The polarizability of a pair of hydrogen atoms at long range

Molecular Interactions Induced by a Static Electric Field in Quantum Mechanics and Quantum Electrodynamics

By means of quantum mechanics and quantum electrodynamics applied to coupled harmonic Drude oscillators, we study the interaction between two neutral atoms or molecules subject to a uniform static electric field. Our focus is to understand the interplay between leading contributions to field-induced electrostatics/polarization and dispersion interactions, as considered within the employed Drude model for both non-retarded and retarded regimes. For the first case, we present an exact solution for two coupled oscillators obtained by diagonalizing the corresponding quantum-mechanical Hamiltonian and demonstrate that the external field can control the strength of different intermolecular interactions and relative orientations of the molecules. In the retarded regime described by quantum electrodynamics, our analysis shows that field-induced electrostatic and polarization energies remain unchanged (in isotropic and homogeneous vacuum) compared to the non-retarded case. For interacting species modeled by quantum Drude oscillators, the developed framework based on quantum mechanics and quantum electrodynamics yields the leading contributions to molecular interactions under the combined action of external and vacuum fields.

The scattering of light. II. The complex refractive index of a molecular fluid

We treat the complex refractive index of a finite molecular fluid on the basis of a classical many-body theory; the surface problem is handled through surfacedependent propagators. We develop a density expansion generalizing the Lorentz-Lorenz relation and sum all two-body terms to a closed form with intermolecular correlations determined by a Lennard-Jones pair potential; the dependence on density, temperature and frequency is discussed and the case when the frequency is near a molecular resonance is considered. The refractive index and the extinction coefficient are compared with experiments for gases. We also derive a generalization of the macroscopic relation of Onsager and Bottcher from the many-body theory, essentially as an expansion in an effective polarizability of a molecule in the many-body system. Exact microscopic expressions for an effective polarizability and for a reaction field are identified, and it is shown that they are related like the Bottcher polarizability and the Onsager reaction field in a well-defined decorrelation approximation. The relation with, and validity of, the macroscopic formulae of Lorentz and Bottcher are analysed in depth.

The density dependence of the refractivity of gases

A sensitive interferometer is described for measuring differentially the effects of molecular interactions on the refractivity of gases. The technique involves changing the density but not the amount of the gas in the optical path. Second refractivity virial coefficients, describing the change in molar refraction due to pairwise interactions, have been determined for neon, argon, nitrogen, carbon dioxide, methane, sulphur hexafluoride and fluoroform.