Limited Formation of CO³⁺ through Strong-Field Ionization and Coulomb Explosion of Formic Acid Clusters

Femtosecond laser pulses are utilized to drive multiple ionization in gas-phase formic acid clusters (FA). 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 C³⁺ and O³⁺ and also increasing the KER values to several hundred electronvolts in magnitude for such ions. At a lower laser intensity (3.8 × 10¹⁴ W/cm²), we record an enhancement in the signal of the (FA)₅(H₂O)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 CO³⁺ is more likely to arise from the molecular case. Thus, the relative production of CO³⁺ is reduced in comparison to the highly charged ions upon clustering and is likely due to the higher ionization levels achieved, which facilitate dissociation.