Exciton Dynamics in Tetracene Single CrystalsStudied Using
Femtosecond Laser Spectroscopy
by
Zephania Birech
Dissertation approved for the degree of
Doctor of Philosophy
at the University of Stellenbosch
Department of Physics,University of Stellenbosch,
Private Bag X1, 7602 Matieland, South Africa.
Promoters:
Prof. Heinrich Schwoerer
Prof. Erich G. Rohwer
December 2012
Required tetracene crystal thickness
Now when determining the appropriate thickness of tetracene crystal required toobtain an absorbance of 0.25 similar to that in solution at 474 nm, we need to firstdetermine the number of molecules in a unit volume. Given that the volume oftetracene unit cell is 583×10−24cm3 and that there are two molecules per unit cellthen we get
ccrystal =2
583× 10−24 = 3.4× 1021 (molecules) cm−3 (A.7)
The thickness which can give absorbance of 0.25 can be obtained from
lcrystal =OD
σ(λ)× ccrystal=
0.25(8.0× 10−19)× 3.4× 1021 = 920 nm (A.8)
The number of molecules excited NcrystalTc in the crystal
Given that the excitation spot diameter was 0.02 cm, one can estimate the numberof molecules excited in the 200 nm, 300 nm and 500 nm thick samples from
NcrystalTc = πr2lccrystal (A.9)
Ncrystal,200nmTc = 2.1× 1013 molecules (A.10)
Ncrystal,300nmTc = 3.2× 1013 molecules (A.11)
Ncrystal,500nmTc = 5.3× 1013 molecules (A.12)
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