  We consider population transfer in open quantum systems, which are described
by quantum dynamical semigroups (QDS). Using second order perturbation theory
of the Lindblad equation, we show that it depends on a weak external field only
through the field's autocorrelation function (ACF), which is phase independent.
Therefore, for leading order in perturbation, QDS cannot support dependence of
the population transfer on the phase properties of weak fields. We examine an
example of weak-field phase-dependent population transfer, and show that the
phase-dependence comes from the next order in the perturbation.
