The Ionization and Dissociation of Formic Acid Monomer by Electron Impact

In HCOOH vapor in a mass spectrometer, low velocity electrons have produced the ionized molecule, one negative and twelve positive fragment ions. For the negative ion and six positive ions the mode of production has not been established. The other fragment ions originate in the reactions requiring the least dissociation energy for the production of the corresponding radicals, although for one of these a more energetic reaction seems preferred at high electron energies. One of the ions of doubtful origin also probably originates in a reaction involving more than the minimum degree of dissociation. The first appearance potential of four ions with established reactions, of one ion of doubtful origin, and probably of another of doubtful origin, as well as the second appearance potential of C$\mathrm{O}_2^{}{}_{}{}^{+}$, all indicate that the corresponding reactions involve no more than 1.0 volt excess energy. Both HC${\mathrm{O}}^{+}$ and COO${\mathrm{H}}^{+}$ utilize the trivalency of ${\mathrm{O}}^{+}$ in their structure. HCOO${\mathrm{H}}^{+}$, COO${\mathrm{H}}^{+}$, and HC${\mathrm{O}}^{+}$ seem to have a second appearance potential close to the first. To explain this, and to make the electron impact and spectroscopic ionization potentials of the molecule compatible, it is assumed that a trans-form of HCOOH exists. Estimates of its abundance relative to the known cis-form give for the energy difference between the two forms the probable limits 1.2-3.0 kcal/mole.