MEASUREMENTS OF ELECTRON-IMPACT IONIZATION CROSS-SECTIONS OF NEON BY COMPARISON WITH PHOTOIONIZATION

A method and apparatus have been developed for precise measurements of the ratios of total cross sections for electron-impact ionization and photoionization in rare gases based on the comparison of the total io n yields of the two ionization processes. Low uncertainties for the cr oss-section ratios are achieved above all by using a cryogenic electri cal substitution radiometer as a primary detector standard in the soft -x-ray and vacuum ultraviolet spectral ranges in order to determine th e impinging photon flux. On the basis of our measured cross-section ra tios and well-known total photoionization cross sections we deduce abs olute total electron-impact ionization cross sections of rare gases. A s a result we present ratios of total electron-impact ionization cross sections to total photoionization cross sections and deduce total ele ctron-impact ionization cross sections of neon in the energy range of electrons from 140 to 4000 eV and of photons from 100 to 1500 eV. Rela tive uncertainties as low as 1.3% for the cross-section ratios and 2.8 % for the total electron-impact ionization cross sections have been ac hieved. A comparison of our cross-section data with published experime ntal and theoretical data is presented. [S1050-2947(98)05310-4].