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

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. © ESO 2005.