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

Abstract

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.

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

“…Given the long laser pulse (200 fs) employed in this experiment, motion is anticipated during ionization. However, the KER measurements from the molecular case reveal that the ionization is sufficiently fast such that the CE samples a frozen geometry . Thus, although the initial ionization likely agrees with the single-active-electron model of ADK, additional electrons may be emitted at similar laser intensities through an ionization enhancement mechanism.…”

“…High-charge-state region of mass spectra recorded at a laser intensity of 1.9 × 10 15 W/cm 2 from FA clusters (black) compared to the molecular beam (blue). The masses are plotted logarithmically to highlight the high charge states.…”

“…Plot comparing relative signal intensities of the high charge states for C, O, and CO with respect to C + for the (a) molecular beam and (b) the clusters.…”

“…Under room-temperature conditions, more than 95% of FA vapor is composed of dimers, supporting the agreement between the measured KER values for all of the high charge states and the simulated values arising from FAD. The strong agreement between experiment and theory for the KER of CO 3+ is an unexpected result given the long pulse width of the laser and suggests that no significant nuclear movement occurs within the FAD during ionization …”

“…The ionization of FA and its decomposition products have been studied using electron impact, X-ray, and strong-field ionization (SFI). , In particular, Coulomb explosion (CE) of formic acid dimer (FAD) has given insight into the interaction between FA and high-intensity lasers. In our previous work, we investigated the SFI and CE of molecular FAD under experimental conditions where clustering did not occur. Our results demonstrated that the extreme laser intensities provided by femtosecond laser pulses enable the multiple ionization and atomization of FAD and facilitate the production of CO 3+ .…”