Doubly 15N-substituted diazenylium: THz laboratory spectra and fractionation models

Context. Isotopic fractionation in dense molecular cores has been suggested as a possible origin of large N-14/N-15 ratio variations in solar system materials. While chemical models can explain some observed variations with di ff erent fractionation patterns for molecules with -NH or -CN functional groups, they fail to reproduce the observed ratios in diazenylium (N2H+). Aims. Observations of doubly N-15-substituted species could provide important constraints and insights for theoretical chemical models of isotopic fractionation. However, spectroscopic data are very scarce. Methods. The rotational spectra of the fully N-15-substituted isopologues of the diazenylium ion, (N2H+)-N-15 and (N2D+)-N-15, have been investigated in the laboratory well into the THz region by using a source-modulation microwave spectrometer equipped with a negative glow discharge cell. An extended chemical reaction network has been used to estimate what ranges of N-15 fractionation in doubly N-15-substituted species could be expected in the interstellar medium (ISM). Results. For each isotopologue of the H-and D-containing pair, nine rotational transitions were accurately measured in the frequency region 88 GHz-1.2 THz. The analysis of the spectrum provided very precise rest frequencies at millimeter and sub-millimeter wavelengths, useful for the radioastronomical identification of the rotational lines of (N2H+)-N-15 and (N2D+)-N-15 in the ISM.