a b s t r a c tA new rovibrational study of the m 6 band of methyl iodide was conducted to obtain a rather complete line list. A new analysis of line positions was accomplished. The spectrum of this band has been first recorded using the Bruker IFS125HR Fourier transform spectrometer (FTS) at the AILES beamline of the SOLEIL Synchrotron facility and later with the Bruker IFS125HR FTS located at the LISA facility in Créteil.Altogether, about 10,000 lines were assigned for the m 6 and 2m 3 bands up to high quantum numbers (J 6 85 and K 6 20). Because of the large value of the 127 I nuclear quadrupole hyperfine constant, a significant portion of these assignments concerns clusters of hyperfine subcomponents, which are easily observable at 11 lm. These infrared data were combined in a least squares fit together with the existing microwave data on rotational transitions within the v 6 = 1 and v 3 = 2 vibrational states to get the upper state rotational constants and interacting parameters for the v 6 = 1 and v 3 = 2 states. Due to the high values of quantum numbers achieved during this infrared analysis, the final energy level calculation accounts for aC x ðD' ¼ AE1; DK ¼ AE1Þ and an a ðD' ¼ Ç1; DK ¼ AE2Þ types of Coriolis interactions coupling the v 6 = 1 energy levels with those from the v 3 = 2 and v 2 = 1 states, respectively. On the other hand, it proved unnecessary to update the existing hyperfine parameters for the v 6 = 1 and v 3 = 2 states.
Context. An astronomical survey of interstellar molecular clouds needs a previous analysis of the spectra in the microwave and sub-mm energy range of organic molecules to be able to identify them. We obtained very accurate spectroscopic constants in a comprehensive laboratory analysis of rotational spectra. These constants can be used to predict the transitions frequencies very precisely that were not measured in the laboratory. Aims. We present the experimental study and its theoretical analysis for two 13 C-methyl formate isotopologues to detect these two isotopologues for the first time in their excited torsional states, which lie at 130 cm −1 (200 K) in Orion-KL. Methods. New spectra of HCOO 13 CH 3 ( 13 C 2 ) methyl formate were recorded with the mm-and submm-wave spectrometer in Lille from 50 to 940 GHz. A global fit for v t = 0 and 1 was accomplished with the BELGI program to reproduce the experimental spectra with greater accuracy. Results. We analysed 5728 and 2881 new lines for v t = 0 and 1 for HCOO 13 CH 3 . These new lines were globally fitted with 846 previously published lines for v t = 0. In consequence, 52 parameters of the RAM Hamiltonian were accurately determined and the value of the barrier height (V 3 = 369.93168(395) cm −1 ) was improved. We report the detection of the first excited torsional states (v t = 1) in Orion-KL for the 13 C 2 and 13 C 1 methyl formate based on the present analysis and previously published data. We provide column densities, isotopic abundances, and vibrational temperatures for these species. Conclusions. Following this work, accurate prediction can be provided. This permits detecting 135 features of the first excited torsional states of 13 C-methyl formate isotopologues in Orion-KL in the 80−280 GHz frequency range, without missing lines.
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