In this paper, we have presented an analytic solution for two 2-level
identical atoms interacting with a single-mode quantized radiation field, taking into
account the level shifts produced by Stark shift. We assume that the two atoms are
initially prepared in the exited state and the field in a coherent state.We investigate the
purity loss of the system and bipartite partitions of the system. The effects of the Stark
shift on the purity loss of the system and different bipartite partitions of the system
(field-two atoms, atom-(field+atom)) through the tangles are considered. In particular,
the effect of the Stark shift on the amount of entanglement between atoms and field is
evaluated by the negativity.