: I don't have their paper in front of me (my old computer died with many of my pdf archives, those that I didn't move to the cloud), but Ives and Stilwell observed an entire spectrum of lines from their experiment, choosing one which was strong and well-separated from the others. The wavelengths of the emitted photons depended, of course, on internal energy level transitions in the H2 and H3 ions. The relative speed of the ions and the ambient media would have affected the spectral distribution (i.e. the relative strengths of the lines at different wavelengths) but not the wavelengths of the emitted lines themselves (that is, in the frame of the ions). I do not know what form of electronic transition the 4861 Angstrom line represented, but it did not necessarily represent recombination of an ion and an electron. I recollect that one of the images in their paper showed quite dramatic changes in the spectra at different voltages, but you should take my my recollections with a grain of salt. I'll see if I can get to the university library in the next few days to verify what I remember of the paper. Good questions!