Electron ionization of acetylene

Abstract

Relative partial ionization cross sections and precursor specific relative partial ionization cross sections for fragment ions formed by electron ionization of C2H2 have been measured using time-of-flight mass spectrometry coupled with a 2D ion-ion coincidence technique. We report data for the formation of H+, H+2, C2+, C+/C2+ 2, CH+/C2H+2, CH+2, C+2, and C2H+ relative to the formation of C2H+2, as a function of ionizing electron energy from 30-200 eV. While excellent agreement is found between our data and one set of previously published absolute partial ionization cross sections, some discrepancies exist between the results presented here and two other recent determinations of these absolute partial ionization cross sections. We attribute these differences to the loss of some translationally energetic fragment ions in these earlier studies. Our relative precursor-specific partial ionization cross sections enable us, for the first time, to quantify the contribution to the yield of each fragment ion from single, double, and triple ionization. Analysis shows that at 50 eV double ionization contributes 2% to the total ion yield, increasing to over 10% at an ionizing energy of 100 eV. From our ion-ion coincidence data, we have derived branching ratios for charge separating dissociations of the acetylene dication. Comparison of our data to recent ab initio/RRKM calculations suggest that close to the double ionization potential C2H2+2 dissociates predominantly on the ground triplet potential energy surface (3Sigma*g) with a much smaller contribution from dissociation via the lowest singlet potential energy surface (1Delta g). Measurements of the kinetic energy released in the fragmentation reactions of C2H2+2 have been used to obtain precursor state energies for the formation of product ion pairs, and are shown to be in good agreement with available experimental data and with theory.

Cross Sections for Electron Collisions with Acetylene

“…Total and partial cross sections are experimental data by Tian and Vidal. 56 The present BEB model and total cross sections of Zheng 66 allow one to obtain the counting ionization cross section, via subtracting half of the cross section for double ionization from the gross total one. This is done in Fig.…”

Electron scattering on molecules: search for semi-empirical indications

“…The experimental data are from: King and Price [54], Tian and Vidal [58] and Feil et al [59]. The unnormalized data of King and Price [54] were normalized to partial cross-sections for C2H + 2 ion reported by Tian and Vidal [58]. The shaded area represents the error bars for BEB results [56] when 10% uncertainties are assumed on the binding and kinetic energies in equation (8).…”

“…The dashed line represents the BEB model (MP2/avtz/wB97XD) from reference [56]. The experimental data are from: King and Price [54], Tian and Vidal [58] and Feil et al [59]. The unnormalized data of King and Price [54] were normalized to partial cross-sections for C2H + 2 ion reported by Tian and Vidal [58].…”

“…We are not aware of such comparisons. The difficulty relies both in determining orbitals and in all possible, post-collisional dynamics of the ionized molecules, see for example [54].…”

Probing the role of excited states in ionization of acetylene

“…This is in part due to its rich electronic structure and in part due to its importance in many areas of chemistry. Initial studies of acetylene considered ionization by electron [9,10] and photon [9,[11][12][13][14][15] impact. More recently, several studies have considered the response to strong fields.…”