Impact radiation from H+ and He+ in noble gases is studied in the wavelength region below 1250 Å. A vacuum spectrograph has been built and absolutely calibrated for this purpose, so that the measurements provided absolute excitation or emission cross sections. We observe strong maxima in the light intensities of some atom and ion lines of noble gases. These maxima appear at remarkably low energies of the impinging particles, and moreover, correspond with large absolute cross sections (~10-17 cm2/atom). For instance, the excitation of the np6 2S1 / 2 states of Ar II (n=3) and Kr II (n=4) induced by the He+ impact on Ar and Kr show maxima at 9 and 7 KeV, respectively. This can be predicted by the adiabatic criterion of Massey. However, for the (n+1)s' 2D5 / 2 states of Ar II (n=3) and Kr II (n=4) the maxima lie below 5 keV, though more energy is needed for the excitation of these states than there is for the 2S1 / 2 ones. Also for the excitation of the 2p5 3s state of Ne I induced by He+ impact on Ne, the maximum lies below 5 keV. There are indications that intermediate molecular states occur which contribute to the formation of these low-energy maxima.

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Journal Phys. Rev.
van Eck, J, de Heer, F. J, & Kistemaker, J. (1963). Excitation of Ne, Ar, and Kr induced by H+ and He+ impact (5-35 KeV). Phys. Rev., 130, 656–658. doi:10.1103/physrev.130.656