High resolution overtone spectroscopy of the acetylene van der Waals dimer, (12C2H2)2

CW-cavity ring down spectroscopy was used to record in a free jet expansion the spectrum of the absorption band in ((C2H2)-C-12)(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 +/- 1 x 10(-3) cm(-1) accuracy) and ring-down time (130 mu s) were improved compared to a previous contribution (Didriche et al. Mol. Phys., 2010, 108, 2158-2164). A line-by-line analysis was achieved. Three series of lines were identified involving levels with A(1)(+), E+ and B-1(+) ground state tunneling symmetries, confirming the spectral and symmetry analyses reported in the literature for the 1CH excitation band (Fraser et al. J. Chem. Phys., 1988, 89, 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 with the same symmetry series. Perturbations affecting the K-a stacks, in particular, are reported. The tunneling frequency in 2CH is estimated to be nu(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 lifetime of the dimer in the 2CH hat state is estimated to be 1 ns from individual line widths.