  The method of perturbative expansion of master equation is employed to study
the dissipative properties of system and of atom in the two-photon
Jaynes-Cummings model (JCM) with degenerate atomic levels. The numerical
results show that the degeneracy of atomic levels prolongs the period of
entanglement between the atom and the field. The asymptotic value of atomic
linear entropy is apparently increased by the degeneration. The amplitude of
local entanglement and disentanglement is suppressed. The better the initial
coherence property of the degenerate atom, the larger the coherence loss.
