10min:

K. DIDRICHE, C. LAUZIN, T. FOLDES, D. GOLEBIOWSKI, M. HERMAN, Service de Chimie quantique et Photophysique CP160/09, Faculté des Sciences, Université Libre de Bruxelles (U.L.B.), Av. Roosevelt, 50, B-1050, Bruxelles, Belgium; C. LEFORESTIER, ACTMM-CC 15.01, Institut Charles Gerhardt, 34095 Montpellier, France.

CW-cavity ring down spectroscopy was used to record in a free jet expansion the spectrum of the absorption band in (¹²C₂H₂)₂ with origin at 6547.6 cm^-1. It is a perpendicular band and corresponds to 2CH excitation in the hat unit of the T-shaped dimer. Calibration (better than \pm1× 10^-3 cm^-1 accuracy) and ring-down time (130 µs) were improved compared to a previous contribution \footnoteDidriche et al. , Mol. Phys. 2010, \textbf108, 2158-2164, and a line-by-line analysis was undertaken. Three series of lines were identified involving levels with A1⁺, E⁺ and B1⁺ tunneling symmetries, confirming the spectral and symmetry analyses reported in the literature for the 1CH excitation band \footnoteFraser et al. , J. Chem. Phys. 1988, \textbf89, 6028-6045. 164 vibration-rotation-tunneling lines were assigned in the K_a'-K_a''= 2-3, 0-1, 2-1 and 4-3 sub-bands and effective rigid rotor vibration-rotation constants were obtained by simultaneously fitting 1CH and 2CH lines from the same symmetry series. Perturbations affecting the K_a stacks, in particular, are reported. The tunneling frequency in 2CH is estimated to be ^2CH_tunn. = 270 MHz for the K_a = 0 stack. The rotational temperature is determined to be 23 K from relative line intensities and the half lifetime of the dimer in the 2CH hat state is estimated to 1 ns from individual line widths.