  We demonstrate that by changing the direction of the chirp in VUV pulses one
can switch between excitation and ionization with very high contrast, if the
carrier frequency of the light is resonant with two bound states. This is a
surprising consequence if rapid adiabatic passage is extended to include
transitions to the continuum. The chirp phase locks the linear combination of
two resonantly coupled bound states whose ionization amplitudes interfere
constructively or destructively depending on the chirp direction under suitable
conditions. We derive the phenomenon in a minimal model and verify the effect
with calculations for helium as a realistic example.
