Ab Initio Computation of Energy Deposition During Electron Ionization of Molecules.

The dissociative ionization of molecules under electron impact forms the basis for analytical mass spectrometry of volatile compounds. It is also important in other situations, notably plasmas. Although qualitative theory for mass spectrometry was developed long ago, progress toward predictive theory has been slow. A major obstacle has been ignorance of the amount of energy deposited in the molecular ion, prior to its fragmentation. Here, we consider the Binary-Encounter Bethe (BEB) theory, which was originally developed for predicting total ionization cross sections. The energy deposition function constructed from BEB molecular-orbital cross sections compares well with the two comparable (e, 2e + ion) experimental measurements. When combined with experimental breakdown data from photoelectron-photoion-coincidence measurements, the BEB energy deposition function successfully reproduces library mass spectra for all but one of the six molecules studied here. This indicates that BEB molecular-orbital cross sections are physically meaningful and are useful for modeling the energy deposition during electron ionization of molecules.