All-optical control of quantum state singlet-triplet character by Autler-Townes splitting.*

Using a four level extended Lambda excitation scheme [1] in $^{7}$Li$_{2}$, we have demonstrated that Autler --Townes splitting can be used to modify the mixing coefficients and thus the amount of singlet or triplet character of a pair of rovibrational A$^{1}\Sigma _{u}^{+}$ and b$^{3}\Pi _{u }$states perturbed by the spin-orbit interaction. The singlet triplet pair of levels is naturally separated by an energy gap of 600 MHz. As indicated earlier [2] such control of molecular valence electron spin polarization can be used to create new gateway levels to the dark manifold of triplet states in molecular systems with a singlet ground state in which the spin orbit interaction is very weak and only a few gateway levels to triplet states exist naturally. In addition, a direct measurement of the spin-orbit interaction strength is possible from the observed AT splitting lineshape [2]. \begin{enumerate} \item E. Ahmed et al., J. Chem. Phys. 124, 084308 --1 to 13 (2006). \item T. Kirova and F. C. Spano, Phys. Rev. A, 71, 063816 (2005). \end{enumerate}