Phase-resolved attosecond near-threshold photoionization of molecular nitrogen

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 $0.9\ensuremath{\pi}$ shift for the electrons produced in the ionization channels leading to the $X\text{ }{^{2}\ensuremath{\Sigma}}_{g}^{+}$, ${v}^{\ensuremath{'}}=1$, and ${v}^{\ensuremath{'}}=2$ 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 often not accessible otherwise.