Eric wrote
“The suggestion is that electrons are more likely to collide with helium atoms (bound electrons and nuclei) than with hydrogen atoms/molecules, for whatever reason.”
Looking at the NIST data base for electron-impact cross-sections I was surprised to find that
HELIUM atoms have a lot SMALLER cross-section (Å2 )than HYDROGEN atoms!
Because the two protons ‘pull’ the outside electrons in in closer than the single proton in hydrogen?
Maybe I have used/interpreted the tables wrong? Wrong tables? First time I’ve seen them.
Because of the smaller crosssection, to get equivalent bremsstrahlung the helium concentration might have to bigger by a factor of 3.3exp(2/3) and 5.6 exp (2/3) fro 100eV and 1000eV respectively.
Electron energy |
Helium 1s2 to 1s2p |
Hydrogen 1s to 2p |
H2 molecule |
100 eV |
0.09 Å2 |
0.5 Å2 |
0.97 Å2 |
1000 eV |
0.03 Å2 |
0.10 Å2 |
0.25 Å2 |
“Electron-Impact Cross Sections for Ionization and Excitation Database” NIST