Enhanced excitation of a driven bistable system induced by spectrum degeneracy

The nonequilibrium statistics and kinetics of a simple bistable system (resonantly driven nonlinear oscillator coupled to reservoir) have been investigated by means of master equation for the density matrix and quasiclas-sical Fokker-Planck equation in quasienergy space. We found that the system's statistical and kinetic properties drastically change when the quasienergy states become nearly degenerate and the occupation of the most excited state is strongly enhanced. It has been revealed that, in nearly degenerate case, a critical quasienergy parameter emerges. Below the critical quasienergy value the eigenstates are superpositions of the quasiclassical states from different phase-space regions, while above this value the eigenstates correspond to only one particular region of the phase space. We have also generalized Keldysh theory for ionization of atoms in the electromagnetic field for bistable systems. It has been demonstrated that Keldysh parameter in the bistability region is large when pumping intensity is smaller than the critical value. It has been shown by direct calculations that multiphoton transition amplitude coincides with the tunneling amplitude. So, multiphoton transitions and tunneling between the regions of the phase space are just the same effects. We also demonstrated that, for bistable systems, the Keldysh parameter logarithmically depends on the external field amplitude.