Deactivation of Excited States of Liquid Alkyl Benzenes at the Entrance Window

Abstract

In a series of early investigations(1) a marked decrease of the fluorescence quantum yield of liquid alkyl benzenes (e.g. toluene, mesitylene) was found when the wavelength of the exciting radiation was decreased from those for excitation of S1 (the first excited singlet level) to those for excitation of S2 and S3 (the second and third excited singlet levels). Further decrease of the excitation wavelength is accompanied by an increase in the fluorescence quantum yield. These results for toluene are reproduced in Fig.1, curve(a), where Φλ represents the fluorescence intensity for excitation at wavelength λ. The low value of Φλ for excitation of S3 may be caused either by a quenching mechanism which acts on the molecule in this state, or by a non-efficient transition from this state to the emitting state S1, or by some quenching mechanism which acts on the molecule in S1 but which is absent when S1 is directly excited from the ground state. (A combination of all three mechanisms or any two of them is, of course not excluded).