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 trapping 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 population. The molecules that reach a = 0 by spontaneous emission are then trapped. while the others are continually repumped. For vibrational cooling to nu = 2 and rotational cooling, a different mechanism operates. The field completely changes the transient eigenstates of the Hamiltonian creating a superposition composed of many states. Finally this superposition is transformed by the field to the target energy eigenstate. (C) 2001 Elsevier Science B.V. All rights reserved.