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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