We investigate the orientation dependence of enhanced ionization (EI) during strong-field-driven nuclear motion in acetylene (${\mathrm{C}}_2{\mathrm{H}}_2$). 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, [${\mathrm{C}}_2{\mathrm{H}}_2{]}^{6+}$. At small interpulse delays ($<20$ fs), this enhancement occurs when the molecule is aligned to the probe pulse. Conversely, at large delays ($>40$ fs), formation of [${\mathrm{C}}_2{\mathrm{H}}_2{]}^{6+}$ occurs when the molecule is aligned to the pump pulse. By analyzing the polarization and time dependence of sequentially ionized [${\mathrm{C}}_2{\mathrm{H}}_2{]}^{6+}$, we resolve two transient alignments 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.

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• Atomic & molecular clusters
• Atomic orbital
• Chemical binding
• Electron & positron scattering
• Electronic excitation & ionization
• Lasers
• Multiphoton or tunneling ionization & excitation
• Nonlinear optics
• Potential energy surfaces
• Relativistic & quantum electrodynamic effects in atoms, molecules,& ions
• Single- and few-photon ionization & excitation
• Ultrafast optics
• Ultrafast phenomena
• Vibrational states