Electron impact partial ionization cross sections of methyl alcohol up to 5 keV using the mass spectrometry data

Abstract

The electron impact partial ionization cross sections (PICSs) of methyl alcohol fragments are reported from ionization threshold to 5 keV. The calculations employ use of electron impact mass spectrometry data and a modified form of binary-encounter-Bethe (BEB) model. In the modified-BEB approach, the binding energy of each occupied molecular orbital is incremented by an amount such that the highest occupied molecular orbital reflects the appearance potential of a particular fragment. The PICSs are then obtained by appropriately scaling the cross section data using the branching ratios (BR). The BRs can either be obtained from the cross section data or using the mass spectrometry data at an energy at which the mass spectrum is recorded. The PICSs are summed to obtain the total ionization cross sections (TICS) of the methyl alcohol. The results are in good agreement with the experimental results and are superior to the existing theoretical results. A simple approach like this in conjunction with mass spectrometry would be of immense help in modeling and interpretation of ionization data. The study reveals the need for more consistent ion energetics and mass spectrometry data for an accurate modeling of PICSs.

Calculated electron impact ionisation fragmentation patterns

“…While this work was being completed Goswami et al [51] proposed a method similar to those discussed here. In their method, the BEB cross section is computed for each fragment and the ions are characterized by appearance energy instead of the ionisation energy.…”

Fragmentation of interstellar methanol by collisions with He˙+: an experimental and computational study

“…Regarding electron ionization, numerous measurements have been reported on relative partial ionization CSs of methanol for ionizing electron energies from 20 to 500 eV (see ref. 44, 54–57 and references therein). In all cases and at all ionizing electron energies, the primary product is CH 3 O + /CH 2 OH + (with a BR equal to 0.37 at an electron energy of 30 eV that decreases to 0.30 at 100 eV, according to ref.…”

Electron impact cross-sections of tetraethyl silicate

“…j R j (E) = 1. The experimental BR can be calculated either from the relative cation abundance data determined from electron ionization mass spectrometry (MS) or from the experimental cross section data [63,69,70].…”

“…The semi-empirical BEB approach of Kim and Rudd [67] has yielded total ionization cross sections (TICS) effortlessly for many molecules [68]. A recently proposed variant of the BEB model named the modified BEB model (m-BEB) has yielded acceptable partial ionization cross sections (PICS) [63,69,70]. Both these models offer several advantages as these are free from fitting parameters, differential oscillator strengths and requires input quantities, which can be obtained from the standard quantum chemistry computer program packages like GAUSSIAN [71] and GAMESS [72].…”