Electron-impact ionization cross sections of atmospheric molecules

Abstract

A theoretical model for electron-impact total ionization cross sections, which has been found to be reliable for a wide range of molecules, is applied to molecules of interest to atmospheric science. The new theory, the binary-encounter-Bethe (BEB) model, combines the binaryencounter theory and the Bethe theory for electron-impact ionization, and uses simple theoretical data for the ground state of the target molecule, which are readily available from molecular structure codes. Total ionization cross sections of 11 molecules, CS, CS 2 , COS, CH 4 , H 2 S, NH 3 , NO 2 , N 2 O, O 3 , S 2 , and SO 2 , are presented for incident electron energies from threshold to 1 keV with an average accuracy of 15% or better at the cross section peak. We also found that the use of vertical ionization potentials (IPs) rather than adiabatic IPs for the lowest IPs significantly improves BEB cross sections between the threshold and cross section peak for molecules whose adiabatic and vertical IPs are different by ~1 eV or more (CH 4 and NH 3 ). The BEB cross sections are presented in a compact analytic form with a small number of constants, making the cross sections suitable for modeling applications. ©1997 American Institute of Physics. History:Received 20 August 1996; accepted 11 October 1996 A theoretical model for electron-impact total ionization cross sections, which has been found to be reliable for a wide range of molecules, is applied to molecules of interest to atmospheric science. The new theory, the binary-encounter-Bethe ͑BEB͒ model, combines the binary-encounter theory and the Bethe theory for electron-impact ionization, and uses simple theoretical data for the ground state of the target molecule, which are readily available from molecular structure codes. Total ionization cross sections of 11 molecules, CS, CS 2 , COS, CH 4 , H 2 S, NH 3 , NO 2 , N 2 O, O 3 , S 2 , and SO 2 , are presented for incident electron energies from threshold to 1 keV with an average accuracy of 15% or better at the cross section peak. We also found that the use of vertical ionization potentials ͑IPs͒ rather than adiabatic IPs for the lowest IPs significantly improves BEB cross sections between the threshold and cross section peak for molecules whose adiabatic and vertical IPs are different by ϳ1 eV or more ͑CH 4 and NH 3 ). The BEB cross sections are presented in a compact analytic form with a small number of constants, making the cross sections suitable for modeling applications.

Electron-impact total ionization cross sections of silicon and germanium hydrides

“…8 Silane has also been detected on Saturn and on several other planets and satellites. As was the case for 28 molecules reported earlier, [1][2][3] the BEB model provides cross sections in good agreement with available experimental data and sometimes enables one to discriminate between conflicting data for the same molecule. To the best of our knowledge, the total ionization cross sections for GeH x , xϭ1 -4, and Ge 2 H 6 presented here are the first of its kind in the literature.…”

“…The experimental data by Tarnovsky et al 16 for SiH and SiH 3 show large fluctuations of the order of 20%-25% above Tϭ50 eV in the cross sections for the production of the parent molecular ions ͑SiH ϩ and SiH 3 ϩ ͒, which dominate the total ionization cross section. The overall experimental accuracy claimed is Ϯ15%, which is indicated in Figs.…”

“…For SiH 2 , the singlet C 2v structure is the ground state using the same basis set. The only experimental data for these molecules are by Tarnovsky et al, 16 3 , and hence the sum of these partial cross sections should be a lower limit to the total ionization cross section. Experimental data for these molecules are compared to BEB cross sections in Figs.…”

“…It has been shown in previous publications that the new theoretical method, the binary-encounter-Bethe ͑BEB͒ model, [1][2][3] produces reliable electron-impact total ionization cross sections for stable molecules and radicals using as inputs simple molecular orbital data for the ground state, all of which can be obtained by using standard molecular wave function codes. There are no adjustable or fitted parameters in the theory.…”

Heats of Vaporization of Room Temperature Ionic Liquids by Tunable Vacuum Ultraviolet Photoionization

“…The narrow energy spread of the light source (0.025 eV) and soft ionization conditions minimize the effect of Although fragmentation of the parent ion can be minimized by lowering the electron impact energy, the electron impact ionization cross section changes rapidly as a function of electron energy near the ionization threshold, 30,31 and may be highly susceptible to thermal effects near the ionization threshold. Also, the energy spread of a typical electron gun is limited to > 0.5 eV, limiting the energy resolution in threshold measurements.…”