The Quantum Adiabatic Approximation in Chaotic Systems

Quantum systems with chaotic classical counterparts cannot be treated by perturbative techniques or any kind of adiabatic approximations. This is so, in spite of the quantum suppression of classical chaos. We explicitly calculate the adiabaticity parameter for the case of a Fermi Accelerator and show that the adiabatic expansion of the evolution operator is divergent. The relevance of this situation for the understanding of quantal effects in the one body dissipation mechanism in nuclear reactions is briefly discussed.