Orientation-Dependent Enhanced Ionization in Acetylene Revealed by Ultrafast Cross-Polarized Pulse Pairs

We investigate the orientation dependence of Enhanced Ionization (EI) during strong-field-driven nuclear motion in acetylene C$₂$H$₂$. Here, we both initiate and probe molecular dynamics in acetylene with intense 6-fs cross-polarized pulse pairs, separated by a variable delay. Following multiple ionization by the first pulse, acetylene undergoes simultaneous elongation of the carbon-carbon and carbon-hydrogen bonds, enabling further ionization by the second pulse and the formation of a very highly charged state, \[C₂H₂\]⁶⁺. At small inter-pulse delays (<20 fs), this enhancement occurs when the molecule is aligned to the probe pulse. Conversely, at large delays (>40 fs), formation of \[C₂H₂\]⁶⁺ occurs when the molecule is aligned to the pump pulse. By analyzing the polarization and time dependence of sequentially ionized \[C₂H₂\]⁶⁺, we resolve two distinct pathways that both contribute to a large increase in the multiple ionization yield. This cross-polarized pulse pair scheme uniquely enables selective probing of deeply bound orbitals, providing new insights on orientation-dependent EI in highly charged hydrocarbons.