Differential electron emission for isotachic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">He</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>impact on helium

Experimental data are presented for differential electron emission from helium induced by isotachic ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}$ impact. Electron emission at 15\ifmmode^\circ\else\textdegree\fi{} was studied in order to investigate maxima in the ${\mathit{d}}^{2}$\ensuremath{\sigma}/${\mathit{dv}}_{\mathit{e}}$d\ensuremath{\Omega} spectra. These maxima were previously observed to shift to lower electron velocities as the projectile charge state increased and the shifts were interpreted as evidence of an independent saddle-point-ionization mechanism. The present data for ${\mathrm{H}}^{+}$ and ${\mathrm{He}}^{2+}$ impact do not confirm any such shifts and indicate that previous ${\mathrm{He}}^{2+}$ measurements, where shifts were observed, are in error.

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