to explain this enhancement in a given harmonic order, the former three-step model was modified and a new four-step model was introduced. this model takes into account the role of autoionization states present in the continuum. the first two steps remain the same i.e the tunnel ionization and the acceleration of this tunnel ionized electron in the continuum. however, in the third step, this tunnel ionization electron gets trapped into the autoionizing state present in the continuum. this autoionizing state usually has a longer lifetime. then in the fourth step, this trapped electron recombines radiatively with the parent ion (ground state) emitting resonantly enhanced high harmonic.