MILLIMETER AND SUBMILLIMETER WAVE SPECTRA OF THE HCOO$^{13}$CH$_{3}$ ISOTOPOLOG OF METHYLFORMATE IN THE GROUND STATE AND IN THE FIRST EXCITED TORSIONAL STATE

Abstract

The detection of nineteen new rotational transitions of the parent molecule of methylformate (HCOOCH$_{3}$) in the second lowest excited torsional mode ($\nu_{t} =2$) was recently reported in Orion-KL nderline{\textbf{64}}, 89, 2012.}, as well as the detection of eighty new lines corresponding to the two $^{18}O$ isotopologs of methylformate in their ground states nderline{\textbf{538}}, A199, 2012.}. The laboratory work on HCOO$^{13}$CH$_{3}$ was continued nderline{\textbf{500}}, 1109, 2009.}. A wide spectral range from 50 to 940 GHz was recorded in Lille with the submillimeter-wave spectrometer based on harmonic generation of a microwave synthesizer source, using a multiplication chain of solid state sources (50-100 and 150-940 GHz) and a backward wave oscillator (100-150 GHz), and coupled to a 2.2 m cell. The absolute accuracy of the line positions is better than 30 kHz up to 630 GHz and 50 kHz above. The two states ($\nu_{t}$ = 0 and 1) were fitted together using the RAM Hamiltonian of the BELGI program and a new set of $45$ parameters was accurately determined. The fit contains 7050 lines corresponding to the ground state up to $J = 78$ and $K_{a} = 34$ and 1907 lines related to $\nu_{t} =1$ up to $J = 59$ and $K_{a} = 24$. The detection of new $\nu_{t} =1$ lines in Orion KL will be reported and discussed. \textit{This work is supported by the French Programme National de Physico-Chimie du Milieu Interstellaire (CNRS), by CNES, and by the Spanish Government through the grants FIS2011-28738-C02-02 and CONSOLIDER 2009-00038.}