POLYENE ELECTRONIC SPECTROSCOPY AT ROTATIONAL $RESOLUTION,^{1}$

$^{1}$ Supported by PRF (23306-B4) and NSF (CHE-9224398). $^{2}$ H. Petek, A. J. Bell, K. Yoshihara, and R. L. Christensen, J. Chem. Phys. 95, 4739 (1991); H. Petek, et al., unpublished.Author Institution: Department of Chemistry, Bowdoin College; Department of Chemistry, University of PittsburghThe symmetry-forbidden nature of the $S_{1}$ state of polyenes has made their electronic spectra somewhat controversial. In this report, we describe the one-photon fluorescence excitation spectra of all-trans-octatetraene (OT), nonatetraene (NT), and decatetraene (DT) at full rotational resolution. In accord with the low resolution results of Petek, $et al.,^{2}$ the lowest frequency mode, an in-plane bend, is assigned as the apparent origin in the spectra of OT and DT. This also is the most intense peak in NT, but there is a weak transition $70 cm^{-1}$ to the red which we assign as the true origin. We discuss the observed differences in the rotational constants and polarization of these bands. We also describe the influence of the methyl group torsional motion in NT and DT on our rotationally resolved spectra