Collisional properties of atoms in high-lying doubly excited states

Using the hyperspherical wave functions of the high-lying doubly excited ${}_{}{}^1{}_{}{}^{}\mathrm{S}_{}^{\mathrm{e}}{}_{}{}^{}$ states of He, we have calculated the generalized oscillator strengths between these excited states and the Born cross sections for electron-impact excitation of He in these excited states within the manifold of ${}_{}{}^1{}_{}{}^{}\mathrm{S}_{}^{\mathrm{e}}{}_{}{}^{}$ states. Based on the rovibrator model of the doubly excited state, we predict theoretically that for these excitation processes a He atom tends to enlarge its size while keeping its shape, as a limber ‘‘e-${\mathrm{He}}^{2+}$-e’’ linear molecule, the same as that in the initial states.