Molecular fragmentation of acetylene by VUV double photoionization

Acetylene is a simple molecule of interest for interstellar medium (ISM) and planetary atmospheres. The presence of C₂H₂ was detected by IR spectroscopic measurements [1]. Acetylene was also found as a minor component in the atmosphere of gas giants like the planet Jupiter, in the atmosphere of Saturn's satellite Titan, and in comets, where photochemical experiments have demonstrated that this simple hydrocarbon is a likely precursor of C₂, a widely observed component in such environments [2-4]. It has to be noted that the presence in planetary atmospheres and ISM of Vacuum Ultra Violet (VUV) light’s photons as well as cosmic rays makes highly probable the double photoionization of molecular species with the production of molecular dications producing subsequent dissociation into ionic fragments having a high kinetic energy content of several eV. This translational energy is sufficient in some cases to allow ions escape from the upper atmosphere of some planet of the Solar System, as Venus, Mars and Titan, into space [5-7]. In this contribution we present the experimental study of the microscopic dynamics of the two-body dissociation reactions of the C₂H₂⁺² dication, induced by the double ionization of acetylene molecules by VUV photons in the energy range of 31.9-50.0 eV. The photoionizing agent was a tunable synchrotron radiation beam with a photon energy resolution of about 1.5 meV, while ion products are revealed by coupling photoelectron-photoion-photoion-coincidence and ion imaging techniques [8]. The measured angular distributions and kinetic energy of product ions exhibit significant changes (as the photon energy increases) for the three leading dissociation reactions producing H⁺+C₂H⁺, C⁺+CH₂⁺, and CH⁺ + CH⁺, providing detailed information on the fragmentation dynamics of the C₂H₂²⁺ dication.

[1] O. Witasse et al. 2002, Geophys. Res. Lett., 29, 1263 1-4.

[2] T. Y. Brooke et al., 1996, Nature, 383, 606-608.

[3] J. Cernicharo et al., 2001, Astrophys. J., 546, L123-L126.

[4] P. M. Woods et al., 2002, Astrophys. J., 574, L167-L170.

[5] R. Thissen et al., 2011, Phys. Chem. Chem. Phys., 13, 18264–18287.

[6] S. Falcinelli et al., 2014, Planetary and Space Science, 99, 149-157.

[7] S. Falcinelli et al., 2016, Atmosphere, 7(9), 112.

[8] S. Falcinelli et al., 2017, Acta Physica Polonica A, 131(1), 112-116.