In the last several years we have discovered a variety of remarkable pulse
strategies for manipulating molecular motion by employing a design strategy
we call "local optimization.'' Here we review the concept of local optimiz
ation and contrast it with optimal control theory. By way of background, we
give highlights from two recent examples of the method: (1) a strategy for
eliminating population transfer to one or many excited electronic states d
uring strong field excitation, an effect we call 'optical paralysis'; (2) a
generalization of the counterintuitive STIRAP (stimulated Raman adiabatic
passage) pulse sequence from three levels to N levels, a strategy we call '
straddling STIRAP.' We then turn to a third example, which is the main subj
ect of this paper: laser cooling of molecular internal degrees of freedom.
We study a model that includes both coherent interaction with the radiation
field and spontaneous emission; the latter is necessary to carry away the
entropy from the molecule. An optimal control calculation was performed fir
st and succeeded in producing vibrational cooling, but the resulting pulse
sequence was difficult to interpret. Local optimization subsequently reveal
ed the cooling mechanism: the instantaneous phase of the laser is locked to
the phase of the transition dipole moment between the excited state amplit
ude and v=0 of the ground state. Thus, the molecules that reach v=0 by spon
taneous emission become decoupled from the field, and no longer absorb, whi
le molecules in all other states are continually repumped. The mechanism co
uld be called "vibrationally selective coherent population trapping,'' in a
nalogy to the corresponding mechanism of velocity selective coherent popula
tion trapping in atoms for sub-Doppler cooling of translations.