Intramolecular non-radiative relaxation from the first excited singlet background manifolds of anthracene and 9,10-dichloroanthracene in partially jet-cooled states

The absorption and fluorescence excitation spectra of anthracene (A) and 9,10-dichloroanthracene (DCLA) were measured simultaneously under various cooling conditions by using the technique of supersonic expansion. The sequence bands observed mostly on the red side of each single vibronic level at low stagnation pressures were ascribed to the transition to the S1 background manifolds. The fluorescence quantum yields for these bands were estimated from the relative intensities of the absorption and fluorescence excitation spectra at their band peaks. The radiations process was found to be enhanced by non-totally symmetric vibrational modes in the S1 background manifolds with low excess energies.