Ejected-electron spectroscopy of autoionizing resonances of helium excited by fast-electron impact

The autoionizing resonances ${\left(2s^2\right)}^{1}S$, ${\left(2p^2\right)}^{1}D$, and ${\left(2s2p\right)}^{1}P$ of helium have been investigated employing ejected-electron spectroscopy by fast-electron impact at incident energies of 250–2000 eV and ejected angles of 26°–116°. Shore parameters of the line shapes for these three resonances have been obtained in such high incident energy regime except at 250 eV. Distinct discrepancies between the present results at 250 eV and those of McDonald and Crowe at 200 eV [D. G. McDonald and A. Crowe, J. Phys. B 25, 2129 (1992); D. G. McDonald and A. Crowe, J. Phys. B 25, 4313 (1992)] and Sise et al. at 250 eV [O. Sise, M. Dogan, I. Okur, and A. Crowe, Phys. Rev. A 84, 022705 (2011)], especially for ${}^{1}D$ and ${}^{1}P$ states, are also observed.

Figure 1

The ejected-electron spectra of helium at 250 eV incident energy for ejected angles from ${26}^{\circ }$ to ${116}^{\circ }$. The solid circles indicate the spectra measured and the solid lines are the overall fit to the data.

Figure 2

As in Fig. 1, except that the incident energy is 1000 eV.

Figure 3

As in Fig. 1, except that the incident energy is 2000 eV.

Figure 4

The direct ionization cross section $f$ from the ejected-electron spectra of helium as a function of ejected angle at incident energies of 250 eV (black square), 500 eV (red circle), 1000 eV (green triangle), 1500 eV (blue downward triangle), and 2000 eV (orange diamond).

Figure 5

The Shore parameters $a$ and $b$ from the ejected-electron spectra as a function of ejected angle for the ${\left(2s^2\right)}^{1}S$ state of helium at incident energies (a) 250 eV, (b) 500 eV, (c) 1000 eV, (d) 1500 eV, and (e) 2000 eV.

Figure 6

As in Fig. 4, but for the ${\left(2p^2\right)}^{1}D$ state of helium.

Figure 7

As in Fig. 4, but for the ${\left(2s2p\right)}^{1}P$ state of helium.