Ultrafast dynamics in acetylene clocked in a femtosecond XUV stopwatch

Few-photon induced ultrafast dynamics in acetylene (C 2 H 2 ) leading to several dissociation channels—deprotonation (H + + C 2 H + and H + + C 2 H 2 + ), symmetric break-up (CH + + CH + ) and isomerization (C + + CH 2 + )—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 + + CH 2 + and H + + C 2 H 2 + 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