Two-dimensional fragment ion momentum distributions for dissociation channels of (a) C₂H₂²⁺ → H⁺+C₂H⁺, (b) C₂H₂²⁺ → CH⁺+CH⁺, (c) C₂H₂²⁺ → C⁺+CH₂⁺ and (d) C₂H₂³⁺ → H⁺+C₂H²⁺

Figure 3. Two-dimensional fragment ion momentum distributions for dissociation channels of (a) C₂H₂²⁺ → H⁺+C₂H⁺, (b) C₂H₂²⁺ → CH⁺+CH⁺, (c) C₂H₂²⁺ → C⁺+CH₂⁺ and (d) C₂H₂³⁺ → H⁺+C₂H²⁺. The polarization vector ε of the light and time-of-flight direction of fragments are marked as P_∥ and P_⊥ directions, respectively. The third momentum component P₃ along the third coordinate (the propagation direction of the FEL) was integrated over |P₃| < 40 au in (a), |P₃| < 20 au in (b), (c) and |P₃| < 15 au in (d). Furthermore, events with |P| < 12 au are removed in (a) due to false H⁺ events resulting from H₂O and H₂ in the vacuum, and events with |P_⊥| < 12 au are removed in (c) due to a strong mixture with the CH⁺+CH⁺ channel.

Abstract

Few-photon induced ultrafast dynamics in acetylene (C₂H₂) leading to several dissociation channels—deprotonation (H⁺+C₂H⁺ and H⁺+C₂H²⁺), symmetric break-up (CH⁺+CH⁺) and isomerization (C⁺+CH₂⁺)-–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⁺+CH₂⁺ and H⁺+C₂H²⁺ 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.