Publisher Summary This chapter discusses molecular refractivity and polarizability. Refractive indices ( n ) of pure substances are more accurately measurable than any other optical properties. The refractive index of a substance varies with its physical state, temperature t , and wave-length λ of the light by which n is observed. The first two of these effects are attributed to the density d . The specific refraction r of a substance multiplied by the molecular weight is referred as molecular refraction. Polarizability of a particle is defined as the dipole moment induced by an electric field of unit intensity. The polarizabilities of monatomic ions and molecules are generally independent of field direction. The principal polarizabilities of molecules can be analyzed in terms of anisotropic bond polarizabilities. Polarizabilities are also responsible for the birefringence, which appears whenever the orientations of anisotropic molecules in an assemblage are derandomized or changed by any disturbing force.

Molecular Refractivity and Polarizability

Accurate values for the orientation-averaged long-range dipole-dipole dispersion energy coefficients, C 6(A, B), have been determined for all possible pair interactions involving ground state H, Li, N, O, H2, N2, O2, NH3, H2O, NO, and N2O. The calculations have been carried out by employing dipole oscillator strength distributions for these species that have been constructed (except in the case of H) by using discrete oscillator strength, photo-absorption, and high energy inelastic scattering data and by requiring the distributions to reproduce the Thomas-Reiche-Kuhn sum rule and, in the case of the molecules, available accurate refractivity and dispersion measurements for the relevant dilute gases. These oscillator strength distributions were also used to evaluate the refractivity R(λ), as a function of wavelength λ in the visible and ultra-violet region below the ultra-violet absorption thresholds, and the dipole oscillator strength sums S -2l , l = 1, 2, …, 7, for each atom and molecule. The calculated...

Dispersion energy constants C 6(A, B), dipole oscillator strength sums and refractivities for Li, N, O, H2, N2, O2, NH3, H2O, NO and N2O

We have recently presented a formalism for calculating zero-point vibrational corrections to molecular properties of polyatomic molecules in which the contribution to the zero-point vibrational correction from the anharmonicity of the potential is included in the calculations by performing a perturbation expansion of the vibrational wave function around an effective geometry. In this paper we describe an implementation of this approach, focusing on computational aspects such as the definition of normal coordinates at a nonequilibrium geometry and the use of the Eckart frame in order to obtain accurate nonisotropic molecular properties. The formalism allows for a black-box evaluation of zero-point vibrational corrections, completed in two successive steps, requiring a total of two molecular Hessians, 6K–11 molecular gradients, and 6K–11 property evaluations, K being the number of atoms. We apply the approach to the study of a number of electric and magnetic properties—the dipole and quadrupole moments, the...

An efficient approach for calculating vibrational wave functions and zero-point vibrational corrections to molecular properties of polyatomic molecules

Electronic and relaxation contribution to linear molecular polarizability. An analysis of the experimental values

Various techniques from the field of surface science have allowed the development of a method for measuring the total absolute cross section for electron-impact-induced dissociation of a number of molecules The technique is particularly reliable for the fluoroalkanes such as CF/sub 4/, CF/sub 3/H, C/sub 2/F/sub 6/, and C/sub 3/F/sub 8/ The total dissociation cross section for CF/sub 4/ has been measured for energies between threshold (approx125 eV) and 600 eV The magnitude of the cross section at its maximum is 55 x 10/sup -16/ cm/sup 2/ Less extensive data are presented for CF/sub 3/H, C/sub 2/F/sub 6/, and C/sub 3/F/sub 8/ Their cross sections at the maxima are 58 x 10/sup -16/ cm/sup 2/, 86 x 10/sup -16/ cm/sup 2/, and 118 x 10/sup -15/ cm/sup 2/, respectively Arguments are presented which suggest that the total dissociation cross section for each of these gases is equal to the sum of the cross sections for excitation to all electronic and ionic states, ie, the total cross section for electronic excitation The results are discussed from the point of view of the Bethe theory It is concluded that the Bethe asymptotic behavior is not yet attained in the energy range of more » these measurements « less

Total dissociation cross section of CF/sub 4/ and other fluoroalkanes for electron impact

In recent years, values of the refractive index of liquids, and sometimes of gases, have been extensively used for determining dipole-moments. Measurements of dielectric coefficients give the sum of the orientation, electronic and atomic polarizations P + Pe +Pa, while the electronic polarization Pe can be calculated approximately from the refractive index. If the dispersion is known, Pe should be given with some accuracy by calculating the refractive index at zero frequency. In order to obtain P, it is necessary to know Pa, a quantity which has only been determined for a few substances, the infra-red spectrum of which has been examined in detail. It is, however, often assumed that Pa is negligible or, at least, small in comparison with Pe. In some cases, an approximation on an empirical basis has been made by assuming that Pa is proportional to Pe, or that Pa + Pe is equal to the value of Pe when calculated from the refractive index for the yellow sodium line. Such assumptions can hardly be regarded as satisfactory. In liquids, determination of the polarization is complicated by the necessity of employing a solvent, but, with gases, no such difficulty exists. Pa can therefore be determined with some accuracy by measuring the refraction for different wave-lengths, and the dielectric coefficient at different temperatures. Numerous measurements of the refractive index of the commoner gases have been made, but the results of different observers are not always in agreement and frequently the dispersion has not been determined. The dielectric coefficient of the same sample of gas seems never to have been measured.

H. E. Watson

Papers

The Refractive Index Dispersion and Polarization of Gases