Reactions in condensed formic acid (HCOOH) induced by low energy (<20 eV) electrons

The interaction of low energy (<20 eV) electrons with a five monolayer (ML) film of formic acid (HCOOH) deposited on a cryogenically cooled monocrystalline Au substrate is studied by electron stimulated desorption (ESD) of negatively charged fragment ions. A comparison with results from gas phase experiments demonstrates the strong effect of the environment for negative ion formation via dissociative electron attachment (DEA). From condensed phase formic acid (FA) a strong H− desorption signal from a resonant feature peaking at 9 eV is observed. In the gas phase, the dominant reaction is neutral hydrogen abstraction generating HCOO− within a low energy resonance, peaking at 1.25 eV. ESD studies on the isotopomers HCOOD and DCOOH indicate effective H/D exchange in the precursor ion at 9 eV prior to dissociation. The evolution of the desorption signals in the course of electron irradiation and the features in the thermal desorption spectra (TDS) of the electron irradiated film suggest the formation of CO2 at electron energies above 8 eV.