Laser cooling of internal degrees of freedom of molecules by dynamically trapped states

Citation
Dj. Tannor et al., Laser cooling of internal degrees of freedom of molecules by dynamically trapped states, FARADAY DIS, (113), 1999, pp. 365-383
Citations number
65
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
Faraday discussions
ISSN journal
13596640 → ACNP
Issue
113
Year of publication
1999
Pages
365 - 383
Database
ISI
SICI code
1359-6640(1999):113<365:LCOIDO>2.0.ZU;2-P
Abstract
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.