  The evolution operator U(t) for a time-independent parity-time-symmetric
systems is well studied in the literature. However, for the non-Hermitian
time-dependent systems, a closed form expression for the evolution operator is
not available. In this paper, we make use of a procedure, originally developed
by A.R.P. Rau [Phys.Rev.Lett, 81, 4785-4789 (1998)], in the context of deriving
the solution of Liuville-Bloch equations in the product form of exponential
operators when time-dependent external fields are present, for the evaluation
of U(t) in the interaction picture wherein the corresponding Hamiltonian is
time-dependent and in general non-Hermitian. This amounts to a transformation
of the whole scheme in terms of addressing a nonlinear Riccati equation the
existence of whose solutions depends on the fulfillment of a certain
accompanying integrabilty condition.
