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 $0.9\pi$ shift for the electrons produced in the ionization channels leading to the $X{{}^2\Sigma }_g^{+}$, $v^{\prime }=1$, and $v^{\prime }=2$ 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.

• Received 21 November 2008

DOI:https://doi.org/10.1103/PhysRevA.80.011404