The investigation of coherent dynamics induced by photoionization of atoms or molecules by extreme ultra-violet (XUV) attosecond laser pulses requires careful consideration of the degree of ion + photoelectron entanglement that results from the photoionization process. Here, we consider coherent H2+ vibrational dynamics induced by photoionization of neutral H2 by a chirped attosecond laser pulse. We show that chirping the attosecond laser pulse leads to ion + photoelectron entanglement and the transition from a pure to a mixed state. This transition is characterized by evaluating the purity, which is close to unity for a transform-limited attosecond laser pulse and which decreases to a value that is determined by the number of vibrational states populated in the photoionization process for increasing values of the chirp parameter. In the calculations, the vibrational dynamics is probed by calculating time-delayed dissociation of the H2+ cation by a short ultra-violet (UV) laser pulse. Independent of the magnitude of the chirp, the coherent vibrational dynamics can be recovered by recording the XUV-UV delay-dependent kinetic energy release in coincidence with the kinetic energy of the accompanying photoelectron.
