Laser-assisted and field-free ionization cross sections of the He(2S1,2)+He(1)13collision system

A study of the laser-assisted and field-free ionization cross sections of the ${\mathrm{He}}^{\mathrm{*}}$(2 ${}_{}{}^1{}_{}{}^{}S$,2 ${\mathrm{}}^3$S)+He(1 ${\mathrm{}}^1$S) collision system is presented as a function of the kinetic energy of the collision. The work of Evans et al. [Phys. Rev. A 4, 2235 (1971)] and Cohen [Phys. Rev. A 13, 86 (1976)] on the ${\mathrm{He}}_2$ ${\mathrm{}}^{\mathrm{*}}$ molecular system is used to calculate the value ${\sigma }_d$ of the field-free ionization cross section and then to normalize our experimental results. Knowing ${\sigma }_d$, we deduce the assisted-ionization cross section ${\sigma }_a$. These data show that within our energy range, the main process below $E_{\mathrm{c}.\mathrm{m}.\mathrm{}=50\mathrm{}}$ eV is laser-assisted ionization, whereas above this kinetic energy, it is a two-step reaction: collisional excitation followed by the photoionization of the level reached.