Microscopic Entropy Flow and Entropy Production in Resonance Scattering

A microscopic dynamical entropy (or { H} function) for a two-level atom interacting with a field is introduced. The excitation process of the atom due to the resonance scattering of a wave packet is discussed. Three stages of scattering process (before, during and after the collision) are described in terms of entropy production and entropy flow. The excitation of the atom may be considered as the construction of a non-equilibrium structure due to entropy flow. The emission of photons distributes the energy of the unstable state among the field modes, leading to an increase of microscopic entropy. In this process, instability in dynamics associated with resonances plays a central role. The { H} function is constructed outside the Hilbert space, which allows strictly irreversible time evolution, avoiding probabilistic arguments associated with ignorance.