Tunneling ionization of carbon dioxide from lower-lying orbitals

Tunneling ionization of molecules from multiple orbitals produces parent ions in different electronic states. The excited parent ion will relax to its ground state through emitting a photon of light. Therefore, the contributions of different lower-lying orbitals can be separated by measuring the corresponding fluorescence emission. Here we report the observation of CO₂⁺(A²Πᵤ→X²Πg) and CO₂⁺(B²Σᵤ→X²Πg) fluorescence emission in the system of neutral CO₂ molecules irradiated by intense femtosecond laser pulses. These observations provide direct evidence that multiple molecular orbitals are simultaneously involved in the process of tunneling ionization. Meanwhile, fluorescence spectroscopy is also demonstrated to be a feasible approach to selectively probe lower-lying orbitals involved in the tunneling ionization of molecules.