Measurements of electron velocity distributions emitted at $0°$ for collisions of 10- and 20-keV ${\mathrm{H}}^{+}$ incident ions on ${\mathrm{H}}_2$ and He show that the electron capture to the continuum cusp formation, which is still possible at these low impact energies, is shifted to lower momenta than its standard position (centered on the projectile velocity), as recently predicted. Classical trajectory Monte Carlo calculations reproduce the observations remarkably well, and indicate that a long-range residual interaction of the electron with the target ion after ionization is responsible for the shifts, which is a general effect that is enhanced at low nuclear velocities.

• Received 7 June 2002

DOI:https://doi.org/10.1103/PhysRevA.67.010704