The quantum corrections to the behavior of a semi-classical gas can be expressed as a power series of the ratio
eta between the cube of De Broglie's thermal wavelength and the specific volume. The connection between
eta and the multioccupancy of quantum states is the aim of the present work. By means of a chemical/physical approach it is possible to associate
eta to the concrete realization of multioccupancy in momentum space, through the formation of what we call "pseudo-molecules", i.e. multiplets of particles sharing momenta whose differences are negligeable in the continuous spctrum approximation. The pseudo-molecules result as casual consequences of multiple scattering processes among non interacting (scattering apart) particles.
We propose a semi-phenomenological Markovian Master equation for describing the quantum dynamics of atom-surface scattering. It embodies the Lindblad-like structure and can describe both damping and pumping of energy between the system and the bath. It preserves positivity and correctly accounts for the vanishing of the interaction of the particle with the surface when the particle is distant from the surface. As a numerical test, we apply it to a model of an Ar atom scattered from a LiF surface, allowing for interaction only in the vertical direction. At low temperatures, we find that the quantum mechanical average energy loss is smaller than the classical energy loss. The numerical results obtained from the space dependent friction master equation are compared with numerical simulations for a discretized bath, using the multi-configurational time dependent Hartree methodology. The agreement between the two simulations is quantitative.