Laser cooling of molecules by dynamically trapped states

Optimal control theory (OCT) is applied to laser cooling of molecules. The objective is to cool vibrations, using shaped pulses synchronized with the spontaneous emission. An instantaneous in time optimal approach is compared to solution based on OCT. In both cases the optimal mechanism is found to operate by a "vibrationally selective coherent population trapping". The tr apping condition is that the instantaneous phase of the laser is locked to the phase of the transition dipole moment of nu = 0 with the excited popula tion. The molecules that reach a = 0 by spontaneous emission are then trapp ed. while the others are continually repumped. For vibrational cooling to n u = 2 and rotational cooling, a different mechanism operates. The field com pletely changes the transient eigenstates of the Hamiltonian creating a sup erposition composed of many states. Finally this superposition is transform ed by the field to the target energy eigenstate. (C) 2001 Elsevier Science B.V. All rights reserved.