LABORATORY MEASUREMENTS AND TENTATIVE ASTRONOMICAL IDENTIFICATION OF H2NCO+

Abstract

The rotational spectrum of H2NCO+, 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 Ka ⩽ 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 H2NCO+ in the centimeter band (00, 0–10, 1 and 11, 0–21, 1) were tentatively identified in absorption in the PRIMOS spectral line survey of Sgr B2(N) with the Green Bank Telescope. The lines of H2NCO+ arise in a region of the Sgr B2(N) halo whose density is low (n < 1 × 104 cm−3). The derived column density of (6–14) × 1011 cm−2 implies that the fractional abundance is ∼10−12. Owing to the ubiquity of HNCO in galactic molecular clouds, H2NCO+ is a good candidate for detection in sources spanning a wide range of physical conditions.