Interactions Between Formic Acid Clusters and Femtosecond Light Pulse(s)

As the simplest carboxylic acid, formic acid (FA) is ubiquitous to Earth’s atmosphere, helping seed cloud nucleation and leading to acid rain. By studying the interactions between FA and high intensity light under high vacuum, conditions similar to the upper atmosphere, on other planets (either in the solar system or beyond), and even in interstellar media are emulated. These results were produced from a home built vacuum chamber system, with a Wiley-McLaren time of flight mass spectrometer and using femtosecond (fs) laser pulses. The laser characteristics were as follows: a pulse width >35 fs, center wavelength of 400 nm (probe pulse was 800 nm for the pump-probe investigation), and laser intensities at ~10¹⁵ W/cm².At high laser intensities, the first direct experimental evidence of CO³⁺ was recorded from the Coulomb explosion (CE) of the formic acid dimer (FAD) from a molecular beam. Theoretical calculations provided further evidence for the formation of CO³⁺ from the vertical ionization of FAD. When (FA)_n(H₂O)_mH⁺ clusters (n = 1-7 and m = 0-1) were exposed to similar laser intensities, the larger clusters (n = 5-7) favored complete atomization from CE, indicating that the repulsive forces within the clusters at those sizes was too great to withstand to form CO³⁺. The protonated nature of the clusters and the peak shapes recorded in the mass spectra suggested that neutral (FA)_n⁺ clusters undergo a dissociation mechanism within the extraction region. A novel technique was created to calculate these dissociation times on the order of 100s of nanoseconds (ns), increasing by ~10 ns for each additional FA molecule. Using pump-probe spectroscopy, it was observed similarly that neutral (FA)_n clusters with n > 1, showed evidence of ion pair formation of the form [(FA)_nH⁺·OOCH^-] on the sub-picosecond timescale, increasing by 70 fs per FA molecule. Both trends indicate that the neutral clusters prefer to form compact 3d structures, but after photoexcitation the clusters have competing pathways to ionization, either through multiphoton ionization (ns dynamics) or ion pair formation (fs dynamics) that inevitably lead to the expansion and subsequent rearrangement into linear chains for the protonated cluster.