THE $1 ^{1}B^{+}_{u} \leftarrow 1 ^{1}A^{-}_{g}$ ABSORPTION OF JET-COOLED TRANS,TRANS-1,3,5,7-OCTATETRAENE

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Ohio State University

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The direct absorption spectrum of the first allowed singlet-singlet electronic transition of octatetraene in a supersonic Argon jet will be presented. Cooling results in an order-of-magnitude decrease in the observed width of the origin band at $35553(3) cm^{-1}$ and a comparable sharpening throughout the spectrum, enabling vibrational frequencies, relative absorption intensities, and bandwidths to be measured with improved accuracy. In addition to the carbon-carbon stretching modes at 1235(5) and $1645(6) cm^{-1}$ and the skeletal modes at 197(5), 348(6), and $547(7) cm^{-1}$ reported for the jet-cooled fluorescence excitation $spectrum^{1}$ and the static gas $spectrum^{2}$, an in-plane bending mode at $1006(7) cm^{-1}$ and a carbon-carbon stretching mode at $1201(8) cm^{-1}$ are observed to be active. The origin is found to have the largest integrated absorption intensity and the narrowest bandwidth. Vibronic bandwidths increase markedly with excess energy in the $1 ^{1}B^{+}_{u}$ state, from $\sim 20 cm^{-1}$ (FWHM) at the origin to $\sim 80 cm^{-1}$ at $3280 cm^{-1}$ above the origin.


$^{1}$ L.A. Heimbrook, J.E. Kenny, B.E. Kohler, and G.W. Scott, J. Chem. Phys., 75, 4338 (1981) $^{2}$ R.M. Gavin, C. Weisman, J.K. McVey, and S.A. Rice, J. Chem. Phys., 68, 522 (1978).
Author Institution: Department of Chemistry, University of Connecticut; Department of Chemistry, Harvard University