Millimeter Wave and Terahertz Spectra and Global Fit of Torsion-Rotation Transitions in the Ground, First and Second Excited Torsional States of 13CH3OH Methanol

Abstract

Methanol is observed in a wide range of astrophysical sources throughout the universe, and comprehensive databases of the millimeter and THz spectra of CH3OH and its principal isotopologues represent important tools for the astronomical community. A previous combined analysis of microwave and millimeter wave spectra of 13CH3OH together with Fourier transform far-infrared spectra was limited to the first two torsional states, v_t = 0 and 1, for J values up to 20. The limits on frequency and quantum number coverage have recently been extended by new millimeter and THz measurements on several different spectrometers in the Cologne laboratory in the frequency windows 34-70 GHz, 75-120 GHz, 240-340 GHz, 360-450 GHz and 1.12-1.50 THz. With the new data, the global treatment has now been expanded to include the first three torsional states for J values up to 30. The current 13CH3OH data set contains about 2,300 microwave, millimeter-wave, sub-millimeter and THz lines and about 17,100 Fourier-transform far-infrared lines, representing the most recent available information in the quantum number ranges J </= 30, K </= 13 and v_t </= 2. The transitions have been successfully fitted to within the assigned measurement uncertainties of +/-50 kHz for most of the frequency-measured (i.e. MW, MMW, Sub-MMW, THz) lines and +/-6 MHz for the FIR lines. A convergent global fit was achieved using 103 adjustable parameters to reach an overall weighted standard deviation of 1.37. Our new C-13 methanol database is improved substantially compared to the existing one (Li-Hong Xu, F.J. Lovas, J. Phys. Chem. Ref. Data 26 (1997) 17-156), and will be available in the Cologne Database for Molecular Spectroscopy, CDMS (http://www.astro.unikoeln.de/cdms/). (Abbreviated)