Laser cooling of internal molecular degrees of freedom

We present a new approach to laser cooling of internal molecular degrees of freedom using a sequence of ultrashort laser pulses. Instead of attempting to maximize the vibrational ground-state population in a single step using an optimal control field, we use an optimally shaped, ultra-short laser pulse to transfer most of the population of the excited vibrational states to an excited electronic surface; then we switch the field off and allow the system to relax until most of the excited electronic state population has decayed due to spontaneous emission. We repeat this procedure a few times until the vibrational ground state population has reached a desired minimum value of ca. 90%. The advantages of this procedure are that it relies mainly on spontaneous emission to reduce the entropy of the system, there is virtually no population loss on the ground surface at the final time, and unlike one-step optimization procedures using coherent control, it is effective even if the lifetime of the excited electronic states is much longer than the length of the pulse