Bright-state expansion and optimal control of highly excited polyatomics

A basis expansion of a quantum state of a molecule in a small orthogonal set of dynamically motivated, active bright states is presented and tested. The active bright states are derived from wavepackets (in a discrete variable representation) corresponding to different times in the time evolution of the molecule in a specified external laser field. Time evolution using the active bright-state representation is computationally less intensive and contains the essential dynamics in the quasiharmonic, quasicontinuum, and continuum regimes. Knowledge of the exact molecular spectroscopy is not needed. The active bright-state representation is thus well suited for optimal-control calculations which require a large number of wavepacket propagations.