We report the first observation of an assisted ${\mathrm{He}}^{+}$ yield arising from single collisions between a $\mathrm{He}(2\mathrm{}{}_{}{}^1{}_{}{}^{}S, 2{}_{}{}^3{}_{}{}^{}S)$) atom beam on a $\mathrm{He}\left(1\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)$ target in the presence of an intense laser field. A time-of-flight analysis shows that the pulsed laser-assisted ion signal is reproducible, energy dependent, and appears in addition to the continuous ion flux coming from a known field-free diabatic channel. From the estimated value of the diabatic ionization cross section ${\sigma }_d$, the order of magnitude of the assisted-ionization cross section ${\sigma }_a$ is given.

• Received 26 November 1984

DOI:https://doi.org/10.1103/PhysRevLett.54.2600