Time-dependent electric fields applied
to ionic systems
can induce
both a dielectric and a conductive response, leading to the generation
of macroscopic polarization and current, respectively. It has long
been recognized that it is not possible to determine the two types
of responses separately. However, this aspect is often not adequately
accounted for in dielectric and absorption spectroscopies of ionic
systems. To clarify this, we theoretically investigate the dielectric
and conductive responses of ionic systems containing polyatomic ions
based on linear response theory. We derive general expressions for
the frequency-dependent dielectric functions, conductivity, and absorption
coefficient, including those measured experimentally. Furthermore,
we show that the dielectric and conductive responses cannot be uniquely
distinguished even at the theoretical level and, therefore, cannot
represent experimentally measured quantities. Instead, dielectric
and absorption spectra of ionic systems should be expressed in terms
of the generalized dielectric function that encompasses both dielectric
and conductive responses. We propose a computational method to calculate
this generalized dielectric function reliably. Model calculations
on concentrated aqueous solutions of NaCl, a monatomic salt, and LiTFSI,
a polyatomic salt, show that the dielectric and linear absorption
spectra of the two systems based on the generalized dielectric function
are significantly different from purely dielectric counterparts in
the far-IR, terahertz, and lower-frequency regions. Moreover, the
spectra are mainly determined by the autocorrelations of total dipole
and total current, but dipole–current cross-correlation can
also significantly contribute to the spectra of the LiTFSI solution.
The present theoretical approach could be extended to nonlinear spectroscopy
of ionic liquids and electrolyte solutions.