Temporal coherence effects in multiple ionization of N2 via XUV pump-probe autocorrelation

Using a split-mirror stage combined with a reaction microscope, nonlinear autocorrelation traces of XUV pulses from the Free-electron LASer at Hamburg were recorded for N-2 multiphoton-induced fragmentation into noncoincident N2+ and coincident N2+ + N2+ channels. We find a pulse duration of 40 +/- 10 fs along with a sharp spike pointing to a coherence time of 4 +/- 1 fs, almost twice as short as in previous observations. Both are well reproduced by a simulation based on the partial-coherence model that includes the molecular dynamics leading to an similar to 12-fs substructure in the trace.