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 restricted to two-level systems. We demonstrate its occurrence in a three-level system where the radiation induces a two-photon absorption. The intermediate level is (slightly) detuned from resonance. For a theoretical descrip tion the dressed-state picture is adopted. The two-photon excitation is performed either by using one laser, yielding a one-color two-photon process, or by using two counterpropagating, spatially overlapping laser beams, yielding a two-color two-photon process. Rabi oscillations are produced by varying the fluence, i.e., by changing either the laser power or the molecule-laser interaction time. For the one-color case the two-photon transition dipole moment for an SF6 transition has been determined from the relation between laser fluence and number of Rabi oscillations.

Progress on the motional Stark effect with laser-induced fluorescence diagnostic (invited) Rev. Sci. Instrum. 77, 10F311 (2006); 10.1063/1.2219432 Development of the motional Stark effect with laser-induced fluorescence diagnostic Rev. Sci. Instrum. 75, 3462 (2004); 10.1063/1.1779616 Influence of the ac Stark effect on multiphoton transitions in moleculesThe laser-induced ac-Stark effect influences the M sublevels of a molecule differently. This provides for a spectroscopical means of isolating individual M sublevels and thereby of producing aligned, or even oriented, excited molecules. We present an experimental study of this phenomenon in the .spherical top SF 6 using narrow bandwidth CO? lasers. One high power C0 2 1aser induces an IMI-dependent ac-Stark shift of the magnetic sublevels of the J=4 level of the vibrational ground state. A weak probe laser induces a two-photon transition starting from the same level and produces aligned, vibrationally excited molecules under appropriate experimental conditions. A theoretical description is provided in the dressed state picture. In principle, our triple-resonance technique is able to produce aligned or oriented molecules in a well-defined (vibrationally) excited state in any molecule possessing a similar three level system.

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A system is described in which a real molecular state is tuned into and out of resonance with the intermediate level of a two-photon transition, by the laser-induced ac-Stark effect. Two narrow-band CO2 wave-guide lasers are utilized to induce rovibrational transitions in the ν3 ladder of SF6. A weak probe laser monitors the absorption for the [Formula: see text] two-photon transition. Simultaneously, a strong pump laser is in resonance with the [Formula: see text] one-photon transition. The ac-Stark splitting is observed and quantum interferences between two-photon amplitudes involving different intermediate states occur. It is demonstrated that destructive interference allows suppression of the two-photon excitation for an appropriate pump laser power. Simulations based on the Dressed State Picture and the optical Bloch equations show a satisfactory agreement with the experiment.

Absolute elastic differential electron scattering cross sections in the intermediate energy region. III. SF6 and UF6 Double-differential cross sections for the interaction of Na͑3 2 S͒ and Na͑3 2 P͒ with SF 6 have been measured in crossed beam experiments for center of mass collision energies between 0.25 and 1.75 eV. In comparison with recently reported experiments the reaction with vibrationally excited SF 6 is found to be more effective than the one with electronically excited Na. Results from an ab initio CASSCF calculation with Na in the ground state and the 3 P state are presented. The experimental findings and the results from the calculation lead us to two different models for the reaction in the ground state and the excited state: While the well known harpooning model is verified for the ground state the reactive collisions with excited Na are mediated by nonadiabatic ͑nonreactive͒ transitions to the ground state surface. For these transitions the vibrational motion of SF 6 is much more efficient than the relative motion in the collision.