Charge Resonance Enhanced Ionization of CO2 Probed by Laser Coulomb Explosion Imaging

Abstract

The process by which a molecule in an intense laser field ionizes more efficiently as its bond length increases towards a critical distance Rc is known as charge resonance enhanced ionization (CREI). We make a series of measurements of this process for CO2, by varying pulse duration from 7 to 200 fs, in order to identify the charge states and time scales involved. We find that for the 4+ and higher charge states, 100 fs is the time scale required to reach the critical geometry ⟨RCO⟩≈2.1 Å and ⟨θOCO⟩≈163° (equilibrium CO2 geometry is ⟨RCO⟩≈1.16 Å and ⟨θOCO⟩≈172°). The CO3+2 molecule, however, appears always to begin dissociation from closer than 1.7 Å indicating that dynamics on charge states lower than 3+ is not sufficient to initiate CREI. Finally, we make quantum ab initio calculations of ionization rates for CO2 and identify the electronic states responsible for CREI.