Ionic Fragments and Clusters Produced by Electron Impact of Acetonitrile and Methanol Mixed Molecular Films

We report the interaction of anhydrous acetonitrile, CH₃CN (ACN), and deuterated methanol, CD₃OD (MeOD), in the condensed crystalline phase by electron impact with 2.3 keV of energy. Theoretical and experimental investigations are focused on fragments and aggregates formed as a result of electron-stimulated ion desorption. Positively charged fragments and aggregates were collected using time-of-flight mass spectrometry (TOF-MS) and temperature-programmed desorption based on quadrupole spectroscopy (TPD). The structures of clusters identified in the TOF spectra were studied by applying density functional theory combined with a global minimum search. Two different deposition methods were used for the formation of the condensed molecular films, bilayer and codeposition, and in a second step, the annealing process was performed. The ionic species released from the surface into the vacuum are highly dependent on the annealing. A discussion of the interaction between the molecules was made. The formation of complex organic species comes from the intermolecular or intramolecular interactions of pure MeOD and ACN molecules. Anhydrous compounds were used, and the background water content was minimized to inhibit caging of the ACN molecules by water molecules.