INTRAMOLECULAR NONRADIATIVE RELAXATION FROM THE FIRST EXCITED SINGLETBACKGROUND MANIFOLDS OF ANTHRACENE AND 9,10-DICHLOROANTHRACENE IN PARTIALLY JET-COOLED STATES

The absorption and fluorescence excitation spectra of anthracene (A) a nd 9,10-dichloroanthracene (DCLA) were measured simultaneously under V arious cooling conditions by using the technique of supersonic expansi on. The sequence bands observed mostly on the red side of each single vibronic level at low stagnation pressures were ascribed to the transi tion to the S-1 background manifolds. The fluorescence quantum yields for these bands were estimated from the relative intensities of the ab sorption and fluorescence excitation spectra at their band peaks. The radiationless process was found to be enhanced by non-totally symmetri c vibrational modes in the S-1 background manifolds with low excess en ergies.