Free-jet fluorescence excitation spectrum of <i>t</i> <i>r</i> <i>a</i> <i>n</i> <i>s</i>, <i>t</i> <i>r</i> <i>a</i> <i>n</i> <i>s</i>-1,3,5,7-octatetraene

Abstract

The fluorescence excitation spectrum from 2980 to 2650 Å of trans, trans-1,3,5,7-octatetraene in a free jet has been measured monitoring emission in the region 3250 to 3500 Å. The vibronic bands of the 1 1Bu←1 1Ag (S2←S0) transition of octatetraene narrow significantly in a seeded free jet of ∼8 ppm octatetraene in helium (FWHM∼200 cm−1 for the room temperature static gas; FWHM∼35 cm−1 in the jet). Although absolute intensities of the vibronic bands are not determined, there is significant intensity in the origin, and the vibronic development involves only a few modes. Estimated translational, rotational, and vibrational temperatures of ∼1,&amp;lt;45, and &amp;lt;170 K, respectively, were achieved in the expansion. The major source of the observed linewidth is attributed to coupling of the 1 1Bu state to the lower 1Ag states.