We have shown that the Jaynes-Cummings model (JCM) with an environment
-induced pure decoherence is an exactly solvable problem. Rabi oscilla
tion of atomic population inversion and oscillation in the field popul
ation get trapped due to decoherence. But the decay of the coherence p
roperties of the atom as well as of the held is comparatively slower t
han that of the population terms. It is also shown that although the f
ast oscillation in nonclassical properties are suppressed due to decoh
erence but an initial Poissonian field acquires a sub-Poissonian chara
cter in the steady state. So unlike the ordinary spontaneous emission
all the nonclassical properties of the system are not erased.