Tests of Born Approximations: Differential and Total $2 {}^{3}S$ , $2 {}^{1}P$ , and $2 {}^{1}S$ Cross Sections for Excitation of He by 100- to 400-eV Electrons

The angular dependence of

2 {}^{3}S

2 {}^{1}P

, and

2 {}^{1}S

excitation of He for incident electron energies from 100 to 225 eV (

2 {}^{3}S

) and 400 eV (

2 {}^{1}P

and

2 {}^{1}S

) has been measured. Apparent generalized oscillator strengths

f (K)

and differential cross sections for the transition

1 {}^{1}S \to 2 {}^{1}P

are obtained by normalizing to Schiff and Pekeris's optical oscillator strength. From the experimental intensity ratios

\frac{2 {}^{3}S}{2 {}^{1}P}

and

\frac{2 {}^{1}S}{2 {}^{1}P}

we then calculate differential

1 {}^{1}S \to 2 {}^{3}S

and

1 {}^{1}S \to 2 {}^{1}S

cross sections. The differential cross sections are integrated to get total cross sections. The

f (K)

found here for

2 {}^{1}P

and

2 {}^{1}S

excitation decrease faster with increasing momentum transfer

K ℏ

than in earlier studies. Departures from the Born approximation appear only below 200 eV for

2 {}^{1}P

excitation, but occur at higher energy and are larger (especially for large momentum transfers) for

2 {}^{1}S

excitation. The angular dependence found for

2 {}^{3}S

excitation disagrees strongly with the Ochkur (Bonham)...

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Tests of Born Approximations: Differential and Total2S3,2P1, and2S1Cross Sections for Excitation of He by 100- to 400-eV Electrons