  A model multilevel molecule described by two sets of rotational internal
energy levels of different parity and degenerate ground states, coupled by a
constant interaction, is considered, by assuming that the random collisions in
a gas of identical molecules, provoke transitions between adjacent energy
levels of the same parity. The prescriptions of the continuous time quantum
random walk are applied to the single molecule, interpreted as an open quantum
system, and the master equation driving its internal dynamics is built for a
general distribution of the waiting times between two consecutive collisions.
The coherence terms and the populations of the energy levels relax to the
asymptotics with inverse power laws for relevant classes of non-Poissonian
distributions of the collision times. The stable asymptotic equilibrium
configuration is independent of the distribution. The long time dynamics may be
hindered by increasing the tail of the distribution density. This effect may be
interpreted as the appearance of the quantum Zeno effect over long time scales.
