Two-center effects in electron emission in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi mathvariant="normal">He</mml:mi></mml:mrow><mml:mrow/><mml:mrow><mml:mn>2</mml:mn><mml:mo>+</mml:mo></mml:mrow><mml:mrow/><mml:mrow/><mml:mprescripts/><mml:mrow/><mml:mrow><mml:mn>3</mml:mn></mml:mrow><mml:mrow/><mml:mrow/></mml:mmultiscripts></mml:mrow></mml:math>-He and<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>-He collisions at intermediate energies

We present measurements of doubly differential cross sections (DDCS) with respect to energy and angle for electron emission produced when 100- and 50-keV/u $^{3}\mathrm{He}^{2+}$ and ${\mathrm{H}}^{+}$ collide with He. An electron spectrometer designed for this purpose enables us to measure electron distributions which, even at small emission angles, do not present appreciable deformations of the measured distributions by extended gas target effects. Using the ratio of the emissions obtained with $^{3}\mathrm{He}^{2+}$ and ${\mathrm{H}}^{+}$, we study the dependence of the DDCS on the projectile charge. We confirm a saturation of this emission ratio that increases with increasing angle. Furthermore, we report a sudden enhancement of the emission ratio that is found at and above the electron capture to the continuum peak. These observations are in basic but not quantitative agreement with calculations performed using a model that incorporates the continuum distorted wave and eikonal initial state and that approximately accounts for a two-center interaction of the active electron.

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