Mass Spectrometry Study Of Microstructures And Dynamics In The Liquid-Vapor Interface

Knowledge of the microstructures of liquid-vapor interface is a prerequisite to understanding the physical,chemical and biological processes involving liquid.It has been the focus of research for over a century.Yet now,the understanding of the most familiar water on molecular-level view is rudimentary and controversial.Structural inhomogeneity of the liquid-vapor interface,such as the spatial orientation of molecular specific groups and the nonuniform distribution of hydrogen-bonded(HB)clusters,is crucial for understanding the physicochemical processes therein.Intermolecular interactions in the liquid-vapor interface are distinctly different from those in the bulk,leading to the asymmetric mechanical and electrical properties of the surface.Molecular orientation,as one asymmetric feature that plays a role in molecular aggregation and assembly,is deemed to exist in the outermost layer or first molecular layer of the liquid surface.In the subsurface,some molecules are potentially oriented due to their interactions with the oriented molecules in the outermost layer.In addition,the hydrogen-bonded(HB)clusters of alcohol,water,and the other organic molecules usually have inhomogeneous distributions in the liquid;It is possible that some of the clusters have a prominent concentration in the outermost layer,the subsurface,or the bulk.Although the molecular orientation at the outermost layer was authenticated,to date,direct experimental evidence of the in situ existence of different-sized HB clusters,as a long-standing theoretical argument,is still lacking.Besides,dissociative electron attachment(DEA)plays a dominant role in ionizing radiation damages,but most knowledge about its dynamics are obtained from the gas-phase studies.Such a process in liquid-vapor interface,more closely related to biological radiation damage,is not directly observed so far,due to the challenges to experimental technique.Our time-delayed time-of-flight mass spectrometer has successfully detected the orientation of molecules on the surface of liquid ethanol.The observations are in line with the widely accepted picture of the molecular orientation on the liquid surface.Therefore,we demonstrate a new mass spectrometry to explore the molecular structures of the liquid surface.On the basis of the original device,we improved the experimental device,further improved the experimental vacuum degree,added 90° ion deflector,and invented TOF(time-of-flight)-QMF(quadrupole mass filter)tandem mass spectrometer.Its powerful ability to identify the local structures of the liquid-vapor interface of 1propanol is demonstrated not only by mapping the molecular orientations both in the outermost layer and in the subsurface but also by validating the existence of the HB molecular dimers in the subsurface by detecting their protonated ions.We further distinguish two different sources of the protonated dimer:the gas-phase protonation of the neutral dimer that evaporates in advance and the time-lag evaporation of the protonated dimer produced in the subsurface.This methodology is a brand-new way to explore the microstructures and the electron-driven chemical reactions in different local regions of the liquid-vapor interface.Using time-delayed mass spectrometry,here we demonstrate an experimental evidence about the DEA processes of liquid methanol by irradiation of the energytunable(up to 28 eV)pulsed electrons.At a proper delaying time after the electron pulse,most of the anionic yields to be collected are produced in the liquid-vapor interface.Besides the dissociative attachment of the low-energy primary electron,we unambiguously show that methoxy anion is also produced with the inelastically scattered or secondary ionized electrons at the high incident energies of the primary electrons.Significantly different from the DEA yields of the gas-phase and molecular solid film of methanol,the methoxy anion is the highly preferred product,which arises from the spatial orientation of the interfacial molecules,namely,the methyl group pointing out of the liquid surface.Our study provides new insight into chemical processes of ionizing radiation.