Twelve-fold increase in the number of usable ThO molecules for the ACME electron electric dipole measurement through STIRAP*

The ACME Collaboration recently reported an order of magnitude improved limit on the electric dipole moment of the electron (eEDM) (ACME collaboration, Science \textbf{343} (2014), 269$-$272), setting more stringent constraints on many time reversal (T) violating extensions to the Standard Model. The experiment was performed using spin precession measurements in a molecular beam of thorium oxide. We report here on a new method of preparing the coherent spin superposition state that serves as the initial state of the spin precession measurement using STImulated Raman Adiabatic Passage (STIRAP). We demonstrate a transfer efficiency of $75\%$, giving a twelve-fold increase in signal. We discuss the particularities of implementing STIRAP in the ACME measurement and the methods we have used to overcome various challenges.