HSCO⁺ and DSCO⁺: a multi-technique approach in the laboratory for the spectroscopy of interstellar ions^⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
e-mail: lattanzi@mpe.mpg.de
² Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, 02138 USA
³ School of Engineering and Applied Sciences, Harvard University, 29 Oxford St., Cambridge, MA, 02138 USA

Received: 28 September 2018
Accepted: 19 October 2018

Abstract

Context. Protonated molecular species have been proven to be abundant in the interstellar gas. This class of molecules is also pivotal for the determination of important physical parameters for the evolution of the interstellar medium (e.g. gas ionisation fraction) or as tracers of non-polar species which are not directly observable. The identification of these molecular species through radioastronomical observations is directly linked to precise laboratory spectral characterisation.

Aims. The goal of the present work is to extend the laboratory measurements of the pure rotational spectrum of the ground electronic state of protonated carbonyl sulfide (HSCO⁺) and its deuterium substituted isotopomer (DSCO⁺). At the same time, we show how implementing different laboratory techniques allows for the determination of different spectroscopical properties of asymmetric-top protonated species.

Methods. Three different high-resolution experiments were used in conjunction to detect for the first time the b-type rotational spectrum of HSCO⁺, and to extend, well into the sub-millimetre region, the a-type spectrum of the same molecular species and DSCO⁺. The electronic ground-state of both ions was investigated in the 273–405 GHz frequency range, allowing for the detection of 60 and 50 new rotational transitions for HSCO⁺ and DSCO⁺, respectively.

Results. The combination of our new measurements with the three rotational transitions previously observed in the microwave region permits the rest frequencies of the most astronomically relevant transitions to be predicted to better than 100 kHz for both HSCO⁺ and DSCO⁺ up to 500 GHz, equivalent to better than 60 m s⁻¹ in terms of equivalent radial velocity.

Conclusions. The present work illustrates the importance of using different laboratory techniques to spectroscopically characterise a protonated species at high frequency. Each instrument addressed a complementary part of the same spectroscopic challenge, demonstrating the potential of such an approach for future studies of similar reactive species.