Absolute intensities measurements in the ν4+ν5 band of 12C2H2:: Analysis of Herman-Wallis effects and forbidden transitions

We measured absolute line intensities in two bands of (C2H2)-C-12 near 7.5 mu m, namely the v(4) + v(5)(Sigma(u)(+))-0(Sigma(g)(+)) and v(4) + v(5)(Delta(g))-0(Sigma(g)(+)) bands, using Fourier transform spectroscopy with an accuracy estimated to be better than 2%. Using theoretical predictions from Watson [J. K. G. Watson, J. Mol. Spectrosc, 188, 78 (1998)], the observation of the forbidden v(4), + v(5)(Delta(g))-0(Sigma(g)(+)) band and the Herman-Wallis behavior exhibited by its rotational lines were studied quantitatively in terms of two types of interactions affecting the levels involved by the band: C-type resonance and Coriolis interaction. In the case of the v(4) + v(5)(Sigma(g)(+))-0(Sigma(g)(+)) band, the influence of e-type resonance is also confirmed. We also attributed the intensity asymmetry observed between the R and P branches of that latter band to a Coriolis interaction with l = 1 levels. We did not observe the v(4) + v(5)(Sigma(g)(-))-0(Sigma(g)(+)) band, consisting only of a Q branch, in agreement with Watson's prediction. (C) 2000 Academic Press.