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 some silicon and germanium compounds have been calculated by applying a new theoretical model that has been found to be reliable for a wide range of molecules. The new theory, the binary-encounter-Bethe (BEB) model, combines the binary-encounter theory and the Bethe theory for electron-impact ionization, and uses simple theoretical molecular orbital data-binding energies, average kinetic energies, and occupation numbers-which are readily available from molecular structure codes. Total ionization cross sections of SiH, SiH 2 , SiH 3 , SiH 4 , Si 2 H 6 , Si(CH 3 ) 4 , GeH, GeH 2 , GeH 3 , GeH 4 , and Ge 2 H 6 are presented for incident electron energies T from threshold to 1 keV, and compared to available experimental data. Theory and experiment agree well for SiH x , x=1-4, from thresholds to T<80 eV, while theoretical peaks occur at lower T than experimental peaks for SiH x , x=1-3. No experimental data are available for germanium hydrides for comparison. The theoretical cross sections are given by a compact analytic form suitable for applications in plasma processing. ©1997 American Institute of Physics. History:Received 27 January 1997; accepted 4 March 1997 Electron-impact total ionization cross sections of some silicon and germanium compounds have been calculated by applying a new theoretical model that has been found to be reliable for a wide range of molecules. 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 molecular orbital data-binding energies, average kinetic energies, and occupation numbers-which are readily available from molecular structure codes. Total ionization cross sections of SiH, SiH 2 , SiH 3 , SiH 4 , Si 2 H 6 , Si͑CH 3 ͒ 4 , GeH, GeH 2 , GeH 3 , GeH 4 , and Ge 2 H 6 are presented for incident electron energies T from threshold to 1 keV, and compared to available experimental data. Theory and experiment agree well for SiH x , xϭ1 -4, from thresholds to T Ͻ80 eV, while theoretical peaks occur at lower T than experimental peaks for SiH x , xϭ1 -3. No experimental data are available for germanium hydrides for comparison. The theoretical cross sections are given by a compact analytic form suitable for applications in plasma processing.
The mechanism of ionization of helium droplets has been investigated in numerous reports but one observation has not found a satisfactory explanation: How are He + ions formed and ejected from undoped droplets at electron energies below the ionization threshold of the free atom? Does this path exist at all? A measurement of the ion yields of He + and He2 + as a function of electron energy, electron emission current, and droplet size reveals that metastable He *-anions play a crucial role in the formation of free He + at subthreshold energies. The proposed model is testable. Research into helium nanodroplets, originally a scientific niche driven by curiosity about the minimum droplet size that supports superfluidity, 1 has matured to a point where 4 He droplets provide a novel method to synthesize and characterize unusual molecules, large aggregates in unusual morphologies, metallic foam, or nanowires from a wide range of materials.2-7 Still, not only do the droplets provide new ways for synthesis but the products also provide new insight into properties of helium droplets. For example, the shape of silver aggregates grown in very large droplets reflects the presence of quantized vortices in superfluid droplets. 7,8 A topic that has been of interest ever since large helium droplets were efficiently produced in supersonic jets 9 is the mechanism by which droplets become charged by ionizing radiation. How do small Hen + ions containing as few as two atoms emerge from a very large neutral, undoped droplet? 10,11 How do monomer or dimer ions form when the energy of the ionizing radiation is below their thermodynamic threshold? [12][13][14] How do large Hen -cluster anions form upon electron impact? [15][16][17][18] What role do metastable electronically excited species play? 12,19 What is the local structure near a positive or negative charge in undoped helium droplets, and how does it compare to that in bulk helium or helium films? [20][21][22][23] Our present work addresses the formation and subsequent ejection of bare He + from undoped droplets. For ionizing radiation exceeding the ionization threshold of atomic He (24.59 eV) small Hen + cluster ions (n > 1) are thought to result from a two-step mechanism. 12,22,[24][25][26][27][28] The process commences with the formation of He + in the droplet. Direct formation of Hen + cluster ions (n > 1) is disfavored by very small Franck-Condon factors. The hole will hop, on the time scale of femtoseconds, by resonant charge exchange with adjacent helium atoms. After about 10 hops the charge will localize by forming a vibrationally excited He2 + . Its excess energy will be large given the large (about 2.4 eV) dissociation energy of He2 + . 29 This energy would be sufficient to boil off thousands of helium atoms (the bulk cohesive energy of helium is 0.62 meV) but a thermal process appears unlikely; evaporation of even thousands of helium atoms from a primary droplet containing »10 4 helium atoms would still result in a helium cluster ion whose size lies outside the range of...
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