Polarization and ionization of carbon monoxide molecular states interacting with a strong laser field

Ionization of CO, CO⁺, and CO²⁺ is measured in a laser field over the intensity span from 10¹⁴ W cm^-2 to 10¹⁷ W cm^-2. The dynamics, including the polarization response of the molecular states and field-driven excitation of molecular vibration, are solved with the time-dependent Schrödinger equation using electron states and ion potentials from Complete Active Space Self-Consistent Field calculations in a strong, classical pulsed laser field. These calculations are used to determine the strong field induced dipole in the molecular bound state. We find the magnitude of the external field induced dipole to be (in atomic units) of order |0.8| q bohr, which exceeds the field free dipole moment of 0.04 q bohr. The Stark shift from the induced dipole changes the ionization energy by ±0.1 hartree. The rearrangement of the charge in the bound states is studied as it impacts ionization. Specifically, we quantify the change in tunneling ionization due to the field polarization and Stark shift of the highest occupied molecular orbital. Tunneling rates of the highest occupied molecular orbital electron states in the field agree with the measured ionization fragment yields near saturation. Rescattering of the electron from the ionization of CO is also observed in the nonsequential ionization yield of CO²⁺. Nonsequential ionization accounts for 4% of the total yield of CO²⁺ fragments. These results for molecular CO are consistent with the recent findings in atomic systems (Jones et al 2023).