Recent theoretical developments have shown how such examples of excitation properties as the electronic band structure and the set of vibrational normal modes of a liquid can be studied by traditional classicalhyphen;liquidhyphen;theory methods. In this paper, we add another example to this collection: the set of polarization modes of a liquid. The basic notion is that in any polarizable but nonpolar fluid, the dynamics of the instantaneous dipoles can be represented as a linear combination of harmonic contributions from independent,microscopicallydefined, polarization modes. We note first how many of the properties one would like to know about the liquidmdash;its full dielectric behavior, its optical absorption spectrum, its effect on the absorption spectrum of a solute, and even how the net polarization of the liquid fluctuates with timemdash;are available from these polarization modes. We then point out how the requisite information about the modes can be ascertained by the same liquid theory methods used to treatphyphen;orbitalhyphen;based electronic problems. These considerations allow us to show how the mean spherical approximation can be used to obtain more accurate versions of optical spectra than was possible heretofore. It also suggests how one might begin to look at the dynamics of polarization in polar liquids.
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