Knowledge Bank

Fluorescence spectra have been obtained from the first excited singlets of gas phase $C6H6$ and $C6D6$ which possess only zero point vibrational energy. These molecules are excited by limiting absorption to the hot $B\circ \circ$ bands. Collisional change of vibrational energy before fluorescence is prevented by use of low gas pressures. Although energy problems limit resolution of the spectra to about $3\backslash AA$, the principal structure is readily apparent. Vibrational assignments are made on the basis of those modes active in gaseous absorption, and the relative intensity of bands correlates well with that predicted from absorption data. This is in marked contrast to the analysis of spectra obtained from excited benzene molecules made vibrationless by use of rigid media at $77\circ K$.