a controlled delay with respect to the second pulse [33] such that the rotational
population time can be varied, 2D rotational spectroscopy with three THz pulses
can enable measurements of the time-dependent evolution of off-diagonal spectral
peaks and spectral diffusion as in other 2D spectroscopies [69, 70]. Such measurements may reveal specific rotational energy transfer and relaxation pathways (due to
dipole-dipole interactions among the molecules under study, collisions with other
species, etc.) by comparing the strengths and line shapes of the 2D spectral peaks
for different population times. Independent control of the THz pulse polarizations
could reveal the dynamics of relaxation among the M sublevels of the rotational J
levels. The use of stronger THz pulses will allow measurement of still higher-order
signal contributions, which will reveal additional correlations among rotational transitions and will allow the method to be used on molecules with moderate or small
dipole moments. In addition to THz excitation pulses, optical excitation can be used
to achieve enhanced control over rotational dynamics in linear and nonlinear molecules [71, 72]. Thus, a wide range of experimental refinements is possible, offering prospects for new insights into molecular rotational dynamics and the molecular
interactions that mediate them.
4 2D THz and Hybrid 2D THz‑Raman Vibrational Spectroscopies
Multidimensional IR vibrational spectroscopy has proved to be a powerful tool to
study complex liquid-state vibrational dynamics, for example, the hydrogen bonds
in water [5, 73] and intramolecular vibrations in proteins and DNA [6, 7]. Multidimensional THz vibrational spectroscopy of liquids could extend the range to
low-frequency molecular vibrations and intermolecular motions that may provide
additional insights, for example, into the structural dynamics of water, proteins, and
DNA and to their chemical properties. However, 2D THz vibrational spectroscopy
has so far been realized only on lattice vibrations in semiconductors. On the other
hand, hybrid 2D spectroscopy combining THz and optical excitation or detection
methods has enabled 2D THz-Raman variations that are complementary to 2D THz
and 2D Raman spectroscopies. As dipole and polarizability interactions are both
involved, the hybrid methods can provide enhanced sensitivity to study some vibrational modes and their interactions. In this section, we discuss some of the 2D THz
and 2D THz-Raman vibrational spectroscopies and their applications to the study
of lattice vibrations in semiconductors, molecular vibrations in halogenated liquids,
and intermolecular dynamics of the hydrogen-bond networks in water and ionic
aqueous solutions.
298
Reprinted from the journal