  We have studied gas-grain chemical models of interstellar clouds to search
for nonlinear dynamical evolution. A prescription is given for producing
oscillatory solutions when a bistable solution exists in the gas-phase
chemistry and we demonstrate the existence of limit cycle and relaxation
oscillation solutions. As the autocatalytic chemical processes underlying these
solutions are common to all models of interstellar chemistry, the occurrence of
these solutions should be widespread. We briefly discuss the implications for
interpreting molecular cloud composition with time-dependent models and some
future directions for this approach.
