Angular Distributions of Fragment Ions Produced by Coulomb Explosion of Simple Molecular Dications of Astrochemical Interest

Abstract

The double photoionization of N₂O and C₂H₂ molecules by linearly polarized light in the 30-50 eV energy range has been studied by coupling ion imaging and electron-ion-ion coincidence techniques. In the case of N₂O, for the two possible dissociative processes leading to N⁺+NO⁺ and O⁺+N₂ ⁺, anisotropic angular distributions of ionic fragments have been measured, indicating that N₂O ionizes when its axis is parallel to the light polarization vector and the fragments are separating in a time shorter than the dication rotational period. In the case of C₂H₂, the two-body dissociation reactions producing C₂H⁺+H⁺ and CH₂ ⁺+C⁺ have shown almost isotropic angular distributions, indicating the double photoionization occurs when acetylene is mainly oriented perpendicularly to the light polarization vector. The analysis of results based on a Monte Carlo trajectory simulation provides: i) the metastable N₂O²⁺ and C₂H₂ ²⁺ dication lifetimes, ii) the kinetic energy release (KER) distribution for the final ions resulting from the Coulomb explosion, and iii) the anisotropy β parameter values as a function of the investigated photon energy extracted from the measured ionic angular distributions.