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 ≲ n ≲ 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 \pm 5) \times \frac{10^{- 17}}{n^{3}}

{cm}^{2}

. This expression predicts a value for

n = 4

that agrees with the optically measured

n = 4

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 \leq 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

σ_{Δ l}^{e} ≅ 5 \times 10^{- (15 \pm 0.3)} \frac{\ln (100 E n^{2}) n^{4}}{E}

{cm}^{2}

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