15min:

MONIKA KOERBER, CHRISTIAN P. ENDRES, FRANK LEWEN, THOMAS F. GIESEN, STEPHAN SCHLEMMER, I. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany; ROLAND POHL, AXEL KLEIN, Institut für Anorganische Chemie, Universität zu Köln, 50939 Köln, Germany.

Dimethylether (DME) is a nearly prolate asymmetric top with two internal rotors (methyl groups) which undergo periodic large amplitude motions and show a complicated torsional splitting of each rotational energy level. Due to its complex spectrum and its high abundance in hot cores such as Orion~KL or Sagittarius~B2 at temperatures exceeding 100~K, DME is very prominent in astronomical line surveys and contributes to spectral line confusion of such sources\footnoteC. Comito et al. , Astrophys. J. Suppl. Ser. \textbf156, 127-167 (2005). The interpretation of astronomical observations therefore depends on the knowledge of accurate rest frequencies and reliable intensities. Precise predictions for the ground state of DME's main isotopologue are now available up to 2.1~THz\footnoteC.~P.~Endres et al. , Astronomy & Astrophysics \textbf504, 635-640 (2009). In contrast, very little is known about ¹³C-substituted DME. Only a few data are available on singly ¹³C-substituted DME, ¹²CH₃O¹³CH₃. However, no data are available on doubly ¹³C-substituted DME, (¹³CH₃)₂O, yet. While in (¹³CH₃)₂O the two internal rotating methyl groups are equivalent and the splitting of rotational energy levels into four substates is comparable to the main isotopologue, singly ¹³C-substituted DME has two non-equivalent internal rotors resulting in torsional splitting of rotational energy levels into five substates. The purpose of our new laboratory measurements is to extend the knowledge on the astrophysically relevant species ¹²CH₃O¹³CH₃. To analyze the complicated spectrum resulting from a ¹³C-enriched sample of DME, containing all different ¹³C-substituted species as well as the main isotopologue, also precise data on doubly ¹³C-substituted DME are inevitable. We performed measurements in the frequency region 35-120~GHz using an all solid state spectrometer. Rotational as well as torsional parameters have been obtained for (¹³CH₃)₂O as well as ¹²CH₃O¹³CH₃ by fitting the assigned transitions to an effective rotational Hamiltonian introduced by Peter Groner.