Stark shift of the $A^2\Pi_{1/2}$ state in $^{174}$YbF

We have measured the Stark shift of the $A^2\Pi_{1/2} - X^2\Sigma^+$ transition in YbF. We use a molecular beam triple resonance method, with two laser transitions acting as pump and probe, assisted by an rf transition that tags a single hyperfine transition of the X state. After subtracting the known ground state Stark shift, we obtain a value of $70.3(1.5)$Hz/(V/cm)$^2$ for the static electric polarizability of the state $A^2\Pi_{1/2}(J=1/2,f)$. From this we calculate a value $\mu_e=2.46(3)$D for the electric dipole moment of the $A^2\Pi_{1/2}(v=0)$ state.