Electron-scattering cross sections for selected alkyne molecules: Measurements and calculations

Abstract

We report cross-section results from experimental and theoretical studies on electron collisions with 1-butyne ($\text{HC}\ensuremath{\equiv}\text{C}--{\text{CH}}_{2}{\text{CH}}_{3}$) and acetylene ($\text{HC}\ensuremath{\equiv}\text{CH}$) molecules and from computations for a propyne ($\text{HC}\ensuremath{\equiv}\text{C}--{\text{CH}}_{3}$) molecule. Absolute $grand\phantom{\rule{0.16em}{0ex}}$-total electron-scattering cross sections (TCSs) were measured at impact energies ranging from about 0.5 to 300 eV using the linear electron-transmission method. The TCS energy curve for 1-butyne has a very broad enhancement on which some distinct features are superimposed, namely a resonant-like maximum (located near 3.2 eV), a broad hump (centered around 7.5 eV), and a shoulder (spanned between 12 and 26 eV). The shape of our experimental TCS curve for acetylene closely resembles that reported earlier, while its magnitude is usually larger. As no previous calculations for electron collisions with 1-butyne and propyne are disclosed in the literature, we computed the elastic (ECS) and ionization (ICS) cross sections for these molecules. Similar calculations were also performed for acetylene molecules. The additivity rule was employed to calculate the ECSs from 50 to 3000 eV, while the binary-encounter-Bethe approach was used for computation of the ICSs, from the threshold up to 3000 eV. The sum (ECS + ICS) of these two computed cross sections reasonably reproduces the TCS measurements above 50 eV. Furthermore, the experimental TCS curves, obtained in our laboratory, for a series of acetylenic compounds---acetylene ($\text{HC}\ensuremath{\equiv}\text{CH}$), propyne ($\text{HC}\ensuremath{\equiv}\text{C}--{CH}_{3}$), and 1-butyne ($\text{HC}\ensuremath{\equiv}\text{C}--{\text{CH}}_{2}{\text{CH}}_{3}$)---are compared to study the substitutional effect. Finally, the influence of the structural differences on the electron-scattering TCS for isomers of the ${\text{C}}_{4}{\text{H}}_{6}$ molecule (1-butyne, 2-butyne, and 1,3--butadiene) is indicated and discussed.