THE REFRACTIVE-INDEX OF COLORLESS LIQUIDS IN THE VISIBLE AND INFRARED - CONTRIBUTIONS FROM THE ABSORPTION OF INFRARED AND ULTRAVIOLET-RADIATION AND THE ELECTRONIC MOLAR POLARIZABILITY BELOW 20500 CM(-1)

This paper addresses the separation of the contributions to the visible refractive index of colorless liquids from electronic (ultraviolet) and vibrational (infrared) absorption. The goal is to find the most accurate infrared values of n(el)(<(nu)over tilde>), the refractive index that results solely from electronic absorption, by fitting and extrapolating currently available visible refractive index data. These values are needed, interalia, to improve the accuracy of infrared real refractive index spectra calculated by the Kramers-Kronig transform of infrared imaginary refractive-index spectra. The electronic molar polarizability alpha(el)(<(nu)over tilde>) is calculated from the values of n(el)(<(nu)over tilde>) at wave numbers between 20 500 and 0 cm(-1). The methods are applied to ten liquids: H2O, D2O, CH3OH, CH3COOH, CH3CN (CH3)(2)CO, CH2Cl2, C6H6, C6H5Cl, and C6H5CH3. The visible refractive indices are expressed as power series in wave number, by expansion of the Kramers-Kronig integral. Terms in <(nu)over tilde>(+2m), m = 1,2, are due to the electronic contribution and terms in <(nu)over tilde>(-2m) are due to the vibrational contribution. The vibrational contribution to the visible refractive index is also calculated from experiment by Kramers-Kronig transformation of the known infrared imaginary refractive index spectrum of the liquid. It is shown that the vibrational absorption contributes greater than or equal to 0.001 to the visible refractive index only for the four hydrogen-bonded liquids, and that, for all ten liquids, at least 25% of the vibrational contribution arises from absorption below 2000 cm(-1). If the vibrational intensities are not known, the available visible refractive indices yield the most accurate infrared values of n(el) for all liquids except H2O if they are fitted to the equation n = a(0) + a(2)<(nu)over tilde>(2) + a(4)<(nu)over tilde>(4). A similar equation, with the additional term a(2)<(nu)over tilde>(2), is theoretically superior because the latter term adequately describes the vibrational contribution to the visible refractive indices, but only for H2O are the currently available visible refractive indices sufficiently accurate and sufficiently extensive to allow the four coefficients in the equation to be determined with useful accuracy. For H2O, D2O, CH3OH, CH2Cl2, C6H6, C6H5Cl3 and C6H5CH3, corrections are given to slightly improve the accuracy of the previously published infrared real refractive-index spectra. (C) 1995 American Institute of Physics.