Destruction of formic acid by soft X-rays in star-forming regions

Abstract

Abstract. Formic acid is much more abundant in the solid state, both in interstellar ices and cometary ices, than in the interstellar gas (ice/gas ∼ 10 4 ) and this point remains a puzzle. The goal of this work is to experimentally study ionization and photodissociation processes of HCOOH (formic acid), a glycine precursor molecule. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons from toroidal grating monochromator TGM) beamline (200-310 eV). Mass spectra were obtained using photoelectron photoion coincidence (PEPICO) method. Kinetic energy distributions and abundances for each ionic fragment have been obtained from the analysis of the corresponding peak shapes in the mass spectra. Photoionization and photodissociation cross sections were also determined. Due to the large photodissociation cross section of HCOOH it is possible that in PDRs regions, just after molecules evaporation from the grain surface, formic acid molecules are almost totally destroyed by soft X-rays, justifying the observed low abundance of HCOOH in the gaseous phase. The preferential path for the glycine formation from formic acid may be through the ice phase reaction.

Ionization and dissociation of the formic acid molecule by protons and electrons

“…represents an intrinsic recombination of some dissociation products of HCOOH also reported in the literature [8]. The base pressure in the vacuum chamber was in the 10 −8 Torr range and during the experiment the chamber pressure was maintained below 10 −5 Torr.…”

“…The formic acid molecule (HCOOH) has been observed in several astronomical sources such as protostellar ices NGC 7538:IRS9, condritic meteorites [3], and dark molecular clouds [4,5] and comets [6,7]. The destruction of the formic acid molecule by soft X-rays, present in star forming regions, has recently been studied in laboratory conditions with synchrotron radiation [8].…”

Photoproduction of H3+ from gaseous methanol inside dense molecular clouds

“…Conventional time-of-flight mass spectra were obtained using photoelectron and photoion coincidence (PEPICO) techniques. The complete description of the experimental setup can be found elsewhere (Boechat-Roberty et al 2005, Pilling et al 2006, 2007b.…”

Employing Soft X-Rays in Experimental Astrochemistry

“…Assuming a negligible fluorescence yield (due to the low carbon atomic number, Chen et al 1981) and anionic fragment production in the present photon energy range, we assumed that all absorbed photons lead to cation formation. The absolute cross section determination is described elsewhere (Boechat Roberty 2005, Pilling et al 2006. Briefly, the non-dissociative single ionization (photoionization) cross-section σ ph − i and the dissociative single ionization (photodissociation) cross section σ ph − d of studied species can be determined by 100…”

“…Details of Time of flight mass spectrometry are given elsewhere (e.g. Boechat-Roberty et al 2005, Pilling et al 2006, Pilling 2006. A schematic diagram of the experimental setup employed in gas-phase experiments is shown in Fig.…”