Single and Double Photoionization and Photodissociation of Toluene by Soft X-rays in Circumstellar Environment

TL;DR

This study investigates how soft X-ray radiation causes ionization and fragmentation of toluene, a precursor to methylated PAHs, providing insights into chemical processes in circumstellar environments like AGB stars.

Contribution

It combines experimental photoionization data with theoretical calculations to identify stable dications and assess their potential observability in space.

Findings

Doubly charged toluene fragments are abundant and detectable.

Stable dications can survive in interstellar conditions.

Photodissociation radius and toluene half-life in AGB star environments were estimated.

Abstract

The formation of polycyclic aromatic hydrocarbons (PAHs) and their methyl derivatives occurs mainly in the dust shells of asymptotic giant branch (AGB) stars. The bands at 3.3 and 3.4 $\mu$m, observed in infrared emission spectra of several objects, are attributed C-H vibrational modes in aromatic and aliphatic structures, respectively. In general, the feature at 3.3 $\mu$m is more intense than the 3.4 $\mu$m. Photoionization and photodissociation processes of toluene, the precursor of methylated PAHs, were studied using synchrotron radiation at soft X-ray energies around the carbon K edge with time-of-flight mass spectrometry. Partial ion yields of a large number of ionic fragments were extracted from single and 2D-spectra, where electron-ion coincidences have revealed the doubly charged parent-molecule and several doubly charged fragments containing seven carbon atoms with considerable abundance. \textit{Ab initio} calculations based on density functional theory were performed to elucidate the chemical structure of these stable dicationic species. The survival of the dications subjected to hard inner shell ionization suggests that they could be observed in the interstellar medium, especially in regions where PAHs are detected. The ionization and destruction of toluene induced by X-rays were examined in the T Dra conditions, a carbon-rich AGB star. In this context, a minimum photodissociation radius and the half-life of toluene subjected to the incidence of the soft X-ray flux emitted from a companion white dwarf star were determined.