No abstract

Formic acid is the simplest carboxylic acid and plays a pivotal role in atmospheric chemistry. It is an intermediate in the Water-Gas-Shift reaction, decomposing into either CO 2 and H 2 or into H 2 O and CO under ionizing radiation. Furthermore, it is important in acid rain and seeding the nucleation of water molecules in cloud formation. Here, I will present our recent work, where femtosecond lasers are applied to study the microsolvation and photodynamics of molecular gas-phase formic acid-water clusters using time-of-flight mass spectrometry. Our cluster distribution confirms the enhanced stability of (FA) 5 (H2O) 1 H + , where the formic acid cluster forms a cage-like structure surrounding the water molecule. Upon exposure to high laser intensities (400 nm, 200 fs, laser intensities of 1.9x10 15 W/cm 2 ), the clusters undergo an enhanced ionization which produces multiply charged ions of C, O, and CO. Coulomb explosion of these ions leads to a large kinetic energy release that is shown to increase with the size of clusters. The measured values are in agreement with a Molecular Dynamics simulation of the Coulomb explosion for the mean size of the clusters within the cluster distribution, suggesting that no movement occurs during ionization. Of particular relevance, we record a large amount of signal for the carbon monoxide trication. KER values were recorded as high as 44 eV for CO 3+ for (FA) 2 , but increases to 75.3 eV when the cluster distribution is shifted toward (FA) 5 as the largest signal. Potential energy curves for CO 3+ are calculated using the multireference configuration interaction (MRCI+Q) method to confirm the existence of metastable states with a large potential barrier with respect to dissociation. This combined experimental and theoretical effort confirms the existence of metastable CO 3+ .