2D density plot for the kinetic energy release (KER) of coincident H<sup>+</sup>+C<sub>2</sub>H<sup>2+</sup> as a function of the delay time (a)

<p><strong>Figure 4.</strong> 2D density plot for the kinetic energy release (KER) of coincident H<sup>+</sup>+C<sub>2</sub>H<sup>2+</sup> as a function of the delay time (a). Three projections (solid lines) collecting the events in the KER bands of 4−8, 8−12 and 12−16 eV are plotted in (b)–(d), respectively. The expected time-delay dependence of the KER for delayed ionization is indicated by dashed lines. See text for more details. Exemplary error bars for the projections are shown for the rightmost data points.</p> <p><strong>Abstract</strong></p> <p>Few-photon induced ultrafast dynamics in acetylene (C<sub>2</sub>H<sub>2</sub>) leading to several dissociation channels—deprotonation (H<sup>+</sup>+C<sub>2</sub>H<sup>+</sup> and H<sup>+</sup>+C<sub>2</sub>H<sup>2+</sup>), symmetric break-up (CH<sup>+</sup>+CH<sup>+</sup>) and isomerization (C<sup>+</sup>+CH<sub>2</sub><sup>+</sup>)-–were investigated employing the (XUV; extreme ultra-violet)-pump–(XUV; extreme ultra-violet)-probe scheme at the free-electron laser in Hamburg, combined with multi-hit coincidence detection. The kinetic energy releases and fragment-ion momentum distributions for various decay channels are presented. The C<sup>+</sup>+CH<sub>2</sub><sup>+</sup> and H<sup>+</sup>+C<sub>2</sub>H<sup>2+</sup> channels reveal clear signatures of ultrafast molecular mechanisms, demonstrating potential applications of our pump-probe technique to complex systems in order to study a large variety of ultrafast phenomena in the XUV regime.</p>