Laboratory studies of electron and ion irradiation of solid acetonitrile (CH 3 CN)

Abstract

The structure and bonding of solid acetonitrile (CH 3 CN) films on amorphous silica are studied, and chemical and physical processes under irradiation with 200 keV protons and 250–400 eV electrons are quantified using transmission infrared spectroscopy, reflection–absorption infrared spectroscopy and temperature-programmed desorption, with the assistance of basic computational chemistry and nuclear materials calculations. The thermal desorption profiles are found to depend strongly on the balance between CH 3 CN–surface and CH 3 CN–CH 3 CN interactions, passing from a sub-monolayer regime (binding energy: 35–50 kJ mol −1 ) to a multilayer regime (binding energy: 38.2±1.0 kJ mol −1 ) via a fractional order desorption regime characteristic of islanding as the coverage increases. Calculations using the SRIM code reveal that the effects of the ion irradiation are dominated by electronic stopping of incident protons, and the subsequent generation of secondary electrons. Therefore, ion irradiation and electron irradiation experiments can be quantitatively compared. During ion irradiation of thicker CH 3 CN films, a cross section for secondary electron-promoted chemical destruction of CH 3 CN of 4 (±1)×10 −18  cm 2 was measured, while electron-promoted desorption was not detected. A significantly higher cross section for electron-promoted desorption of 0.82–3.2×10 −15  cm 2 was measured during electron irradiation of thinner CH 3 CN films, while no chemical products were detected. The differences between the experimental results can be rationalized by recognizing that chemical reaction is a bulk effect in the CH 3 CN film, whereas desorption is a surface sensitive process. In thicker films, electron-promoted desorption is expected to occur a rate that is independent of the film thickness; i.e. show zeroth-order kinetics with respect to the surface concentration.