Rates of Electron-Impact Transitions between Excited States of Helium

Measurements of electron densities and population densities of excited states of neutral helium are presented for afterglows in the C stellarator. Electron densities range from 10¹¹ to 3×{}10¹³ cm^-3, and electron temperatures range from 0.04 to 1 eV. Semiempirical cross-section formulas for transitions among excited levels are deduced by comparing observed excited-state population densities with solutions of detailed transition-rate equations. The agreement between computed and measured values is good, when the assumed cross sections are comparable with impact-parameter approximation results; the agreement is poor - especially at low electron densities and temperatures - when classical cross sections are assumed.