  We photoionize nitrogen molecules with a train of extreme ultraviolet
attosecond pulses together with a weak infrared field. We measure the phase of
the two-color two-photon ionization transition (molecular phase) for different
states of the ion. We observe a 0.76 $\pi$ shift for the electrons produced in
the ionization channels leading to the X$^2 \Sigma_g^+$, $v'=1$ and $v'=2$
states. We relate this phase shift to the presence of a complex resonance in
the continuum. By providing both a high spectral and temporal resolution, this
general approach gives access to the evolution of extremely short lived states,
which is hardly accessible otherwise.
