Magic Numbers and Stabilities of Photoionized Water Clusters: Computational and Experimental Characterization of the Nanosolvated Hydronium Ion

A Vacuum Ultraviolet Photoionization Mass Spectrometry and Density Functional Calculation Study of Formic Acid-Water Clusters

The interaction between formic acid (FA) and water (W) holds significant importance in various chemical processes. Our study combines vacuum-ultraviolet photoionization mass spectrometry with density functional calculations to investigate formic acid water clusters generated in supersonic molecular beams. The mass spectra obtained reveal the formation of protonated clusters as the major product. Enhanced intensities are observed in the mass spectra for a number of clusters holding the following composition, FA1W5H+, FA2W4H+, FA3W3H+, FA4W2H+, FA5W1H+ and FA6W2H+ compared to their neighbors with one less or one more water component. Our calculations shed light on these potentially stable structures, highlighting cyclic arrangements with molecules enclosed within the ring as the most stable structures, and demonstrate a decrease in the stability upon the addition of a water molecule. Comparing experimental appearance energies with calculated ionization energies suggests that fragmentation can occur from clusters of various sizes.

Fragmentation of water clusters formed in helium nanodroplets by charge transfer and Penning ionization

Helium nanodroplets ("HNDs") are widely used for forming tailor-made clusters and molecular complexes in a cold, transparent, and weakly interacting matrix. The characterization of embedded species by mass spectrometry is often complicated by the fragmentation and trapping of ions in the HNDs. Here, we systematically study fragment ion mass spectra of HND-aggregated water and oxygen clusters following their ionization by charge transfer ionization ("CTI") and Penning ionization ("PEI"). While the efficiency of PEI of embedded clusters is lower than for CTI by about factor 10, both the mean sizes of detected water clusters and the relative yields of unprotonated cluster ions are significantly larger, making PEI a "soft ionization" scheme. However, the tendency of ions to remain bound to HNDs leads to a reduced detection efficiency for large HNDs containing >104 helium atoms. These results are instrumental in determining optimal conditions for mass spectrometry and photoionization spectroscopy of molecular complexes and clusters aggregated in HNDs.