Exciton Dynamics in Tetracene Single Crystals Studied 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