ABSOLUTE INTENSITIES MEASUREMENTS IN THE $\nu_{4} + \nu_{5}$ BAND OF $^{12}C_{2}H_{2}$: ANALYSIS OF HERMANWALLIS EFFECTS AND FORBIDDEN TRANSITIONS

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2000

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Ohio State University

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We have measured absolute line intensities in two bands of $^{12}C_{2}H_{2}$ near $7.5\mu m$, namely the $\nu_{4} + \nu_{5} (\Sigma^{+}_{u}) - 0(\Sigma^{+}_{g})$ and $\nu_{4} + \nu_{5}(\Delta_{u}) - 0(\Sigma^{+}_{g})$ bands, using Fourier transform spectroscopy with an accuracy estimated to be better than 2%. Using theoretical predictions from $Watson ^{a}$, the observation of the forbidden $\nu_{4} + \nu_{5}(\Delta_{u}) - 0(\Sigma^{+}_{g})$ 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: $\ell$-type resonance and Coriolis interaction. In the case of the $\nu_{4} + \nu_{5} (\Sigma^{+}_{u}) - 0(\Sigma^{+}_{g})$ band, the influences of $\ell$-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 $\ell = 1$ levels. We did not observe the $\nu_{4} + \nu_{5}(\Sigma^{-}_{u})- 0(\Sigma^{+}_{g})$ band, consisting only of a Q branch, in agreement with Watson's prediction.

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$^{a}$ J.K.G. Watson, J. Mol. Spectrosc. 188, 78 (1998)
Author Institution: Laboratoire de Chimie Physique Mol\'eculaire C. P. 160/09, Universit\'e Libre de Bruxelles

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