Photodissociation of organic molecules in star-forming regions

Abstract

The presence of methyl alcohol or methanol (CH 3 OH) in several astrophysical environments has been characterized by its high abundance that depends on both the production rate and the destruction rate. In the present work, the photoionization and photodissociation processes of methanol have been experimentally studied, employing soft X-ray photons (100-310 eV) from a toroidal grating monochromator (TGM) beamline of the Brazilian Synchrotron Light Laboratory (LNLS). Mass spectra were obtained using the photoelectron photoion coincidence (PEPICO) method. Kinetic energy distribution and abundances for each ionic fragment have been obtained from the analysis of the corresponding peak shapes in the mass spectra. Absolute photoionization and photodissociation cross sections were also determined. We have found, among the channels leading to ionization, about 11-16% of CH 3 OH survive the soft X-rays photons. This behavior, together with an efficient formation pathways, may be associated with the high column density observed in star-forming regions. The three main photodissociation pathways are represented by COH + (or HCO + ) ion release (with ejection of H 2 + H), the dissociation via C-O bond rupture (with strong charge retention preferentially on the methyl fragment) and the ejection of a single energetic (2-4 eV) proton. Since methanol is very abundant in star forming regions, the produced protons could be an alternative route to molecular hydrogenation or a trigger for secondary dissociation processes or even to promote extra heating of the environment.

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

“…This large difference between the yield of these two fragments, after C–O bond rupture indicates that, during the dissociation process, there is a significant preference in the charge retention by methyl over the formyl. This behaviour has also been seen in the photodissociation of acetic acid (Pilling et al 2006) and methanol (Pilling et al 2007a).…”

Communication: Protonation process of formic acid from the ionization and fragmentation of dimers induced by synchrotron radiation in the valence region

“…Expressions for the branching ratio and its associated error are obtained in Ref. 27, where the total area of the spectrum was calculated excluding the peak of the ion with m/q = 50, because it is originated from the dimer.…”

Soft X-Ray Irradiation of Methanol Ice: Formation of Products as a Function of Photon Energy

“…The experiments by Hempelmann et al (1999) show that fragmentation process evidences relevant differences at the C and O K-edges. The irradiation at 292 eV of gas-phase CH 3 OH produces fragments caused by C−H bond rupture as testified by the high yields (∼34%) of COH + or HCO + , CH 2 OH + , and H + ions, followed, with a lower efficiency, by the C−O bond rupture that produces species such as CH 3 + (Pilling et al 2007). Irradiation with 537 eV photons of a CH 3 OH ice leads to the break of the C−O and O−H bonds (Andrade et al 2010).…”