Mass-spectrometric study of energy characteristics of methanol and ethanol ions during ionization by a strong electric field

Abstract

lished. The present work deals with the study of the mechanisms of formation of field ions of al- cohols. Calculation of the adiabatic energies of formation EA, required for comparison with standard energies of appearance S, requires information on molecular constants such as ioniza- tion potentials, bond cleavage energies, or affinity to proton. We therefore studied the simplest alcohols, i.e., methanol and ethanol, whose constants are considered to be reliably established. Figure 2 shows the appearance curves for all the methanol ions observed in the mass spec- trum at two values of the ionization voltage. An analysis of the data obtained shows that on- ly the molecular ion M + is formed by one single mechanism, and this mechanism represents di- rect ionization. Thus, the energy S of H + ion is equal to 10.5 eV, which agrees with the tab- ular value of the ionization potential of methanol. In all the remaining cases, the decrease in the electric field leads to a decrease in the appearance energy, as observed for water and ammonia. The decrease does not take place by a smooth shifting of the curve, but it is pos- sible to assume that a new mechanism for the formation of these ions, giving anomalously low readings, occurs. Thus, at a narrow field, not less than two, and for the (M- i) + ion even three, mechanisms of formation of the corresponding ion take place. During the determination of the standard energy of appearance S, in the case of several mechanisms of formation of ions there are difficulties in the separation of the appearance curves corresponding to these mechanisms. The energies of appearance of one and the same ion formed by different mechanisms are determined more accurately the greater the difference be- tween these energies. A complete separation of the appearance curves is possible only if the difference in the energies is greater than 1.5-2 eV. In the case of smaller differences, a satisfactory accuracy ( eV) can be obtained if the intensities of the superposed curves