In nonhydrogenic atoms in a dc electric field, the finite size of the ionic
core introduces a coupling between quasibound Stark states that leads to
avoided crossings between states that would otherwise cross. Near an avoided
crossing, the interacting states may have decay amplitudes that cancel each
other, decoupling one of the states from the ionization continuum. This well-
known interference narrowing effect, observed as a strongly electric field-
dependent decrease in the ionization rate, was previously observed in several
atoms. Here we use photoionization microscopy to visualize interference
narrowing in helium atoms, thereby explicitly revealing the mechanism by which
Stark states decay. The interference narrowing allows measurements of the
nodal patterns of red Stark states, which are otherwise not observable due to
their intrinsic short lifetime