Spectra Induced by 200-kev Proton Impact on Helium

Abstract

Spectra induced by 200-kev proton impact on helium have been observed in the spectral region of $\lambda 3500$ A to $\lambda 6000$ A. ${}_{}{}^1{}_{}{}^{}S$ states appear to be strongly excited. Absolute cross-sections for the direct excitation of the $4{}_{}{}^1{}_{}{}^{}S$ and $5{}_{}{}^1{}_{}{}^{}S$ states of neutral helium were determined as well as the simultaneous ionization and excitation cross-section for helium into the $n=4\mathrm{}$ state of ${\mathrm{He}}^{+}$. Of the more intense lines, only the $2{}_{}{}^1{}_{}{}^{}P-n{}_{}{}^1{}_{}{}^{}S$ lines and the He ii $\lambda 4686$ line behaved linearly with pressure within experimental error. Triplet spectra were observed in which the dominant feature was the $2{}_{}{}^3{}_{}{}^{}P-n{}_{}{}^3{}_{}{}^{}D$ lines. The populations of the $4{}_{}{}^3{}_{}{}^{}D$ and $4{}_{}{}^1{}_{}{}^{}D$ states, in particular, were analyzed as a function of a direct mechanism and collision of the second kind which seem to fit the data fairly well. A very weak Doppler-shifted ${\mathrm{H}}_{\beta }$ line was detected. If this is interpreted to be produced by charge exchange, then the cross section for electron capture into the $n=4\mathrm{}$ state of hydrogen is estimated to be of the order of 8×${10}^{-21\mathrm{}}$ ${\mathrm{cm}}^2$.