Evidence for a deep Cooper minimum in the 3s-np channel of ND4

NH4 and ND4 have been the subject of many studies, experimental and theoretical. Fuke et al. have recently recorded the photoionization cross-section at threshold for ND4 revealing unusual behavior. Based on this observation, theoretical calculations, and comparison with isoelectronic sodium and related alkali atoms, the existence of a deep Cooper minimum slightly below the ionization threshold is postulated for the 3s-np channel of the NH4 radical and its ND4 isotope.