Collective Mechanisms for Atomic Processes in Plasmas.

New mechanisms for atomic processes in plasmas induced by the collective behavior of the system are proposed, the collective behavior being a consequence of the long range of the Coulomb forces between the (charged) particles. These new mechanisms are hydrogen recombination with plasmon emission, ionization by plasmon absorption, and bound-bound and free-free atomic transitions with emission or absorption of plasmons. The Bohm-Pines Fock-Tani Hamiltonian for a proton at rest immersed in a finite temperature plasma (in the electron gas model), is obtained from first principles by a sequence of canonical transformations. This Hamiltonian shows explicitly the new proposed reaction and scattering mechanisms, in which, the emission or absorption of plasmons allow the process to occur. An effective potential for the hydrogen atom in a plasma is obtained which is very similar to the Ecker-Weizel potential used to interpret the plasma shifts of the discrete and continuous spectra of hydrogen. We interpret the negative shift of the proton as arising from emission and absorption of virtual plasmons. Expressions for plasmonic recombination matrix elements in the orthogonalized Born approximation (OBA) and the distorted wave Born approximation (DWBA) are given in terms of 6- or 12-dimensional integrals which are reduced analytically to 2-dimensional integrals. The generalization to the case of a slowly moving proton introduces an extra momentum conservation Kronecker delta factor in the matrix elements. Explicit evaluations of the cross section for the plasmonic and radiative recombination in the (OBA) are obtained for n = 10^{18} cm^{-3} and kT = 0.5 eV and for the states n = 8,9, ~l = 0,1, and m = 0. The results indicate that the radiative mechanism is negligible compared to the plasmonic mode for the value of the parameters and quantum numbers indicated above. This result points to the fact that the plasmonic mode should be compared to the mechanism usually assumed to be dominant for the regime under consideration, namely three-body recombination. Finally by using a generalized Schroedinger equation for composite particles in a medium we recalculate the energy shift of a hydrogen atom produced by the exchange between a free electron and the bound atomic electron. The result shows that the shifts are important for the highly excited states of atoms in plasmas.