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The understanding of interstellar nitrogen chemistry has improvedsignificantly with recent results from the Herschel Space Observatory. To seteven better constraints, we report here on deep searches for the NH+ groundstate rotational transition J=1.5-0.5 of the ^2Pi_1/2 lower spin ladder, withfine-structure transitions at 1013 and 1019 GHz, and the para-NH2- 1_1,1-0_0,0rotational transition at 934 GHz towards Sgr B2(M) and G10.6-0.4 usingHerschel-HIFI. No clear detections of NH+ are made and the derived upper limitsare <2*10^-12 and <7*10^-13 in Sgr B2(M) and G10.6-0.4, respectively. Thesearches are complicated by the fact that the 1013 GHz transition lies only-2.5 km/s from a CH2NH line, seen in absorption in Sgr B2(M), and that thehyperfine structure components in the 1019 GHz transition are spread over 134km/s. Searches for the so far undetected NH2- anion turned out to be unfruitfultowards G10.6-0.4, while the para-NH2- 1_1,1-0_0,0 transition was tentativelydetected towards Sgr B2(M) at a velocity of 19 km/s. Assuming that theabsorption occurs at the nominal source velocity of +64 km/s, the restfrequency would be 933.996 GHz, offset by 141 MHz from our estimated value.Using this feature as an upper limit, we found N(p-NH2-)<4*10^11 cm^-2. Theupper limits for both species in the diffuse line-of-sight gas are less than0.1 to 2 % of the values found for NH, NH2, and NH3 towards both sources.Chemical modelling predicts an NH+ abundance a few times lower than our presentupper limits in diffuse gas and under typical Sgr B2(M) envelope conditions.The NH2- abundance is predicted to be several orders of magnitudes lower thanour observed limits, hence not supporting our tentative detection. Thus, whileNH2- may be very difficult to detect in interstellar space, it could, bepossible to detect NH+ in regions where the ionisation rates of H2 and N aregreatly enhanced.

Upper limits to interstellar NH+ and para-NH2- abundances. Herschel-HIFI observations towards Sgr B2 (M) and G10.6-0.4 (W31C)