82
2 The Interaction of Electromagnetic Waves with Water
LO–TO splitting assumes long-range correlations, we should consider corresponding changes to the Bernal–Fowler water model, as the model does not assume any
long-range order for water. Finally, the intense far-IR part of the water spectrum indicates strong molecular correlations. This part of the spectrum, insensitive to isotopic
substitution, requires intermolecular correlations that go beyond the model of the
network of bounded H 2 O molecules.
2.6 The Terahertz Spectrum of Water
2.6.1 Between the Infrared and Dielectric Spectra
Vibrational spectroscopy studies intramolecular oscillations and dielectric spectroscopy is a reflection of collective dynamics, while terahertz spectroscopy, which
lies at the frequencies between these two methods, probes the transition between
oscillatory and diffusion motion in water and ice. That is why the continuous spectrum
of water was missing until terahertz data appeared. Time-domain laser spectroscopy
made the terahertz data widely spread; however, there are still some technical difficulties.
The first comprehensive collection of spectral data points was made by Segelstein
in 1981 [69] and is still used for analysis. Mid-IR and far-IR data were measured
by Zelsmann in 1995 [65] and updated by Bertie et al. in 1996 [70], where the
attenuated total reflection (ATR) technique was used. A more recent collation of
data was made by Artemov in 2014 [71], and new experimental data from 500 MHz
to 400 THz were provided by Shiraga et al. in 2018 [72]. Despite the great work
on data collection and the availability of many independent measurements, the data
points still vary significantly depending on the sample geometry, the method used,
and the data processing procedure.
Figure 2.18a collates spectral data points for water, which were obtained over
the past few decades. The terahertz–infrared part of the spectrum is enlarged on the
right-hand side of the graph. Although a generally good correlation between the data
points has been achieved, there is still a large degree of uncertainty in the terahertz
frequency region (mainly in the real part of the dielectric permittivity). In particular,
there is uncertainty in both the peak center and integrated absorption cross section
of the ν s mode centered at about 5 THz (200 cm
−1 ), and there is a divergence of the
data in between 0.1 and 1 THz, which contain both secondary relaxations: ν D2 and
ν D3 .
The terahertz region covers the intermolecular vibrational modes, which appear
in the frequency window from 1 THz up to about 10 THz, corresponding to 30–
300 cm
−1 . This part of the broadband spectrum reflects the dynamics in the first
and the second molecular coordination spheres at distances of about 6–7 Å. The
long-range sensitivity of terahertz absorption to the dynamic properties of water
2 The Interaction of Electromagnetic Waves with Water
LO–TO splitting assumes long-range correlations, we should consider corresponding changes to the Bernal–Fowler water model, as the model does not assume any
long-range order for water. Finally, the intense far-IR part of the water spectrum indicates strong molecular correlations. This part of the spectrum, insensitive to isotopic
substitution, requires intermolecular correlations that go beyond the model of the
network of bounded H 2 O molecules.
2.6 The Terahertz Spectrum of Water
2.6.1 Between the Infrared and Dielectric Spectra
Vibrational spectroscopy studies intramolecular oscillations and dielectric spectroscopy is a reflection of collective dynamics, while terahertz spectroscopy, which
lies at the frequencies between these two methods, probes the transition between
oscillatory and diffusion motion in water and ice. That is why the continuous spectrum
of water was missing until terahertz data appeared. Time-domain laser spectroscopy
made the terahertz data widely spread; however, there are still some technical difficulties.
The first comprehensive collection of spectral data points was made by Segelstein
in 1981 [69] and is still used for analysis. Mid-IR and far-IR data were measured
by Zelsmann in 1995 [65] and updated by Bertie et al. in 1996 [70], where the
attenuated total reflection (ATR) technique was used. A more recent collation of
data was made by Artemov in 2014 [71], and new experimental data from 500 MHz
to 400 THz were provided by Shiraga et al. in 2018 [72]. Despite the great work
on data collection and the availability of many independent measurements, the data
points still vary significantly depending on the sample geometry, the method used,
and the data processing procedure.
Figure 2.18a collates spectral data points for water, which were obtained over
the past few decades. The terahertz–infrared part of the spectrum is enlarged on the
right-hand side of the graph. Although a generally good correlation between the data
points has been achieved, there is still a large degree of uncertainty in the terahertz
frequency region (mainly in the real part of the dielectric permittivity). In particular,
there is uncertainty in both the peak center and integrated absorption cross section
of the ν s mode centered at about 5 THz (200 cm
−1 ), and there is a divergence of the
data in between 0.1 and 1 THz, which contain both secondary relaxations: ν D2 and
ν D3 .
The terahertz region covers the intermolecular vibrational modes, which appear
in the frequency window from 1 THz up to about 10 THz, corresponding to 30–
300 cm
−1 . This part of the broadband spectrum reflects the dynamics in the first
and the second molecular coordination spheres at distances of about 6–7 Å. The
long-range sensitivity of terahertz absorption to the dynamic properties of water