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 $\ensuremath{\lambda}3500$ A to $\ensuremath{\lambda}6000$ 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 $n=4$ state of ${\mathrm{He}}^{+}$. Of the more intense lines, only the $2^{1}P\ensuremath{-}n^{1}S$ lines and the He ii $\ensuremath{\lambda}4686$ line behaved linearly with pressure within experimental error. Triplet spectra were observed in which the dominant feature was the $2^{3}P\ensuremath{-}n^{3}D$ 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 ${\mathrm{H}}_{\ensuremath{\beta}}$ line was detected. If this is interpreted to be produced by charge exchange, then the cross section for electron capture into the $n=4$ state of hydrogen is estimated to be of the order of 8\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}21}$ ${\mathrm{cm}}^{2}$.