The Hamiltonian of a system of charged particles interacting with the electromagnetic field is investigated. For an arbitrary system the multipole expansion of the interaction between the system and the field is derived by means of a suitable canonical transformation. The transformed Hamiltonian is obtained from the Hamiltonian of the system by replacing the momenta by the transformed kinetic momenta and by adding to the Hamiltonian a term representing the interaction of the system with the electric component of the field. By expanding this interaction term, as well as the transformed momenta, in powers of the dimension of the system over the wavelength, the multipole expansion of the Hamiltonian is obtained. For a system interacting with a classical field the multipole form of the Hamiltonian is exactly equivalent to the original Hamiltonian. For a quantized field this is not true, and the multipole form of the transformed Hamiltonian is shown to be equivalent to the original Hamiltonian only for first-order radiation processes.

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The impact theory of Anderson for the pressure broadening of absorption and emission spectra is extended to the Raman spectra. Expressions are derived for the line shape and the optical cross sections in the classical path approximation. The central problem is the calculation of the average value of the evolution operator of the molecular system. It is shown that a simple derivation of the usual impact formula for this average value is obtained by averaging over all collisions as well as over all collision times. The perturbations of the intermediate states of the radiation processes are of importance only for resonant Raman scattering and may be neglected for non-resonant scattering. For freely rotating molecules the Raman scattering arising from electric dipole interaction can be decomposed into "isotropic", "magnetic dipole", and "electric quadrupole" scattering, corresponding to the j = 0, 1, and 2 irreducible parts of the Raman tensor. The optical cross sections for these three types of Raman scattering are different and are given by the reduced matrix elements, corresponding to j = 0, 1, and 2, of the optical cross-section operator, where j is the sum of the angular momenta in the initial and the complex conjugate of the final state of the radiation process.

Atoms, Molecules ze,,oo r,, and ClustersThis work is an extension of the previously developed description [1, 2] of the pulsed-light near-resonant scattering on an optically active atom --inert gas system for the case of an alkali metal atom. The alkali metal atom is assumed to be three level system, i.e. the ground state Sa/2 and two excited states Pa/2 and P3/2. The results, expressed in terms of the cross-sections and energy spectrum, are partially compared with some experimental results and theoretical calculations for stationary beam scattering.

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