Production of Metastable CO³⁺ through the Strong-Field Ionization and Coulomb Explosion of Formic Acid Dimer

Femtosecond laser pulses are utilized to drive multiple ionization of formic acid dimers and the resulting ions are studied using time-of-flight mass spectrometry. The interaction of formic acid dimer with 200 fs linearly polarized laser pulses of 400 nm with intensities of up to 3.7 × 10¹⁵ W/cm² produces a metastable carbon monoxide trication. Experimental kinetic energy release (KER) measurements of the ions are consistent with molecular dynamics simulations of the Coulomb explosion of a formic acid dimer and suggest that no significant movement occurs during ionization. KER values were recorded as high as 44 eV for CO³⁺, in agreement with results from a classical Molecular Dynamics simulation of fully ionized formic acid dimers. Potential energy curves for CO³⁺ are calculated using the multireference configuration interaction (MRCI+Q) method to confirm the existence of an excited metastable ²Σ state with a significant potential barrier with respect to dissociation. This combined experimental and theoretical effort reveals the existence of metastable CO³⁺ through direct observation for the first time.