Two-dimensional fragment ion momentum distributions for dissociation channels of (a) C<sub>2</sub>H<sub>2</sub><sup>2+</sup> → H<sup>+</sup>+C<sub>2</sub>H<sup>+</sup>, (b) C<sub>2</sub>H<sub>2</sub><sup>2+</sup> → CH<sup>+</sup>+CH<sup>+</sup>, (c) C<sub>2</sub>H<sub>2</sub><sup>2+</sup> → C<sup>+</sup>+CH<sub>2</sub><sup>+</sup> and (d) C<sub>2</sub>H<sub>2</sub><sup>3+</sup> → H<sup>+</sup>+C<sub>2</sub>H<sup>2+</sup>

<p><strong>Figure 3.</strong> Two-dimensional fragment ion momentum distributions for dissociation channels of (a) C<sub>2</sub>H<sub>2</sub><sup>2+</sup> → H<sup>+</sup>+C<sub>2</sub>H<sup>+</sup>, (b) C<sub>2</sub>H<sub>2</sub><sup>2+</sup> → CH<sup>+</sup>+CH<sup>+</sup>, (c) C<sub>2</sub>H<sub>2</sub><sup>2+</sup> → C<sup>+</sup>+CH<sub>2</sub><sup>+</sup> and (d) C<sub>2</sub>H<sub>2</sub><sup>3+</sup> → H<sup>+</sup>+C<sub>2</sub>H<sup>2+</sup>. The polarization vector ε of the light and time-of-flight direction of fragments are marked as P<sub>∥</sub> and P<sub>⊥</sub> directions, respectively. The third momentum component P<sub>3</sub> along the third coordinate (the propagation direction of the FEL) was integrated over |P<sub>3</sub>| < 40 au in (a), |P<sub>3</sub>| < 20 au in (b), (c) and |P<sub>3</sub>| < 15 au in (d). Furthermore, events with |P| < 12 au are removed in (a) due to false H<sup>+</sup> events resulting from H<sub>2</sub>O and H<sub>2</sub> in the vacuum, and events with |P<sub>⊥</sub>| < 12 au are removed in (c) due to a strong mixture with the CH<sup>+</sup>+CH<sup>+</sup> channel.</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>