First detection of the ammonium ion in space supported by an improved determination of the 10-00 rotational frequency of NH3D+ from the nu4 infrared band

Abstract

The detection of a narrow and isolated feature at 262816.7 MHz toward the sources Orion-IRc2 and the cold prestellar core B1-bS has been recently reported [1]. The observations were performed with the IRAM 30 m radio telescope (Sierra Nevada, Spain). This emission line is very close to the predicted frequency [2] for the JK=10-00 line of the deuterated ammonium ion, NH3D+, 262807±9 MHz (±3¿). This deuterated species of the ammonium ion was first observed in the laboratory [2] through its ¿4 vibrational mode from which rotational constants for the ground state were derived. In our laboratory we have measured the high-resolution spectrum of the¿¿4 band of NH3D+ by difference frequency IR laser spectroscopy in a multipass hollow cathode discharge cell [3] . The main experimental improvement of our study is due to the availability of a more precise frequency scale and a more favorable ratio of precursors in the discharge. Based on our observations we propose a value of 262817±6 MHz (±3¿) for the frequency of the 10-00 pure rotational transition of NH3D+, in good agreement with that of the feature detected in Orion-IRc2 and B1-bS, thus supporting the identification of this molecular ion in the Interstellar Medium (ISM).