Laboratory Measurements and Tentative Astronomical Identification of H_2NCO^+

The rotational spectrum of H_2NCO^+, the ground-state isomer of protonated HNCO, has been measured in a molecular beam in the centimeter band with a Fourier transform microwave spectrometer and in a low-pressure laboratory discharge in absorption in the millimeter band. Spectroscopic constants, including the nitrogen-14 hyperfine coupling constant, derived from 30 a-type transitions between 20 and 367 GHz with J ≤ 18 and K_a ≤ 3 allow the principal rotational transitions to be calculated to 1 km s^(–1) or better in equivalent radial velocity well into the far IR. Two low-lying rotational transitions of H_2NCO^+ in the centimeter band (0_(0,0)-1_(0,1) and 1_(1,0)-2_(1,1)) were tentatively identified in absorption in the PRIMOS spectral line survey of Sgr B2(N) with the Green Bank Telescope. The lines of H_2NCO^+ arise in a region of the Sgr B2(N) halo whose density is low (n < 1 × 10^4 cm^(–3)). The derived column density of (6-14) × 10^(11) cm^(–2) implies that the fractional abundance is ~10^(–12). Owing to the ubiquity of HNCO in galactic molecular clouds, H_2NCO^+ is a good candidate for detection in sources spanning a wide range of physical conditions