Figure 1: Generation of the acetylene dication and the deprotonation fragments.

(a) 1-D cut of potential energy curves of the considered states in the ionization and deprotonation: ground () and first excited state (A¹Π_u) of the neutral molecule, the cation ground (X²Π_u) and degenerate first excited state (A²Π_u), and the dication ground () and first excited state (A³Π_u) along the anti-symmetric stretching coordinate R_anti at R_sym=0. The red vertical line indicates the tunnelling ionization and the grey curved line marks the second ionization step by electron recollision. (b) Density plot of the acetylene dication longitudinal recoil momentum as a function of the experimental CEP, φ, for 4 fs pulses at an intensity of 2 × 10¹⁴ W cm⁻². CEP-integrated momenta are shown in the right panel. The dashed (dotted) lines shown correspond to CEP’s with maximum positive (negative) asymmetry in the ion emission, respectively. (c) Density plot for the asymmetry parameter, A(p_||,φ), for C₂H₂ ²⁺ ions. (d) Density plot of the vector momentum sum of the recoil of the two (H⁺,C₂H⁺) deprotonation fragments. (e) Density plot of the asymmetry parameter for the momentum sum of the deprotonation fragments (H⁺,C₂H⁺).