LASER COOLING OF INTERNAL DEGREES OF FREEDOM .2.

Theoretical progress in the cooling of internal degrees of freedom of molecules using shaped laser pulses is reported. The emphasis is on ge neral concepts and universal constraints. Several alternative definiti ons of cooling are considered, including reduction of the von Neumann entropy, -tr{<(rho)over caplog<(rho)over cap>} and increase of the Ren yi entropy, tr{<(rho)over cap>(2)}. A distinction between intensive an d extensive considerations is used to analyse the cooling process in o pen systems. It is shown that the Renyi entropy increase is consistent with an increase in the system phase space density and an increase in the absolute population in the ground state. The limitations on cooli ng processes imposed by Hamiltonian generated unitary transformations are analyzed. For a single mode system with a ground and excited elect ronic surfaces driven by an external field it is shown that it is impo ssible to increase the ground state population beyond its initial valu e. A numerical example based on optimal control theory demonstrates th is result. For this model only intensive cooling is possible which can be classified as evaporative cooling. To overcome this constraint, a single bath degree of freedom is added to the model. This allows a hea t pump mechanism in which entropy is pumped by the radiation from the primary degree of freedom to the bath mode, resulting in extensive coo ling. (C) 1997 American Institute of Physics.