Optical control the singlet-triplet mixing via ac Stark effects in a spin-orbit mixed five-level molecular system coupled by three lasers.

The spin-orbit mixed rovibrational levels carry both characteristics of the singlet (the total spin quantum number S=0) and triplet(S=1) states. The mixing coefficient of the mixed states varies from case to case, which depends on the spin-orbit interaction energy. When the mixed states are coupled to auxiliary quantum states with lasers, the mixing coefficient of the singlet-triplet states can be modified by ac Stark effects via controlling the Rabi frequency, and the frequency detuning of the coupling lasers. We use density matrix equations in a five-level molecular system to show the Autler-Townes effects in a mixed singlet-triplet pair of rovibrational levels. The absorption of the probe laser and the populations of the two upper excited spin-orbit mixed rovibrational states can be controlled by the two coupling lasers. This technique can enhance the accessibility to the lower triplet rovibrational states which can't be directly accessed from the ground state due to the selection.