Femtosecond Photoionization and Fragmentation Mechanisms of Aqueous Acetone, Aniline, and Cytidine

The aqueous-phase mass spectra of acetone, aniline, and cytidine were measured by our liquid-microjet time-of-flight mass spectrometer equipped with a femtosecond laser ionization source. In comparison with their gas-phase mass spectra, their aqueous-phase mass spectral peaks exhibit much broader and richer features. Parent fragment clusters with H 2 O molecules [e.g., (CO-H 2 O) + , (C 5 H 4 NH 2 -H 2 O) + , (C 4 H 3 NH 2 -(H 2 O) 2 ) + , and (NH 2 -(H 2 O) 5 ) + ] for aqueous-phase acetone, aniline, and cytidine were assigned and analyzed, indicating a distinctive fragmentation pattern of parent ions in their native aqueous environments. By controlling the energies of the femtosecond ionization laser, parent fragment clusters are verified to be mainly ejected from the bulk side of the liquid microjet rather than the gas region around the liquid microjet. From a mechanistic perspective, femtosecond multiphoton dissociative ionization is tentatively proposed to explain the mechanism of production and fragmentation of parent ions in bulk aqueous solution. Additionally, ion diffusion and ion Coulomb repulsion are also applicable to respectively explain the mechanisms of the ion migration from bulk to surface and ensuing ion ejection from surface into vacuum. To our knowledge, this is the first time that we demonstrate the experimental feasibility of the measurement of aqueous-phase mass spectra and propose mechanistic insights into femtosecond photoionization and fragmentation of bulk aqueous molecules.