Separation energy is nothing but the energy of the electron in any excited state.
For, H− atom first excited state(n=2), Es1=3.4 eV
For He+ ion, fourth excited state (n=5), Es2=13.6×2252
=2.17 eV
where, Es is separation energy
Now, from photoelectric equation,
K.E.max=Ei−E0
where, Ei− incident energy
E0− Threshold energy (work function)
So, stopping potential =K.E.maxe=(3.4−2.17) V
≅1.2 V