Knowledge Bank

Dimethyl ether (DME) was first detected in the interstellar medium more than 30 years (1974) ago towards Orion} {\it et al.} \newblock {\em Astrophys. J.}, {\bf 191}, L79$-$L82 (1974)}. It is highly abundant in hot cores such as Orion or Sagittarius B2 at temperatures exceeding 100 K. A large amount of ground state transitions up to 900 GHz have been detected in several line surveys}, P. {Schilke}, T.G. {Philips}, D.C. {Lis}, F. {Motte}, and D. {Mehringer}. \newblock {\em Astrophys. J. Suppl. Ser.}, {\bf 156}, (2005) 127$-$167\}.\; Due\; to\; high\; excitation\; temperatures\; in\; hot\; cores,\; rotational\; levels\; of\; torsional\; excited\; states\; are\; expected\; to\; be\; sufficiently\; populated\; to\; show\; transitions\; in\; these\; surveys.\; New\; telescopes,\; such\; as\; the\; Herschel\; Space\; Observatory\; will\; grant\; observations\; with\; greatly\; improved\; sensitivity\; at\; even\; higher\; frequencies\; reaching\; far\; into\; the\; THz\; regime.\; This\; will\; drastically\; increase\; the\; request\; of\; accurately\; known\; rest\; frequencies\; for\; the\; interpretation\; of\; astronomical\; observations.\; Since\; precise\; predictions\; for\; the\; ground\; state\; of\; DME\; are\; available\; only\; up\; to\; 600\; GHz\},\; S.\; \{Albert\},\; E.\; \{Herbst\},\; and\; F.C.\; \{De\; Lucia\}.\; \backslash newblock\; \{\backslash em\; Astrophys.\; J.\},\; \{\backslash bf\; 500\},\; 1059$-$1063 (1998)}, and the datasets for the excited torsional states are very limited, we decided to record spectra into the THz region by using frequency multiplier techniques. Weaker transitions have been additionally obtained with highly sensitive spectrometers using phase-locked backward wave oscillators. The greatly extended ground state dataset currently includes transitions up to 1.2 THz and transitions accessing $J$ and $K$ rotational quantum numbers up to $J=69$ and $K=18$. An increased number of $P$-branch transitions improves the quality of the predictions for line frequencies. Equally, the very limited datasets for the first two excited torsional states of DME have been greatly enlarged and analyzed to allow predictions for the $mm$- and sub$mm$-range.