Electron Impact with the Liquid–Vapor Interface

Reaction dynamics in the liquid-vapor interface is one of the crucial physical sciences but is still starving for in-depth exploration. It is challenging to selectively detect the interfacial species or the yields of chemical reaction therein, meanwhile shielding or reducing the interference from the vapor and liquid bulk. Mass spectrometry is a straightforward method but is also frustrated in such a selective detection. Using a liquid microjet in combination with a pulsed electron beam, a linear time-of-flight mass spectrometer, and a quadrupole mass filter, we recently innovated time-delayed mass spectrometry for investigations of the liquid-vapor interface. In this Account, we illustrate how this unique method succeeds in disentangling different sources, i.e., the vapor and liquid-vapor interface, of the ionic yields of the electron impacts with a liquid beam of alcohol in vacuum. These achievements are basically attributed to the application of an onion-peeling strategy in the ion detection. Concretely, the microsecond time scale of molecular volatilization can be resolved well by tuning the delay time between the nanosecond pulses of incident electron bunch and ion attractor. First, the specific orientation of the interfacial molecule, i.e., a well-known fact about the hydrophobic hydrocarbon groups pointing outside the liquid surface of alcohol, is validated again. More importantly, the dynamic features of time-delayed mass spectra, in particular, for the ionic yields from the liquid-vapor interface, are rationalized explicitly. Moreover, we demonstrate evidence of in situ molecular dimers in the liquid-vapor interface of 1-propanol. As the first example of electron-induced reaction in the liquid-vapor interface, dimethyl ether can be synthesized in the liquid methanol interface due to local interfacial acidification by high-energy electron impacts. On the contrary, the low energy electron can lead to local basicity through dissociative electron attachment (DEA). Besides the primary low-energy electrons, the low-energy secondary and inelastically scattered electrons in the higher-energy impacts of the primary electrons can also participate in the DEA process. In contrast to the gas- or solid-phase DEAs, that in the liquid-vapor interface shows distinct differences in both the types and efficiencies of anionic products. With these and efforts in the future, we develop a molecular-level understanding of how the chemical reactions happen in the liquid-vapor interface.