Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

12.3 The radiative lifetime 0 and the fluorescence quantum yield Φ 243
12.3 The radiative lifetime τ 0 and the fluorescence quantum yield Φ
I Determination of the radiative lifetime τ 0 : Assuming that there are no competing
nonradiative processes, we can determine the radiative lifetime 0 by
(1) time-resolved measurements of the (exponential) fluorescence decay after pulsed
excitation,
(2) high-resolution measurement of the natural (Lorentzian) linewidth according to
∆ ∆ = ~ (12.8)
y
0 =
1
2 ∆ 0
(12.9)
(3) evaluation from the molar absorption coefficient (Lambert-Beer) using the relationship
between the Einstein coefficients 21 and 21 , 51
21 = 83
3 21 (12.10)
(4) calculate the value of ¯¯ ¯¯2 (⇒ 12 21 21 )fromfirst principles.
I Fluorescence quantum yield: Apart from quenching, the fluorescence is reduced by
other radiationless processes (IC, ISC, R, . . . ). The experimentally measured fluorescence
lifetime is therefore smaller (often much smaller) than the radiative lifetime 0 .
The ratio is the fluorescence quantum yield:
Φ =
+ + + + [Q] + = 0
(12.11)
51 The actual procedure, which involves integration over the absorption band, corrects for the influence
of the refraction index of the solvent, etc., is called the Strickler-Berg analysis.