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 þ and higher charge states, 100 fs is the time scale required to reach the critical geometry hRCO i �� 2:1 �� A and h��OCO i �� 163�� (equilibrium CO2 geometry is hRCO i �� 1:16 �� A and h��OCO i �� 172�� ). The CO2 3 þ molecule, however, appears always to begin dissociation from closer than 1.7 A˚ indicating that dynamics on charge states lower than 3 þ 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.