Dissociation of the benzene molecule by UV and soft X-rays in circumstellar environment

TL;DR

This study investigates how UV and soft X-ray radiation from a star can ionize and break apart benzene molecules in space, revealing that X-rays cause more fragmentation than UV light, with implications for organic molecule formation.

Contribution

It provides quantitative analysis of benzene ionization and dissociation under UV and X-ray radiation using synchrotron experiments, including cross sections and molecule half-lives in space.

Findings

Benzene is more efficiently fragmented by soft X-rays than UV radiation.

Approximately 50% of benzene molecules survive UV exposure, only 4% resist X-ray exposure.

Half-lives of benzene in the nebula environment are calculated based on photon fluxes.

Abstract

Benzene molecules, present in the proto-planetary nebula CRL 618, are ionized and dissociated by UV and X-ray photons originated from the hot central star and by its fast wind. Ionic species and free radicals produced by these processes can lead to the formation of new organic molecules. The aim of this work is to study the photoionization and photodissociation processes of the benzene molecule, using synchrotron radiation and time of flight mass spectrometry. Mass spectra were recorded at different energies corresponding to the vacuum ultraviolet (21.21 eV) and soft X-ray (282-310 eV) spectral regions. The production of ions from the benzene dissociative photoionization is here quantified, indicating that C6H6 is more efficiently fragmented by soft X-ray than UV radiation, where 50% of the ionized benzene molecules survive to UV dissociation while only about 4% resist to X-rays. Partial ion yields of H+ and small hydrocarbons such as C2H2+, C3H3+ and C4H2+ are determined as a function of photon energy. Absolute photoionization and dissociative photoionization cross sections have also been determined. From these values, half-life of benzene molecule due to UV and X-ray photon fluxes in CRL 618 were obtained.