Lecture Note for Oct. 30, 2002 (Wed.)

 

VII.  ELECTRON-IMPACT IONIZATION OF MOLECULES

 

     The BEB model has been found to produce reliable total ionization cross sections for many large and small molecules, from threshold to several keV in T

 

     The BEB model is the only ab initio theory that can generate molecular ionization cross sections using only the ground-state molecular wave functions

      

     The BEB model works well for neutral molecules, its neutral fragments (radicals), and singly charged molecular ions

 

     Unlike atoms, when a molecule receives energy transfer high enough to ionize,  the molecule may dissociate into neutral fragments without ionization

 

     However, the BEB model assumes, as most other theories do, that the target molecule ionizes whenever it receives energy high enough to ionize, i.e., it does not allow for the possibility of dissociation without ionization


     Hence, if the cross section for dissociation without ionization is substantial—this happens often in large molecules near the ionization threshold—the BEB model will overestimate the total ionization cross section

 

     There are several powerful computer codes for molecular wave functions.  Among them, GAMESS is free, and provides the orbital kinetic energy U as an optional output

 

     For atoms, excitation-autoionization is an important channel for indirect ionization.  For molecules, however, the BEB cross section—which accounts only for direct ionization—works well for a large number of molecules, their fragments and ions

 

     One of the reasons that the BEB model works better for molecules than atoms may be that the rotational and vibrational excitation channels effectively smooth threshold behavior, i.e., molecular ionization cross sections have fewer sharp thresholds than atoms

 

     Some molecular ionization cross sections show clear signs of EA (e.g., OH).  There is an urgent need to resurrect computer codes to calculate PWB cross sections for molecules

     The BEB model is for the total ionization cross sections only.  There is no reliable theory for calculating partial cross sections for producing ion fragments (= dissociative ionization)

 

     Experimental partial cross sections may be normalized by comparing their sum to the total ionization cross section calculated from the BEB model

 

     There are some practical problems in measuring partial cross sections with a mass spectrometer as is commonly done

 

     Most mass spectrometers are tuned to detect very slow charged fragments

 

     Light ion fragments, such as protons generated by dissociation, may have kinetic energies exceeding a few eV.  Such fast ions cannot be detected efficiently by a mass spectrometer

 

     This is why the sum of experimental partial cross sections is often lower than total ionization cross section either from theory or from a separate measurement of the total ionization cross section