LASER COOLING OF MOLECULAR INTERNAL DEGREES OF FREEDOM BY A SERIES OFSHAPED PULSES

Laser cooling of the vibrational motion of a molecule is investigated. The scheme is demonstrated for cooling the vibrational motion on the ground electronic surface of HBr. The radiation drives the excess ener gy into the excited electronic surface serving as a heat sink. Thermod ynamic analysis shows that this cooling mechanism is analogous to a sy nchronous heat pump where the radiation supplies the power required to extract the heat out of the system. In the demonstration the flow of energy and population from one surface to the other is analyzed and co mpared to the power consumption from the radiation field. The analysis of the flows shows that the phase of the radiation becomes the active control parameter which promotes the transfer of one quantity and sto ps the transfer of another. In the cooling process the transfer of ene rgy is promoted simultaneously with the stopping population transfer. The cooling process is defined by the entropy reduction of the ensembl e. An analysis based on the second law of thermodynamics shows that th e entropy reduction on the ground surface is more than compensated for by the increase in the entropy in the excited surface. It is found th at the rate of cooling reduces to zero when the state of the system ap proaches an energy eigenstate and is therefore a generalization of the third law of thermodynamics. The cooling process is modeled numerical ly for the HBr molecule by a direct solution of the Liouville von Neum an equation. The density operator is expanded using a Fourier basis. T he propagation is done by a polynomial approximation of the evolution operator. A study of the influence of dissipation on the cooling proce ss concludes that the loss of phase coherence between the ground and e xcited surface will stop the process.