Photodissociation of methyl formate in circumstellar environment: stability under soft X-rays

The methyl formate molecule (HCOOCH3) is one of the most abundant molecules observed in hot molecular cores. Together with acetic acid and the sugar glycolaldehyde, these C2H4O2 molecules are the first triad of isomers detected in interstellar clouds. However, the observations point to a high enhancement in the abundance of methyl formate, which can reach up to 100 times the amount of its second abundant isomer, acetic acid. The observational abundance difference between these isomers has been subject of several works, invoking different formation pathways or survival rates. In this work we present an experimental and theoretical study of photoionization and photodissociation processes of HCOOCH3, to point out clues about its survival to high-energy photons observed in some interstellar environments. In order to obtain the most stable C2H4O2 cations and dications dissociation pathways, spectroscopic and computational methods were used. The measurements were taken employing soft X-ray photons with energies around carbon 1s and oxygen 1s resonances, while the calculations were performed at B3LYP/cc-pVDZ level, at the experimental pressure and temperature. Mass spectra of the photoproduced fragments were obtained using photoelectron photoion coincidence method. The main photodissociation channels are in agreement with the most favourable calculated pathways. Photoionization and photodissociation cross-sections were also determined. The results suggest that the observed high abundance of methyl formate could not be attributed to photodissociation processes.