Optical Ionization-Excitation Functions of Helium by Electron Impact

The cross sections for the production of the 3203 Å ($n=5\mathrm{}\to 3$), 2733 Å ($n=6\mathrm{}\to 3$), 2511 Å ($n=7\mathrm{}\to 3$) lines of ${\mathrm{He}}^{+}$ by ionization-excitation collisions between electrons and neutral atoms have been measured over the energy range of 0 to 350 eV. All three excitation functions have their maximum values near 230 eV and show the same general shape as that of the 4686 Å ($n=4\mathrm{}\to 3$) line. The Born approximation was used to compute the direct ionization-excitation cross sections of the $\mathrm{nS}$, $\mathrm{nP}$, and $\mathrm{nD}$ states of the ${\mathrm{He}}^{+}$ ions ($n=5, 6, \mathrm{and} 7$) from which the theoretical excitation cross sections of the 3203, 2733, and 2511 Å lines are obtained. In all cases the $\mathrm{nS}\to 3P$ transition accounts for 90% of the total radiation of the $n\to 3$ transitions. The theoretical ionization-excitation cross sections are found to exhibit reasonable agreement with the experimental values. The cascade contributions of the observed intensities of the three lines are estimated to be less than 5%.