Laser-assisted ionization in the He(21,3S)+He(11S) system: Nonparticipation of the two-step process via the 2P1level

A specific experiment demonstrates unambiguously that the ${\mathrm{He}}^{+}$ ion signal coming from single collisions between a ${\mathrm{He}}^{\mathrm{*}}$(2 ${}_{}{}^1{}_{}{}^{}$,3S) beam on a He${(1\mathrm{}}^1$S) target in the presence of a laser field cannot be produced by the two-step process involving 2 ${}_{}{}^1{}_{}{}^{}2$ ${}_{}{}^1{}_{}{}^{}P$ collisional excitation followed by $2^1$P photoionization. This evidence gives support to the conclusion that the laser-assisted collision is the ionizing mechanism.