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.

Femtosecond laser pulses are utilized to drive multiple ionization in gas-phase formic acid clusters (FA) n . Experimental measurements of the kinetic energy release (KER) of the ions through Coulomb explosion are studied using time-of-flight mass spectrometry and compared to the values recorded from molecules. Upon interacting with 200 fs linearly polarized laser pulses of 400 nm, formic acid clusters facilitate the formation of higher charge states than the formic acid dimer, reaching both C 3+ and O 3+ and also increasing the KER values to several hundred electronvolts in magnitude for such ions. At a lower laser intensity (3.8 × 10 14 W/cm 2 ), we record an enhancement in the signal of the (FA) 5 (H 2 O)H + cluster, which suggests that it has a higher stability, in agreement with previous studies. A molecular dynamics simulation of the Coulomb explosion shows that the highly charged atomic ions arise from larger clusters, whereas the production of CO 3+ is more likely to arise from the molecular case. Thus, the relative production of CO 3+ is reduced in comparison to the highly charged ions upon clustering and is likely due to the higher ionization levels achieved, which facilitate dissociation.