Electron-impact excitation of helium: Cross sections,n, andldistributions of high Rydberg states

Helium atoms in highly excited Rydberg states are produced by electron impact and are detected by electric field ionization. The principal quantum numbers $n$ are determined to lie in the range $20\lesssim n\lesssim 80$ by ionizing the excited He atoms in a uniform, swept electric field. The absolute excitation cross section for 100-eV incident electrons is found to be $(9\mathrm{}\pm 5)\times \frac{{10}^{-17\mathrm{}}}{n^3}$ ${\mathrm{cm}}^2$. This expression predicts a value for $n=4\mathrm{}$ that agrees with the optically measured $n=4\mathrm{}$ cross section. The 108-μsec time of flight requires that the high Rydberg He atoms have lifetimes much longer than those calculated for low-angular-momentum ($l\le 2$) states, those expected to be populated by electron impact. The proposed explanation is that the excited He atoms suffer collisions with electrons and/or ground-state He atoms causing longer-lived higher $l$ states to be populated. This explanation is supported by measurements of signal as a function of incident-electron energy, electron current, and He pressure. The cross section for $l$-changing collisions between electrons and high Rydberg helium atoms is given by ${\sigma }_{\Delta l}^e\cong 5\times {10}^{-(15\mathrm{}\pm 0.3)}\frac{\ln \left(100E{n}^2\right)n^4}{E}$ ${\mathrm{cm}}^2$.