2-PHOTON RABI OSCILLATIONS

Rabi oscillations reflect the existence of a coherent superposition of two molecular eigenstates during the interaction of molecules with a strong, resonant electromagnetic field. This phenomenon is not restric ted to two-level systems. We demonstrate its occurrence in a three-lev el system where the radiation induces a two-photon absorption. The int ermediate level is (slightly) detuned from resonance. For a theoretica l description the dressed-state picture is adopted. The two-photon exc itation is performed either by using one laser, yielding a one-color t wo-photon process, or by using two counterpropagating, spatially overl apping laser beams, yielding a two-color two-photon process. Rabi osci llations are produced by varying the fluence, i.e., by changing either the laser power or the molecule-laser interaction time. For the one-c olor case the two-photon transition dipole moment for an SF6 transitio n has been determined from the relation between laser fluence and numb er of Rabi oscillations. For the two-color cases the detuning with the intermediate level is varied by tuning the respective laser frequenci es while keeping the sum frequency constant. A dramatic change in the degree of excitation as a function of the intermediate detuning has be en observed.