Oscillations, Waves, and Interactions - GWDG

338 R. Pottel, J. Haller and U. Kaatze
Figure 3. Real part ɛ ′ (ν) and negative imaginary part ɛ ′′ (ν) of the complex dielectric
spectrum of a solution of 0.051 mol/l NaCl in water at 20 ◦ C [47,48]. Open symbols represent
the dielectric part ɛ ′′ = ɛ ′′
tot − ɛ ′′
σ in the total loss. Triangles on the one hand and points as
well as circles on the other hand indicate results from two institutes [47].
2.2 Measurement techniques
Wide frequency ranges exist for both spectroscopic methods in which absolute measurements
of the liquid properties are enabled by variation of the sample thickness.
In dielectric measurements of aqueous systems the wavelength λ within the liquid at
frequencies roughly above 1 GHz is sufficiently small to enable variable path length
techniques for the determination of the complex propagation constant γ = α+i2π/λ
[38]. In ultrasonic spectrometry the limiting parameter is the absorption coefficient
which only above about 1 MHz is sufficiently large to enable absolute α measurements
[36,37]. At lower frequencies quasistatic ɛ (ν) measurements are performed
with the aid of suitable sample cells, whereas the ultrasonic attenuation coefficient is
obtained from resonator techniques in which the path length of interactions between
the sample and the sonic field is virtually increased by multiple reflections.
Figure 5 shows the scheme of a semi-automatic double-beam interferometer for
dielectric measurements in the microwave region [52]. It may be constructed from