15min:

SHANSHAN YU, BRIAN J. DROUIN, JOHN C. PEARSON AND HERBERT M. PICKETT, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109; VALERIO LATTANZI AND ADAM WALTERS, Centre d'Etude Spatiale des Rayonnements, Université de Toulouse [UPS], CNRS [UMR 5187], 9 avenue du Colonel Roche, BP 44346, F-31028 Toulouse Cedex 4, France.

Symmetric molecules have no permanent dipole moment and are undetectable by rotational spectroscopy. Their interstellar observations have previously been limited to mid-infrared vibration-rotation spectroscopy. Although relatively weak, vibrational difference bands provide a means for detection of non polar molecules by terahertz techniques with microwave precision. Herschel, SOFIA, and ALMA have the potential to identify a number of difference bands of light symmetric species, e.g., C₂H₂, CH₄ and C₃. This paper reports the results of the laboratory study on ¹²C₂H₂ and ¹²C₂D₂. The symmetric isotopomers of acetylene have two bending modes, the trans bending ₄ (¹ _g), and the cis bending ₅ (¹ _u). For ¹²C₂H₂, the two bending modes occur at 612 and 729 cm^-1, respectively. For ¹²C₂D₂, the two bending modes occur at 511 and 538 cm^-1. The ₅- ₄ difference bands are allowed and occur in the microwave, terahertz, and far-infrared wavelengths, with band origins at 117 cm^-1 (3500 GHz) for ¹²C₂H₂ and 27 cm^-1 (900 GHz) for ¹²C₂D₂.

Two hundred and fifty-one ¹²C₂D₂ transitions, which are from ₅- ₄, ( ₅+ ₄)-2 ₄ and 2 ₅-( ₅+ ₄) bands, have been measured in the 0.2-1.6 THz region, and 202 of them were observed for the first time. The precision of these measurements is estimated to be from 50 kHz to 100 kHz. A multistate analysis was carried out for the bending vibrational modes ₄ and ₅ of ¹²C₂D₂, which includes the lines observed in this work and prior microwave, far-infrared and infrared data on the pure bending levels. Significantly improved molecular parameters were obtained for ¹²C₂D₂ by adding the new measurements to the old data set which had only 10 lines with microwave measurement precision. The experiments on ¹²C₂H₂ are in progress and ten P branch lines have been observed. We will present the ¹²C₂H₂ results to date.