The understanding of interstellar nitrogen chemistry has improved significantly with recent results from the Herschel Space Observatory .
To set even better constraints , we report here on deep searches for the NH ^ { + } ground state rotational transition J = 1.5 - 0.5 of the ^ { 2 } \Pi _ { 1 / 2 } lower spin ladder , with fine-structure transitions at 1 013 and 1 019 GHz , and the para-NH ^ { - } _ { 2 } 1 _ { 1 , 1 } -0 _ { 0 , 0 } rotational transition at 934 GHz towards Sgr B2 ( M ) and G10.6 -0.4 ( W31C ) using the Herschel Heterodyne Instrument for the Far-Infrared ( HIFI ) .
No clear detections of NH ^ { + } are made and the derived upper limits relative to the total number of hydrogen nuclei are \lesssim 2 \times 10 ^ { -12 } and \lesssim 7 \times 10 ^ { -13 } in the Sgr B2 ( M ) molecular envelope and in the G10.6 -0.4 molecular cloud , respectively .
The searches are , however , complicated by the fact that the 1 013 GHz transition lies only - 2.5 km s ^ { -1 } from a CH _ { 2 } NH line , which is seen in absorption in Sgr B2 ( M ) , and that the hyperfine structure components in the 1 019 GHz transition are spread over 134 km s ^ { -1 } .
Searches for the so far undetected NH ^ { - } _ { 2 } anion turned out to be unfruitful towards G10.6 -0.4 , while the para-NH ^ { - } _ { 2 } 1 _ { 1 , 1 } -0 _ { 0 , 0 } transition was tentatively detected towards Sgr B2 ( M ) at a velocity of 19 km s ^ { -1 } .
Assuming that the absorption occurs at the nominal source velocity of +64 km s ^ { -1 } , the rest frequency would be 933.996 GHz , offset by 141 MHz from our estimated value .
Using this feature as an upper limit , we found N ( p-NH ^ { - } _ { 2 } ) \lesssim 4 \times 10 ^ { 11 } cm ^ { -2 } , which implies an abundance of \lesssim 8 \times 10 ^ { -13 } in the Sgr B2 ( M ) molecular envelope .
The upper limits for both species in the diffuse line-of-sight gas are less than 0.1 to 2 % of the values found for NH , NH _ { 2 } , and NH _ { 3 } towards both sources , and the abundance limits are \lesssim 2 - 4 \times 10 ^ { -11 } .
An updated pseudo time-dependent chemical model with constant physical conditions , including both gas-phase and surface chemistry , predicts an NH ^ { + } abundance a few times lower than our present upper limits in diffuse gas and under typical Sgr B2 ( M ) envelope conditions .
The NH _ { 2 } ^ { - } abundance is predicted to be several orders of magnitudes lower than our observed limits , hence not supporting our tentative detection .
Thus , while NH _ { 2 } ^ { - } may be very difficult to detect in interstellar space , it could , on the other hand , be possible to detect NH ^ { + } in regions where the ionisation rates of H _ { 2 } and N are greatly enhanced .