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 Φ_λ represents the fluorescence intensity for excitation at wavelength λ. The low value of Φ_λ 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).

© 1984 Optical Society of America