Article révisé par les pairs
| Résumé : | The evolution of strongly coupled systems is much clarified by the introduction of quasiparticles in the Boltzmann representation. These quasiparticles are described by a distribution function which is obtained by a new transformation from the asymptotic bare particle distribution function. They are entities which from the points of view of both their dynamical (kinetic equation) and thermodynamical (entropy) properties, behave like weakly coupled objects. The aim of the present work is to discuss the application of these concepts to the problem of interaction between radiation and bound electrons. The kinetic equation for quasiparticles, i.e. electron in perturbed levels, is derived up to order λ4. Whereas the Pauli equation (Born approximation) only describes one-photon processes, two-photon processes are now included. The meaning of the perturbed states, from the points of view of equilibrium properties and comparison with Rayleigh Schrödinger perturbation expansion, is briefly discussed. The kinetic equation is then used to discuss the problem of spontaneous decay of an excited state, taking into account the contributions of one- and two-photon processes. First, a rather general model is used to exhibit the main differences between the bare and quasiparticles pictures. Then, we restrict ourselves to a generalized Wigner-Weisskopf model and discuss mainly the sequential decay. A brief comparison with the T-matrix formalism is sketched; a more detailed comparison is left out for another paper. © 1968. |
