Knowledge Bank

This preliminary work is the first to demonstrate that narrowing of vibronic bands in linear polyenes can be accomplished by using the free-jet expansion technique. A mixed-gas free-jet expansion was used to produce cold, gas phase octatetraene for laser-induced fluorescence excitation studies. The results indicate a high degree of vibrational cooling and significant narrowing of vibronic bands in the $S0\to S2(11Ag\to 11Bu$) electronic transition. In the free-jet excitation spectrum the vibronic bands of the $S0\to S2$ electronic transiton were narrowed ($fwhm\prime s=20cm-1$) relative to the static gas spectrum ($fwhm\prime s=20cm-1$) when a mixture of 8ppm of octatetraene in Helium expanded through a 50$\mu$ diameter nozzle into a vacuum. In the expansion, a translational temperature of less than 1k and a vibrational temperature of less than 137k were achieved respectively; at this time a rotational temperature can not be made. The $11Bu$ origin for trans, trans-octatetraene in the gas phase has now been precisely assigned at $35547.8cm-1$. The prominence of this $11Bu$ origin shows that there is no twist in the geometry of the second excited state of this molecule. The observed linewidths have been compared to those reported for octatetraene and other linear polyenes in static gas, low-temperature matrix and free-jet environments.