X-ray photochemistry of carbon hydride molecular ions

Abstract

Hydride molecular ions are key ingredients of the interstellar chemistry since they are precursors of more complex molecules. In regions located near a soft X-ray source these ions may resonantly absorb an X-ray photon which triggers a complex chain of reactions. In this work, we simulate ab initio the X-ray absorption spectrum, Auger decay processes and the subsequent fragmentation dynamics of two hydride molecular ions, namely CH₂⁺ and CH₃⁺. We show that these ions feature strong X-ray absorption resonances which relax through Auger decay within 7 fs. The doubly-charged ions thus formed mostly dissociate into smaller ionic carbon fragments: in the case of CH₂⁺, the dominant products are either C⁺/H⁺/H or CH⁺/H⁺. For CH₃⁺, the system breaks primary into CH₂⁺ and H⁺, which provides a new route to form CH₂⁺ near a X-ray source. Furthermore, our simulations provide the branching ratios of the final products formed after the X-ray absorption as well as their kinetic and internal energy distributions. Such data can be used in the chemistry models of the interstellar medium.