Photoabsorption and direct dissociation cross sections of C2H2 in the 1530–1930 Å region: A temperature dependent study

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Abstract
Using synchrotron radiation as a continuum light source, the absolute photoabsorption and direct dissociation cross sections of C2 H2 in the 1530–1930 Å region have been measured at 295 and 155 K. In comparison with cross sections obtained at room temperature, the cross sections at absorption peaks typically increase by 10%–40% while those at absorption valleys decrease by as much as 30% at 155 K. Further, the absorption band profiles become narrower as the gas temperature decreases. In the presently studied wavelength region, there are at least three electronic states, namely, the à 1 Au , the B̃ 1 Bu , and a continuum. Using the present low temperature data we have constructed the repulsive potential energy curve of the direct dissociation state according to the Franck–Condon principle. The potential energy curve thus constructed can be more accurate than that constructed by using room temperature cross section data because only the absorption cross section from the ν’=0 level contributes. The present cross section data have also been used in the analysis of oscillator strength distribution.