UC Berkeley

Ionic liquids (ILs) are room temperature molten salts with unique properties and broad range of applications. The properties are tunable based on the cation-anion (C⁺-A^-) combination. Studying the ion pairs provides understanding of the interaction between the isolated cation and anion, which primarily determines the electronic and nuclear structure.

New velocity map imaging experiments were initiated to study the dissociative pathways upon photoexcitation of isolated ion pairs. Dissociative ionization and ion pair dissociation pathways initiated by 54 nm light of an IL, 1-ethy-3-methylimidazolium ([Emim]⁺) bis(trifluoromethyl-sulfonyl)imide ([Tf₂N]^-), are revealed. Furthermore, the ultrafast excited dynamics of an IL, [Emim]⁺[Br]^-, has been studied using time-resolved photoion spectroscopy. Momentary depletion of the intact cation and rise of the alkyl-loss fragments are observed, suggesting a possibility for a short-lived electronically or internally excited state that promotes the formation of alkyl-loss fragments upon probe excitation.

The volatilization process of thermally stable and unstable ionic liquids is studied using tunable vacuum ultraviolet light (VUV). Heats of vaporization for imidazolium-based ionic liquids are measured and the ionization threshold for the neutral ion pair is directly measured for the first time. The thermal decomposition mechanism for [Emim]⁺[Br]^-is explored to understand the volatilization process better, showing that alkyl abstraction upon heating occurs via an S_N2 type mechanism.

Moreover, an IL hypergolic reaction mechanism is studied using an aerosol flow tube method combined with tunable VUV light to understand the reaction chemistry of a potentially less corrosive and toxic rocket fuel system. Isolated ion pairs are monitored from IL aerosols with the advantage of less photofragmentation. Observation of early stage reaction intermediates, cyanamide, N₂O, and CO₂ from a hypergolic IL reaction with nitric acid reveals the existence of a nitro-substituted intermediate.