In this Reply, it is shown how our experimental results lead us to the conclusion that the two-step process proposed in the recent Comment by Gillen [Phys. Rev. A 39, 2248 (1989)] cannot explain the ion signals observed for $E_{\mathrm{c}.\mathrm{m}.\mathrm{}\mathrm{\le }50}$ eV, whereas laser-assisted ionization is the only mechanism consistent with our observations in this energy range. The measured amplitude of the signals can be interpreted in terms of a transition-dipole enhancement induced by the collision. Such an enhancement has been ignored in the estimates of Gillen.

• Received 21 November 1988

DOI:https://doi.org/10.1103/PhysRevA.40.1147