Diagrammatic representation of optical test plant 100W halogen lamp for PLE measurement 100W halogen lamp for OR measurement Lasers HeNe laser  632.8 nm,  10 mW/cm2 Ar laser  514 nm,  100 mW/cm2 HeCd laser  325 nm,  25 mW/cm2 N2 laser  337.1 nm,  1 MW/cm2 Laser beam chopper Helium closed loop cryostat (CTI-Cryogenics) N2 liquid cryostat Monochromator (MDR-41 - LOMO) for PLE excitation Monochromator (MDR-23 - LOMO) for PL and PLE measurement Cooled PMT, optical range 300-1100nm Cooled Ge p-i-n photodiode (Edinburgh Instruments), optical range 800-1700nm Analogue preamplifier (for synchronous detection mode) and comparator circuit (for photon counting mode) Lock-in amplifier (Stanford Research Systems SR510 and SR810, Unipan type 232B) Microprocessor controller for datalogging and monochromators remote control personal computer rotated mirrors lenses