Holding molecular dications together in strong laser fields

Metastable channel of doubly ionized carbon monoxide, CO²⁺, was scantly seen in previous strong-field experiments at the visible wavelength region, but was commonly observed using single high-energy photon or electron excitation. For the first time with near-IR ultrashort-pulse radiation, we observe an abundance of CO²⁺. We show that CO²⁺ results from nonsequential double ionization, while its dissociation counterpart, C⁺+O⁺, results from sequential processes, and CO²⁺ can be obtained through either single high-energy photon or electron excitation or multiphoton ionization with ultrashort pulses before a critical internuclear distance is reached. Our study demonstrates the experimental conditions to converge the outcomes from two vastly different regimes, namely, multiphoton excitation and ionization in strong fields and single high-energy photon or electron excitation and ionization in weak fields.