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

TL;DR

This study reveals how the orientation of acetylene molecules influences enhanced ionization during strong laser fields, using ultrafast cross-polarized pulses to probe and control molecular dynamics and ionization pathways.

Contribution

It introduces a novel cross-polarized pulse pair technique to selectively investigate orientation-dependent ionization in highly charged hydrocarbons.

Findings

Enhanced ionization depends on molecule alignment and pulse delay.

Two distinct ionization pathways contribute to high charge states.

The method enables probing of deeply bound molecular orbitals.

Abstract

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, [C$_2$H$_2]^{6+}$. At small inter-pulse delays ($<$20 fs), this enhancement occurs when the molecule is aligned to the probe pulse. Conversely, at large delays ($>$40 fs), formation of [C$_2$H$_2]^{6+}$ occurs when the molecule is aligned to the pump pulse. By analyzing the polarization and time dependence of sequentially ionized [C$_2$H$_2]^{6+}$, 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.