  The Autler-Townes effect due to near resonance transition between 4s-4p
states in potassium atoms is mapped out in the photo-electron-momentum
distribution and manifests itself as a splitting in the photo-electron kinetic
energy spectra. The energy splitting fits well with the calculated Rabi
frequency at low laser intensities and shows clear deviation at laser
intensities above 1.5x10^11 W/cm^2. An effective Rabi frequency formulae
including the ionization process explains the observed results. Our results
reveal the possibility to tune the effective coupling strength with the cost of
the number of level-populations.
