Photophysics of isolated molecules: para-substituted benzenes

Fluorescence quantum yield and decay times of p-fluorotoluene and p-fluorobenzotrifluoride in the gas phase at low pressures are reported. The variation in rate constant for radiative and non-radiative decay with increasing vibrational energy in the singlet manifold is discussed in relation to results for benzene and to current theories of non-radiative decay of aromatic molecules.