Exploring binary formic acid-water cluster formation and ionization. The study clarifies the impact of electron collisions on the detection of hydrated formic acid clusters, with cluster fragmentation and ion formation being strongly influenced by the original cluster composition, degree of hydration, and collision energy.

Abstract

We investigate ion formation in hydrated formic acid (FA) clusters upon collision with electrons of variable energy, focusing on electron ionization at 70 eV (EI) and low-energy (1.5–15 eV) electron attachment (EA). To uncover details about the composition of neutral clusters, we aim to elucidate the ion formation processes in FA _M  ⋅ W _N clusters initiated by interaction with electrons and determine the extent of cluster fragmentation. EI predominantly produces protonated [FA _m +H]⁺ ions, and in FA-rich clusters, the stable ring structures surrounding H₃O⁺ ions are formed. In contrast, EA leads to a competition between the formation of intact [FA _m  ⋅ W _n ]⁻ and dissociated [FA _m  ⋅ W _n −H]⁻ fragment ions, influenced by the cluster size, level of hydration, and electron energy. Our findings reveal a predisposition of low-energy EA towards forming [FA _m  ⋅ W _n ]⁻, while higher electron energies tend to favor the formation of [FA _m  ⋅ W _n −H]⁻ due to intracluster ion-molecule reactions. The comparison of positive and negative ion spectra suggests that the mass spectra of FA-rich clusters may indicate their actual size and composition. On the other hand, the more weakly bound water evaporation from the clusters depends strongly on the ionization. Thus, for the hydrated clusters, the neutral cluster size can hardly be estimated from the mass spectra.